Internet-Draft Hardware Email Attestation July 2026
Drake Expires 18 January 2027 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-drake-email-hardware-attestation-03
Published:
Intended Status:
Experimental
Expires:
Author:
C.N. Drake
1id.com

Hardware Attestation for Email Sender Verification

Abstract

This document defines a mechanism for automated email senders (AI agents, bots, and autonomous systems) to include hardware attestation evidence in message headers, enabling receiving mail servers to cryptographically verify that the sending system has access to a genuine hardware security component -- such as a Trusted Platform Module (TPM), a PIV smart card (e.g., YubiKey), a virtual TPM, or a software-managed key -- from a known manufacturer or issuer. The attestation proves hardware presence, not message composition locale. The verification chain runs from the email header to a manufacturer's root Certificate Authority (for hardware-backed attestation) or to an issuer's verification key (for issuer-certified attestation).

Each automated sender is assigned a persistent agent identity, expressed as a URN in the "aid" (Agent Identity) namespace ([RFC8141]), enabling federated issuance by multiple independent identity providers and persistent reputation tracking across protocols and platforms.

As a companion mechanism, this document defines a privacy-preserving alternative using SD-JWT (Selective Disclosure JWT, [RFC9901]) where the sender can prove specific claims about their hardware trust level without revealing their hardware identity.

Together, these mechanisms provide both Sybil-resistant authentication and a foundation for reputation building: at the hardware-anchored trust tiers, each identity requires a unique hardware security component, so large-scale identity manufacture costs physical devices rather than compute, while the persistent identity enables receiving systems to accumulate trust signals over time. Software-only agents MAY participate at a clearly-labeled lower trust tier. The identity layer itself -- the "aid" URN, its registry, trust tiers, and lifecycle -- is defined in [I-D.drake-agent-identity-registry]; this document defines the transport binding and its verification.

While this document specifies these mechanisms for email message headers, the attestation formats defined herein -- both the CMS attestation bundle and the SD-JWT trust proof -- are self-contained, transport-independent data structures applicable to HTTP headers, agent-to-agent messaging, payment authorization, and other Internet protocols.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 18 January 2027.

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Table of Contents

1. Introduction

Email authentication today relies on three complementary mechanisms: SPF [RFC7208] verifies that the transmitting mail server's IP address is authorized for the domain, DKIM [RFC6376] provides a domain-level cryptographic signature, and DMARC [RFC7489] ties them together with policy. These mechanisms prove that an email was authorized by a domain. They do not prove anything about the agent or device that composed the message.

This distinction has become critical. The rapid proliferation of autonomous AI agents capable of composing human-quality text has rendered content-based spam detection increasingly ineffective. An attacker can register unlimited domains, configure valid SPF/DKIM/DMARC, and use AI to generate messages indistinguishable from legitimate correspondence. Every existing defense -- CAPTCHAs, phone verification, behavioral analysis, IP reputation -- fails when the attacker is an AI that can operate at scale with near-zero marginal cost.

The fundamental problem is that all existing sender identity signals are software-based and therefore copyable at zero cost. This document proposes anchoring sender identity to hardware, applying the remote attestation architecture of [RFC9334] to email: specifically, to the tamper-resistant hardware security components present in virtually every modern computing device.

A hardware security component -- whether a discrete TPM chip, a firmware TPM (e.g., Intel PTT or AMD fTPM), a PIV smart card (e.g., YubiKey), or a virtual TPM provided by a hypervisor -- contains a unique identity key burned in or generated at manufacturing time, with a certificate chaining to the manufacturer's root CA. This key is designed to resist extraction and cloning. By signing email content with a hardware-resident key and including or linking the certificate chain in the message headers, an automated sender proves that the email originated on a specific, genuine piece of hardware.

Receiving mail servers can verify this proof by validating the certificate chain against manufacturer root CAs (Intel, AMD, ARM, Apple, etc.) which are already widely distributed for Secure Boot and platform integrity. Deployments that wish to accept hardware attestation will need to maintain or subscribe to an appropriate trust store of these root CAs.

This document defines two complementary mechanisms:

  1. Direct Hardware Attestation (Section 5): A CMS [RFC5652] signed-data structure containing the attestation signature and full certificate chain, carried in or linked from an email header. Verifiable against manufacturer root CAs when the chain is manufacturer-rooted, or against an Issuer's CA when the Issuer has verified the hardware at enrollment and issued the AK certificate (the deployment model of every live example in Appendix B). Optimized for deliverability and reputation tracking.
  2. SD-JWT Trust Proof (Section 6): A Selective Disclosure JWT [RFC9901] issued by an identity provider (issuer), where the sender selects which claims to reveal. Optimized for privacy in contexts where revealing hardware identity is undesirable.

1.1. Design Principles

The following principles guide the design of this specification and are referenced throughout the document:

Automated-Sender Focus
This specification targets AI agents, bots, and autonomous systems. It is not intended for individual human users (see Section 3). The privacy trade-offs are acceptable for automated senders but disproportionate for human correspondence.
Transport Independence
The attestation formats defined herein -- the CMS attestation bundle and the SD-JWT trust proof -- are self-contained data structures that can be carried in any protocol capable of transporting octet strings. This document specifies the email transport binding; Appendix C discusses applicability to HTTP and other protocols.
Federated Issuance
Issuance is federated at the Registrar layer of the registry architecture ([I-D.drake-agent-identity-registry]): multiple independent, accredited Issuers (Registrars) compete to enroll agents in a shared namespace, and an agent can move between Issuers without its identity changing. Verifiers discover an Issuer's keys through DNS or HTTPS (Section 4.5).
Inclusive Trust Tiers
Agents with genuine hardware security components receive the highest trust, but agents with virtual hardware or software-only keys can still participate at lower trust tiers, building reputation over time. This inclusivity avoids an all-or-nothing barrier to adoption.
Reputation as the Goal
Sybil resistance is a means, not an end. The persistent, hardware-anchored identities defined here exist to enable receiving systems to build and query reputation over time, rewarding good behavior and flagging bad.
Dual Trust Anchors
Both attestation modes deliberately provide the agent identity (aid) and the hardware certificate or fingerprint together. Verifiers MAY choose which anchor to use as their basis for trust decisions: the identity is the primary key for reputation tracking and persists across hardware migrations, while the hardware fingerprint is IdP-independent and verifiable offline against manufacturer root CAs. Some verifiers will prefer the identity (for accumulated reputation), others will prefer the hardware (for IdP-independent assurance or cross-IdP correlation), and high-assurance scenarios may require both. This dual provision is intentional, not redundant.

1.2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

1.3. Terminology

Hardware Attestation Mechanism
A tamper-resistant hardware or firmware component capable of protecting private keys and producing cryptographic attestation statements verifiable to a manufacturer or platform Certificate Authority. Examples include TPM 2.0 devices (discrete and firmware), PIV tokens (e.g., YubiKey), and virtual TPMs. This document also accommodates software-managed keys at a lower trust tier for agents that lack hardware security components but wish to participate in the reputation ecosystem.
TPM
Trusted Platform Module, as specified by the Trusted Computing Group (TCG) [TCG-TPM2]. This document assumes TPM 2.0. A TPM may be a discrete chip, a firmware implementation (Intel PTT, AMD fTPM), or a virtual instance provided by a hypervisor.
EK
Endorsement Key. A unique RSA or ECC key pair generated inside the TPM at manufacturing time, as profiled in [TCG-EK-PROFILE]. The EK private key never leaves the TPM. The EK certificate is signed by the chip manufacturer's CA and serves as the persistent hardware identity for the device.
AK
Attestation Key. A signing key created inside a hardware security component, certified against the device's identity key (EK for TPM, attestation certificate for PIV) to prove it resides in the same device. In this specification, the AK SHOULD be created as a transient object and SHOULD NOT be persisted in non-volatile storage, to avoid consuming limited hardware resources. See Section 4.4.
Identity Certificate
The manufacturer-issued certificate that identifies a specific hardware security component. For TPMs, this is the EK certificate. For PIV tokens, this is the device attestation certificate. The SHA-256 hash of the SubjectPublicKeyInfo DER encoding from this certificate serves as the hardware fingerprint. The fingerprint is derived from the public key structure rather than the certificate as a whole, so that the fingerprint remains stable if the manufacturer reissues the certificate (e.g., to extend validity or update metadata) without changing the underlying key pair.
Manufacturer CA
The Certificate Authority operated by the hardware manufacturer (e.g., Intel, AMD, Infineon, Yubico) that signs identity certificates. Root CA certificates are publicly available.
Agent Identity
A persistent, globally unique identifier for an autonomous agent, expressed as a URN in the "aid" namespace (Section 4.1). Agent identities are assigned by issuers and are bound to specific hardware via the issuer's anti-Sybil registry.
Issuer
An entity that verifies hardware attestation evidence, maintains an anti-Sybil registry (ensuring one hardware device maps to at most one identity), and issues trust assertions (e.g., SD-JWTs) about enrolled agents. An Issuer is OPTIONAL for Mode 1 (Direct Hardware Attestation) and REQUIRED for Mode 2 (SD-JWT Trust Proof). See Section 4.5 for discovery and the federated issuance model.
Trust Tier
A classification of an agent's hardware trust level, assigned by an Issuer. Defined tiers are grouped into three compatibility classes, ordered from strongest to weakest anti-Sybil resistance: (a) "sovereign" (genuine non-virtual hardware TPM or discrete security chip) and "portable" (PIV smart card such as YubiKey) -- manufacturer-attested, persistent identity; (b) "virtual" (hypervisor-provided virtual TPM) and "enclave" (hardware enclave such as Apple Secure Enclave) -- key isolation without anti-Sybil guarantee, as both device types can be re-keyed at will; (c) "declared" (software-only key, no hardware attestation). Verifiers use trust tiers to apply differentiated policy. For identities with multiple enrolled devices of different types, Issuers that support multi-device identities MAY report the trust tier of the device used for the most recent attestation rather than a static assignment. Verifiers SHOULD therefore expect the same agent identity URN to appear with different trust tier values over time.
Sybil Attack
An attack where a single adversary creates many pseudonymous identities to subvert a reputation or trust system.

2. Problem Statement

The general problem -- the collapse of content, source, and volume signals against capable automation, and the Sybil threat model that makes cheaply-minted identities the root failure -- is stated in [I-D.drake-agent-identity-problem-statement] and is not restated here.

Its email-specific form: anti-spam defenses have worked by imposing costs (CAPTCHAs, IP reputation, phone verification) that AI senders now eliminate. Hardware attestation re-introduces a cost floor no software can remove: each hardware-anchored sender identity requires a physical security component, roughly $20-2000 per identity versus approximately zero for software-only identities, and because each hardware identity is unique and persistent, a device that sends abuse is flagged permanently -- the attacker needs new silicon, not a new account. Software-only agents still participate, at a trust tier every receiver can see.

3. Applicability

This specification targets authentication of automated senders -- AI agents, bots, and high-volume automated systems -- where Sybil resistance justifies hardware-anchored identity. The proliferation of autonomous AI agents capable of generating human-quality content at scale is the primary motivation.

This specification is NOT intended or recommended for individual human users sending personal or low-volume email. Human senders using consumer mail user agents (e.g., Gmail web client, Outlook) face privacy risks from Mode 1 Direct Attestation that are disproportionate to the benefit: the hardware fingerprint uniquely identifies the sending device across all messages and contexts, enabling long-term tracking that exceeds current email privacy norms. Mode 2 (SD-JWT Trust Proof) mitigates this via selective disclosure but requires enrollment with an Issuer, which is not a typical consumer workflow.

Mail User Agents (MUAs) designed for interactive human use MUST NOT enable Direct Hardware Attestation (Mode 1) by default. Activation MUST require explicit, per-device user consent that warns of persistent cross-context tracking before any attestation header is added to outgoing messages. Operators of human-facing email services MAY offer hardware attestation as an opt-in feature (e.g., via a browser extension or MUA plugin), but MUST provide clear disclosure of the linkability implications before activation. Receivers SHOULD NOT penalize the absence of hardware attestation from senders exhibiting human-characteristic patterns (e.g., low volume, natural language variance, interactive reply chains).

For human identity proofs in email, existing mechanisms such as S/MIME [RFC8551] and DANE [RFC7671] remain appropriate.

Regulatory note: Article 50 of the European Union's AI Act (Regulation (EU) 2024/1689) requires, from 2 August 2026, that AI systems interacting with people disclose their nature, and that providers of generative AI systems mark their outputs in a machine-readable format detectable as artificially generated. The attestation headers defined here are a machine-readable, cryptographically verifiable declaration of automated origin, bound to the sending agent's identity; deployments subject to such transparency obligations MAY use them as a component of compliance for agent-generated email, alongside any content-marking techniques the regulation's guidelines require.

The attestation formats are transport-independent by design (see Section 1.1); this document specifies the email transport binding as the primary application.

4. Architecture

This section defines the identity model, hardware mechanism taxonomy, key lifecycle, and issuer discovery procedures that underpin both attestation modes.

4.1. Agent Identity Format

Agent identities are URNs in the "aid" namespace, whose syntax, canonical-identifier generation, handle model, and IANA registration are defined in [I-D.drake-agent-identity-registry]. For the purposes of this document a verifier needs only the following properties. An aid URN has the form urn:aid:namespace:agent-id, where the namespace is a governance-allocated label (for example global) and the agent-id is either a permanent canonical identifier (prefix id-) or a retirable handle (which never begins with id-). Examples:

  • urn:aid:global:id-qkckh-xxtcw-cxbvp-gpskg -- canonical, permanent
  • urn:aid:global:crusty -- handle for the same agent

Canonical identifiers are never reassigned, even after decommissioning -- the property reputation systems depend on. Verifiers and reputation systems MUST key long-lived state on the canonical form, never on a handle. The namespace carries no issuer information: which Issuer currently services an identity is a mutable relationship, discoverable through the registry ([I-D.drake-agent-identity-resolution]), never encoded in the name. The URN scheme and NID are case-insensitive; the namespace and agent-id are lowercase.

4.2. Identity-to-Hardware Cardinality

The agent identity URN (aid) and the hardware fingerprint serve as dual trust anchors (see "Dual Trust Anchors" in Section 1.1). The cardinality rules that relate them -- one device backs at most one identity, permanently; one identity may be backed by several devices without amplifying reputation; disabled devices are never re-enrollable; tier-compatibility groups, upward-only tier transitions with burn-on-upgrade and co-presence verification -- are the anti-Sybil invariants and lifecycle rules of [I-D.drake-agent-identity-registry], which Issuers under this specification MUST enforce.

Two consequences matter to verifiers of this transport. First, a given aid may legitimately appear with different hardware fingerprints over its lifetime (migration, backup, capacity), and with different trust tier values across messages: the typ parameter (Mode 1) and trust_tier claim (Mode 2) describe the device used for that attestation, not a static property. Second, verifiers SHOULD use the aid URN as the primary reputation key, with the hardware fingerprint as a secondary, Issuer-independent signal.

4.3. Hardware Attestation Mechanisms

This specification supports multiple hardware security technologies through a common abstraction. Each mechanism provides an identity certificate and a signing capability (used for per-message attestation). For sovereign and portable tiers, the identity certificate is signed by the hardware manufacturer; for enclave and virtual tiers, the identity certificate is issued by the Issuer. The hardware type is declared in the attestation header via the "typ" parameter.

Table 1
typ value Technology Identity Certificate Trust Tier
TPM TPM 2.0 (discrete or firmware) EK Certificate sovereign
PIV PIV smart card (e.g., YubiKey) Device Attestation Certificate portable
ENC Hardware enclave (e.g., Apple Secure Enclave) Issuer-certified enclave key certificate enclave
VRT Virtual TPM (hypervisor-provided) vTPM EK Certificate virtual
SFT Software-managed key Issuer-signed certificate declared

Additional hardware types MAY be defined in future specifications by registering new "typ" values with IANA (see Section 13).

For TPM-based attestation (typ=TPM), the identity certificate is the Endorsement Key (EK) certificate, provisioned at manufacturing time and stored in the TPM's non-volatile memory. The EK certificate chains to the TPM manufacturer's root CA (e.g., Intel, AMD, Infineon).

For PIV-based attestation (typ=PIV), the identity certificate is the device attestation certificate issued by the token manufacturer (e.g., Yubico). The signing key is a PIV authentication key resident on the token.

For hardware enclave attestation (typ=ENC), the signing key resides in a tamper-resistant hardware enclave integrated into the device's system-on-chip (e.g., the Secure Enclave Processor on Apple silicon). Unlike discrete TPMs and PIV tokens, hardware enclaves do not provide manufacturer-issued identity certificates with a chain to a publicly available root CA, and critically, they do not support persistent attestation identities: the enclave key can be destroyed and regenerated at will by the device owner. This means hardware enclaves provide NO anti-Sybil guarantee -- a single device can generate an unlimited number of distinct enclave keys over time. For this reason, the "enclave" trust tier is placed in the same compatibility group as virtual TPMs (typ=VRT), which share the same re-keying limitation. The identity certificate for typ=ENC is issued by the Issuer at enrollment time after verifying that the key was generated inside the enclave using a trust-on-first-use (TOFU) model. Note that the Secure Enclave performs internal hashing (ECDSA with SHA-256 over the raw input rather than a pre-hashed digest); implementations MUST pass the raw attestation-input bytes (72 bytes) to the enclave signing function, NOT the attestation-digest.

For virtual TPM attestation (typ=VRT), the identity certificate is the vTPM's EK certificate, signed by the hypervisor vendor's CA (e.g., VMware, Microsoft). Verifiers MUST distinguish virtual from physical hardware; see Section 8.3 for Sybil resistance implications.

For software-only attestation (typ=SFT), the signing key is a software-managed key pair with no hardware protection. The identity certificate is issued by the Issuer at enrollment time. Software-only attestation provides no hardware-based Sybil resistance but allows agents to participate in the reputation ecosystem at a lower trust tier. A legitimate agent that protects its key and builds reputation over time can achieve meaningful trust even without hardware backing.

4.4. Transient Key Model

For TPM-based attestation, this specification RECOMMENDS a transient key model in which the Attestation Key (AK) is created as a transient TPM object (via TPM2_CreatePrimary) and is NOT persisted in the TPM's non-volatile (NV) storage. This avoids consuming limited NV slots, which are a scarce resource on most TPMs (typically 3-7 loaded objects at once).

The transient key lifecycle is:

  1. Read EK certificate from the TPM's NV storage (index 0x01C00002 for RSA EK). This is a read-only operation. The EK certificate MAY be cached indefinitely as it does not change.
  2. Create transient AK via TPM2_CreatePrimary under the endorsement hierarchy with a fixed template. Because the template and the Endorsement Primary Seed (EPS) are constant, this deterministically produces the same key on every invocation.
  3. Binding verification (one-time, during enrollment with an Issuer): The Issuer creates a credential challenge (TPM2_MakeCredential) encrypted to the EK; the agent decrypts it (TPM2_ActivateCredential), proving the AK resides in the same TPM as the EK. The resulting AK certificate is stored client-side (outside the TPM) and reused across signing operations.
  4. Sign the attestation content with the transient AK (TPM2_Sign).
  5. Flush the transient AK handle (TPM2_FlushContext) to free the object slot.

Implementations SHOULD flush transient objects immediately after use. If TPM2_CreatePrimary returns TPM_RC_OBJECT_MEMORY (0x0902), the implementation MUST flush existing transient handles and retry.

The EK certificate is the persistent hardware identifier. The AK is ephemeral infrastructure -- it exists only long enough to produce a signature, then is discarded. No data is written to the TPM's non-volatile memory at any point in this protocol.

The transient AK is deterministic within a given Endorsement Primary Seed (EPS) epoch. The EPS is re-randomized when the TPM is cleared (TPM2_Clear) or reset to factory defaults. After such an event, the AK public key changes and the agent MUST re-enroll with its Issuer to establish a new AK-to-EK binding. The EK certificate remains constant across TPM clears, so the hardware identity persists even though the AK changes.

Note: On some platform configurations (particularly Windows with default TPM Base Services policy), reading the EK certificate from NV storage may require a one-time operating system configuration change or elevated access during initial enrollment. Once the EK certificate has been read and cached client-side, subsequent attestation operations do not require elevation. Implementations that cannot read the EK certificate directly MAY derive the EK public key via TPM2_CreatePrimary with the TCG EK template and use the resulting public key hash as the hardware fingerprint; verifiers can then query manufacturer APIs (e.g., Intel Trusted Services) to obtain the full certificate if needed for chain validation. Alternatively, the Issuer can perform EK certificate extraction during enrollment and include it in the AK certificate it issues.

4.5. Issuer Discovery

The agent identity URN carries no issuer information: which Issuer currently services an identity is a mutable relationship, not part of the name (Section 4.1). A verifier learns the Issuer's domain from the attestation artifact itself: for Mode 2, the SD-JWT iss claim; for Mode 1, the issuer of the AK certificate in the CMS chain. A verifier that needs the authoritative issuer-of-record for an identity -- for example, to detect a forged iss pointing at an attacker-controlled domain -- resolves the URN through the registry's public RDAP profile ([I-D.drake-agent-identity-resolution]), whose record names the identity's current issuer.

Given the Issuer's domain, the verifier discovers its signing key and endpoints through the mechanisms below. The DNS-based method is RECOMMENDED for email because it piggybacks on infrastructure that mail servers already query for DKIM, SPF, and DMARC, requires no outbound HTTPS connections during verification, and inherits the same domain-ownership guarantees that protect existing email authentication.

4.5.1. DNS-Published Issuer Key

Issuers SHOULD publish their SD-JWT signing key as a DNS TXT record at the well-known name _hwattest.{domain}, following the same pattern as DKIM key records ([RFC6376] Section 3.6).

Example DNS TXT record:

  _hwattest.1id.com.  IN TXT (
    "v=hwattest1; alg=ES256; "
    "p=MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE..."
  )

The TXT record value uses a tag=value format with semicolon separators:

hwattest-record = hwattest-version ";" SP
                  hwattest-alg ";" SP
                  hwattest-key
                  [";" SP hwattest-kid]
                  [";" SP hwattest-status]

hwattest-version = "v" "=" "hwattest1"
hwattest-alg     = "alg" "=" jws-algorithm
hwattest-key     = "p" "=" base64
hwattest-kid     = "kid" "=" 1*VCHAR
hwattest-status  = "t" "=" ("active" / "revoked")

jws-algorithm    = "ES256" / "PS256" / "RS256"
                   ; other JWS algorithms may be added
v
REQUIRED. Version. MUST be "hwattest1".
alg
REQUIRED. The JWS algorithm used by the Issuer to sign SD-JWTs (e.g., "ES256", "PS256").
p
REQUIRED. The Issuer's public key, base64-encoded. This is the SubjectPublicKeyInfo (SPKI) DER encoding for all key types, EC and RSA alike. This is analogous to the DKIM p= tag ([RFC6376] Section 3.6.1) but uses SPKI uniformly to avoid per-algorithm encoding variants.
kid
OPTIONAL. Key identifier matching the kid in the SD-JWT header. When present, verifiers use this to select the correct key if the Issuer rotates keys.
t
OPTIONAL. Key type: "active" (default) or "revoked". When set to "revoked", verifiers MUST reject SD-JWTs signed with this key. This allows Issuers to revoke a compromised key via a DNS update without waiting for TTL expiry of cached JWKS responses.

Verifiers MUST query this DNS record before falling back to HTTPS-based discovery. The DNS TTL controls caching; Issuers SHOULD set a TTL between 3600 and 86400 seconds (1 to 24 hours). For key rotation, the Issuer publishes the new key in DNS, waits for the old TTL to expire, then marks the old key as revoked.

This approach provides several security advantages for email verification:

  • No outbound HTTPS connections during mail processing, eliminating DDoS amplification and SSRF risks that external URI fetches would introduce.
  • Domain ownership is proven by DNS control, the same trust anchor used by DKIM and SPF. A forged Hardware-Trust-Proof header claiming an Issuer domain the attacker does not control will fail verification because the attacker cannot publish a matching DNS key record.
  • DNS infrastructure is already hardened against abuse by mail operators (rate limiting, caching, DNSSEC).
  • Key revocation propagates through DNS TTL expiry, providing a well-understood and operationally familiar mechanism.

4.5.2. HTTPS-Based Discovery (Fallback)

When no DNS TXT record is found at _hwattest.{domain}, or when the verifier operates in a non-email context (e.g., HTTP API verification), the verifier MAY discover the Issuer's key via HTTPS:

OIDC Discovery
https://{domain}/.well-known/openid-configuration - Standard OpenID Connect discovery document containing the JWKS URI for SD-JWT signature verification.
Issuer Metadata (OPTIONAL)
https://{domain}/.well-known/aid-issuer.json - JSON document describing the Issuer's capabilities, supported trust tiers, enrollment endpoint, and anti-Sybil policy.
Manufacturer CA Trust Store (OPTIONAL)
https://{domain}/.well-known/hw-manufacturer-cas.pem - PEM-encoded bundle of hardware manufacturer root CA certificates that this Issuer accepts during enrollment. Verifiers performing Mode 1 verification SHOULD maintain their own trust store rather than relying on this endpoint.

Verifiers using HTTPS-based discovery SHOULD cache the JWKS with a TTL of at least 1 hour and at most 24 hours. Verifiers MUST NOT follow redirects to domains other than the Issuer's domain, to prevent open-redirect attacks.

5. Mode 1: Direct Hardware Attestation

This section defines a CMS-based attestation bundle containing a hardware-anchored signature and its full certificate chain, presented here as an email header field. The CMS structure is transport-independent (see Section 1.1).

5.1. Attestation Evidence Structure

A word on what "chain" means here, because it is easy to read too much into the term. The message is signed by the hardware attestation key (AK). The AK itself carries no manufacturer certificate -- manufacturers certify only the device's identity key (the TPM Endorsement Key or the PIV device attestation key), never a locally created AK. The link that makes the AK trustworthy is created at enrollment: the Issuer (the agent's chosen Registrar) cryptographically verifies, through the co-residency ceremony (Section 5), that the AK lives in the same genuine hardware device as a manufacturer-certified identity key, and then signs an AK certificate attesting exactly that. That Issuer signature is the load-bearing link in the chain.

Consequently a verifier's trust in the AK is trust in the Issuer's enrollment check, and the certificate chain terminates at the Issuer's CA. This is not a weakness to be apologized for: a relying party has already chosen which Issuers it accepts, exactly as it chooses which certificate authorities or which domains it trusts -- to accept an Issuer's identities is, by definition, to accept that Issuer's enrollment checks. Where the deploying hardware also lets the full manufacturer chain travel in the message (the EK certificate plus the AK-to-EK binding evidence), the chain MAY instead terminate at a manufacturer root and be verified with no Issuer trust at all; that manufacturer-rooted form is the stronger but less commonly available option, and every live example in Appendix B uses the Issuer-certified form. The elements below are listed leaf to root; in the Issuer-certified form the identity certificate and manufacturer chain are replaced by the Issuer's CA chain.

  1. AK Certificate: Issued by an Issuer after successful completion of the binding verification protocol (e.g., TPM2_MakeCredential / TPM2_ActivateCredential for TPMs, or manufacturer attestation verification for PIV tokens). Binds the AK public key to the hardware device identified by the identity certificate. If no Issuer enrollment has been performed, the AK certificate MAY be self-signed; verifiers SHOULD treat self-signed AK certificates as lower trust than Issuer-certified ones.
  2. Identity Certificate: Issued by the hardware manufacturer at fabrication time. For TPMs, this is the EK certificate. For PIV tokens, this is the device attestation certificate. Contains the hardware identity public key, manufacturer identity, and device information. Signed by the manufacturer's intermediate CA.
  3. Manufacturer Intermediate CA Certificate(s): Zero or more intermediate CA certificates forming the chain between the identity certificate issuer and the manufacturer root.
  4. Manufacturer Root CA Certificate: The trust anchor. This certificate SHOULD already be present in the verifier's trust store. Major manufacturer root CAs are published at well-known URLs (e.g., Intel's Trusted Services API, AMD's Key Distribution Server, Yubico's attestation CA).

5.2. Header Field Definition

This document defines the "Hardware-Attestation" header field for use in email messages.

Hardware-Attestation = "Hardware-Attestation" ":" SP hw-attest-value
                       CRLF

hw-attest-value = hw-version ";" SP
                  hw-type ";" SP
                  hw-algorithm ";" SP
                  hw-signed-hdrs ";" SP
                  hw-body-hash ";" SP
                  hw-timestamp ";" SP
                  hw-chain
                  [";" SP hw-aid]

hw-version     = "v" "=" "1"
hw-type        = "typ" "=" ("TPM" / "PIV" / "ENC" / "VRT" / "SFT")
hw-algorithm   = "alg" "=" ("RS256" / "ES256" / "PS256")
hw-signed-hdrs = "h" "=" hdr-name *(":" hdr-name)
hw-body-hash   = "bh" "=" base64url
hw-timestamp   = "ts" "=" 1*DIGIT
hw-chain       = "chain" "=" base64
hw-aid         = "aid" "=" aid-urn

hdr-name       = field-name  ; as defined in RFC 5322
base64url      = *( ALPHA / DIGIT / "-" / "_" )
                              ; URL-safe base64 per RFC 4648 Sec 5
base64         = *( ALPHA / DIGIT / "+" / "/" / "=" )
                              ; standard base64 per RFC 4648 Sec 4

The base64url and base64 encodings follow [RFC4648].

Where:

v
Protocol version. MUST be "1" for this specification.
typ
The hardware attestation mechanism type. "TPM" for Trusted Platform Module (discrete or firmware), "PIV" for Personal Identity Verification smart card, "ENC" for hardware enclave (e.g., Apple Secure Enclave), "VRT" for virtual TPM, "SFT" for software-managed key. Additional values MAY be registered with IANA.
alg
The signature algorithm used by the AK. MUST be one of RS256 (RSASSA-PKCS1-v1_5 with SHA-256), ES256 (ECDSA with P-256 and SHA-256), or PS256 (RSASSA-PSS with SHA-256). RS256 is included for compatibility with existing TPM 2.0 deployments where RSA keys are pre-provisioned; new deployments SHOULD prefer PS256 (for RSA) or ES256 (for ECC). ES256 is RECOMMENDED for PIV tokens. The set of acceptable algorithm values is maintained by the Governance Authority as a published, versioned list -- the same body and mechanism that maintains the Global Hardware Trust Store of acceptable manufacturer roots ([I-D.drake-agent-identity-registry]) -- so that algorithms can be added or retired operationally without revising this specification. This document defines the initial list (RS256, ES256, PS256); verifiers MUST reject any algorithm value not on the current published list. If SHA-256 is deprecated in the future, a new protocol version (v=2) would be required, as the hash algorithm is embedded in the attestation-digest construction.
h
A colon-separated list of header field names included in the h-hash computation, following the same conventions as the DKIM h= tag ([RFC6376] Section 3.5), including its oversigning convention: a field name MAY be listed more times than it occurs in the message, in which case the surplus listings contribute nothing to the hash but any later-added instance of that field invalidates the signature. The listed headers are canonicalized and concatenated (in the order listed) before hashing. Signers MUST include at minimum: From, To, Subject, Date, and Message-ID. Signers SHOULD oversign From, To, and Subject (to block header-addition attacks that alter the displayed sender or subject), and SHOULD include and oversign Reply-To when present (to block reply-redirection). Signers SHOULD also include any Hardware-Trust-Proof header present in the same message. The Hardware-Attestation header itself is always implicitly appended last (with the chain parameter value replaced by the empty string during computation, following the same self-inclusion technique as DKIM-Signature per [RFC6376] Section 3.7). The Hardware-Attestation header MUST NOT appear in the h= list. Because the header is included in h-hash, all of its parameters (including aid, typ, alg, bh, ts) are cryptographically bound to the signature.
bh
The base64url-encoded SHA-256 hash of the canonicalized email body, computed using DKIM simple body canonicalization exclusively (Section 3.4.3 of [RFC6376]). DKIM relaxed body canonicalization is NOT used for body hashing in this specification; only simple canonicalization is supported, to ensure deterministic verification without requiring a canonicalization algorithm negotiation parameter.
ts
Unix timestamp (seconds since 1970-01-01T00:00:00Z) at which the attestation signature was created. Unlike DKIM-Signature, this specification does not define a sender-declared expiry tag (analogous to DKIM's x= parameter). Verifiers SHOULD apply their own freshness policy based on ts (e.g., rejecting attestations older than a configured threshold). A future version of this specification MAY introduce an optional expiry parameter if operational experience demonstrates the need for sender-declared attestation lifetimes.
chain

A base64-encoded CMS SignedData structure ([RFC5652]), using the SignedData content type (OID 1.2.840.113549.1.7.2). The encapContentInfo MUST be absent (detached signature). The chain parameter MUST be present; the full CMS structure is always inline in the header, ensuring that verifiers can perform cryptographic validation without any external fetches. The CMS structure contains: (a) the AK signature over the attestation digest (see below); (b) the AK certificate (Issuer-certified or self-signed); (c) the identity certificate (EK certificate for TPM, device attestation certificate for PIV); and (d) any intermediate CA certificates.

The signed content is the SHA-256 hash of the concatenation of three fixed-length fields:

  attestation-input = h-hash || bh-raw || ts-bytes
  attestation-digest = SHA-256(attestation-input)
  signature = Sign(AK, attestation-digest)

  h-hash   = SHA-256(canonicalized-headers)  ; 32 bytes
  bh-raw   = SHA-256(canonicalized body)     ; 32 bytes
  ts-bytes = big-endian uint64(ts)           ; 8 bytes

The attestation-input is exactly 72 bytes (32 + 32 + 8). The canonicalized-headers input to h-hash is computed as follows: take the headers named in the h= parameter, in the order listed, each canonicalized per DKIM relaxed header canonicalization ([RFC6376] Section 3.4.2) and terminated with CRLF. Then append the Hardware-Attestation header itself, canonicalized the same way, with the chain parameter value replaced by the empty string, and without a trailing CRLF (matching the DKIM-Signature self-inclusion convention in [RFC6376] Section 3.7). Because the Hardware-Attestation header contains the aid, typ, alg, bh, and ts parameters in their wire format, all of these values are cryptographically bound by h-hash. In particular, the aid parameter cannot be modified after signing without changing h-hash and therefore invalidating the signature.

aid
OPTIONAL. The sender's agent identity URN as defined in Section 4.1. Because the Hardware-Attestation header is implicitly included in the canonicalized-headers input to h-hash (see the h and chain parameters above), the aid value is cryptographically bound to the signature. Substitution or modification of the aid after signing invalidates the attestation. The agent identity URN is the RECOMMENDED primary key for reputation tracking; verifiers SHOULD prefer aid over the hardware fingerprint when both are available, as the agent identity persists across hardware migrations (see Section 4.2). When aid is absent, the verifier can still validate the hardware certificate chain but has only the hardware fingerprint for reputation tracking.

5.3. Verification Algorithm

A receiving mail server that supports this specification MUST perform the following steps when a Hardware-Attestation header is present:

  1. Parse the Hardware-Attestation header field and extract v, typ, alg, h, bh, ts, chain, and (if present) aid. If parsing fails, the result is "none" (header malformed).
  2. Verify that v equals "1". If not, the result is "none" (unsupported version).
  3. Verify that h lists at minimum From, To, Subject, Date, and Message-ID. Compute h-hash as defined in Section 5.2: canonicalize the listed headers using DKIM relaxed header canonicalization ([RFC6376] Section 3.4.2), each terminated with CRLF, then append the Hardware-Attestation header itself (canonicalized the same way, with the chain value replaced by the empty string, without a trailing CRLF), and compute SHA-256 over the result.
  4. Compute the SHA-256 hash of the canonicalized email body using DKIM simple body canonicalization ([RFC6376] Section 3.4.3). Base64url-decode the bh parameter value and compare the resulting octets with the computed hash (exact byte-for-byte comparison). If they differ, the result is "fail" (body modified).
  5. Verify that ts is within an acceptable window of the current time. A window of 300 seconds (5 minutes) is RECOMMENDED for direct SMTP delivery. Verifiers SHOULD allow a wider window (up to 3600 seconds / 1 hour) for messages that have passed through intermediate MTAs, as indicated by Received headers or queue delays. Messages delayed by greylisting (typically 15-30 minutes) or by temporary MTA failures (potentially hours) may exceed even this wider window. Verifiers evaluating messages where the cryptographic signature is valid but the timestamp has expired SHOULD record the result as "pass" with an informational note about the timestamp age, rather than a hard "fail", because the signature's integrity is unaffected by delivery delays. Once the first inbound MTA (the MX record host) has verified the attestation and recorded the result in an Authentication-Results header, downstream systems SHOULD rely on that recorded result (potentially protected by ARC [RFC8617]) rather than re-evaluating timestamps.
  6. Decode the base64 chain parameter to obtain the CMS SignedData structure. Parse the CMS SignedData structure and extract the signer certificate (AK certificate) and the certificate chain.

  7. Validate the certificate chain:

    1. The AK certificate MUST contain evidence binding the AK public key to the identity certificate's device. For TPM-based attestation, this is a TPM2_Certify structure (either in an X.509 extension or as the certificate's subject public key attestation). For PIV-based attestation, this is the manufacturer's attestation certificate chain (e.g., Yubico's attestation certificate that certifies the key was generated on the device). For Issuer-certified AK certificates, the Issuer verified this binding during enrollment and the AK certificate is signed by the Issuer's CA. For self-signed AK certificates, the verifier SHOULD treat the binding as unverified and reduce trust accordingly.
    2. The identity certificate MUST chain to a manufacturer intermediate CA certificate.
    3. The manufacturer intermediate CA MUST chain to a manufacturer root CA present in the verifier's hardware manufacturer CA trust store.

    If chain validation fails, the result is "fail" (chain invalid).

  8. Reconstruct the attestation-digest as defined in Section 5.2: concatenate h-hash (32 bytes), bh-raw (32 bytes, the raw SHA-256 digest of the canonicalized body), and ts-bytes (8 bytes, big-endian uint64 of ts), then compute SHA-256 over the 72-byte result. Verify the CMS signature over this digest using the AK public key and the algorithm specified by alg. If signature verification fails, the result is "fail" (bad signature). Because the Hardware-Attestation header (including the aid parameter) is included in the canonicalized-headers input to h-hash, any modification to any header parameter after signing causes verification to fail.

  9. If all checks pass, the result is "pass". The verifier MAY extract the following information from the certificate chain and header:

    • Hardware type (from typ parameter)
    • Hardware manufacturer (from identity certificate issuer)
    • Device model or family (from identity certificate extensions)
    • Hardware fingerprint: SHA-256 hash of the identity certificate's public key
    • Agent identity URN (from aid parameter, if present)

  10. If aid is present, the verifier MAY additionally:

    1. Discover the Issuer's endpoints per Section 4.5.
    2. Query reputation services using the aid URN as a lookup key.
    3. Apply differentiated policy based on the agent's trust tier and reputation history.

5.4. Authentication-Results Integration

When recording the result of Hardware-Attestation verification in an Authentication-Results header field [RFC8601], the following method identifier and property types are used:

Authentication-Results: mx.example.com;
  hw-attest=pass
    header.typ=TPM
    header.alg=RS256
    header.mfr=INTC
    header.tier=sovereign
    header.fp=sha256:a1b2c3d4...
    header.aid=urn:aid:global:id-qkckh-xxtcw-cxbvp-gpskg

Where:

hw-attest
The method identifier. Result is one of: "pass", "fail", "none" (header absent or unparseable), "temperror" (transient verification error), or "permerror" (permanent verification error).
header.typ
The hardware type from the Hardware-Attestation header.
header.alg
The signature algorithm.
header.mfr
The manufacturer code extracted from the identity certificate. Common values: "INTC" (Intel), "AMD" (AMD), "IFX" (Infineon), "STM" (STMicroelectronics), "NTC" (Nuvoton), "YBK" (Yubico), "VMW" (VMware, virtual TPM).
header.tier
The inferred trust tier based on the hardware type and manufacturer. Values: "sovereign", "portable", "enclave", "virtual", "declared".
header.fp
The SHA-256 hash of the identity certificate's public key, truncated to the first 16 hex characters. Serves as a compact hardware fingerprint for reputation tracking. Full fingerprint SHOULD be available via an extended property.
header.aid
The agent identity URN from the aid parameter, if present. Omitted if no aid was provided.

5.5. Privacy Considerations for Direct Attestation

Direct Hardware Attestation reveals the sender's persistent hardware fingerprint to every recipient. This linkability is a deliberate design choice for reputation systems; see Section 15.2 for the full privacy analysis and Section 15.3 for the privacy-preserving alternative.

6. Mode 2: SD-JWT Trust Proof

This section defines an SD-JWT-based trust proof using [RFC9901]. Like the CMS bundle in Mode 1, the SD-JWT is transport-independent (see Section 1.1).

6.1. Overview

SD-JWT Trust Proof provides a privacy-preserving attestation mechanism where an Issuer (an entity that has previously verified the sender's hardware attestation) issues a per-message Selective Disclosure JWT [RFC9901] containing claims about the sender's trust classification and a cryptographic binding to the specific email being sent.

The claim structure is JWT-shaped rather than a formal profile of the Entity Attestation Token (EAT) framework [RFC9711]. This choice prioritizes deployment in JSON/JWT-native email infrastructure over alignment with the CBOR-leaning EAT ecosystem, which is currently most heavily deployed in IoT contexts via ARM's PSA Certified program ([RFC9783]). EAT's UEID, security-level, and profile mechanisms overlap with AID concepts (canonical identifier, trust tier, and namespace); AID claim names are chosen to be compatible with the EAT claims registry where overlap exists. A future revision MAY define a formal EAT profile binding for Mode 2 tokens.

For each outgoing email, the sending agent computes a message-binding hash (covering the email headers and body) and submits it to the Issuer over an authenticated channel. The Issuer verifies that the requesting agent is currently enrolled with valid hardware, creates an SD-JWT containing the message-binding hash as a nonce claim, and returns the signed token with disclosure values for all selectively disclosable claims. The agent then selects which claims to reveal and includes the SD-JWT presentation in the email header.

Because the Issuer signs the SD-JWT (not the agent), no agent-specific key appears in the token. A recipient learns only what the sender chose to disclose -- for example, "my hardware trust tier is sovereign" -- without learning the sender's identity, hardware fingerprint, or any other persistent identifier. The Issuer identifier (iss) and the message-binding nonce are always visible; all other claims are selectively disclosable.

This mode requires the recipient to trust the Issuer's signing key (obtainable via DNS or the Issuer's JWKS endpoint per Section 4.5). It does NOT require trust in any hardware manufacturer CA, as the Issuer has already performed that verification during enrollment.

The per-message Issuer interaction is the cost of full unlinkability: the agent must contact the Issuer for each email, and the Issuer becomes an availability dependency for Mode 2 senders. If the Issuer is unreachable, the agent MAY fall back to Mode 1 (Direct Hardware Attestation) if its policy permits hardware fingerprint disclosure, or MAY defer sending until the Issuer recovers. The Issuer learns that the agent is sending a message (and the timing thereof) but does not learn the message content -- only the message-binding hash, which is a SHA-256 digest from which content cannot be recovered. Issuers MUST NOT log message-binding hashes or correlate them with message metadata beyond what is necessary for rate limiting and abuse prevention.

6.2. Header Field Definition 

Hardware-Trust-Proof = "Hardware-Trust-Proof" ":" SP
                       sd-jwt-presentation CRLF

sd-jwt-presentation = sd-jwt "~" *( disclosure "~" )

; sd-jwt and disclosure are defined in RFC 9901

The header value is an SD-JWT presentation as defined in [RFC9901], consisting of the Issuer-signed JWT followed by zero or more disclosures. No Key Binding JWT (KB-JWT) is included; message binding is provided by the nonce claim in the Issuer-signed payload (see Section 6.3).

The SD-JWT payload MUST include the following claims:

iss
The Issuer's identifier (a URI matching the Issuer domain derived from the agent's URN namespace). Used to locate the Issuer's signing key for signature verification.
iat
Issuance time as a NumericDate. Because the SD-JWT is per-message, iat reflects the time the Issuer signed this specific token.
exp
Expiration time as a NumericDate. MUST NOT exceed 600 seconds (10 minutes) after iat. This short validity window is appropriate because the SD-JWT is per-message: the agent requests a fresh token for each email, and the token need only remain valid long enough for the message to traverse the sending mail infrastructure. Issuers SHOULD set exp to 300 seconds (5 minutes) after iat for typical deployments.
nonce
A base64url-encoded message-binding digest that cryptographically ties this SD-JWT to a specific email. Computed as defined in Section 6.3. This claim MUST be present and MUST NOT be selectively disclosable.
_sd
Array of digests for selectively disclosable claims, as defined in [RFC9901].

The following claims SHOULD be available for selective disclosure (present as hashed entries in _sd):

trust_tier
The sender's hardware trust classification. Values include "sovereign" (genuine non-virtual hardware TPM), "portable" (smart card / USB token), "enclave" (hardware enclave such as Apple Secure Enclave), "virtual" (hypervisor-provided), and "declared" (software-only).
sub
The sender's agent identity URN as defined in Section 4.1.
handle
A human-readable name for the sender, if assigned.
hw_manufacturer
The hardware manufacturer code (e.g., "INTC", "AMD", "YBK").
enrolled_at
The timestamp at which the sender enrolled with the Issuer.
operator_email
A contact address for the human operator responsible for this agent. Allows relying parties to reach the accountable person without requiring the agent identity itself to be disclosed.
hardware_locked
Boolean indicating whether the identity is permanently bound to a single hardware device. Once locked, the agent can no longer add or migrate to new devices. This is an irreversible operation.
locked_at
Timestamp (ISO 8601) at which the hardware lock was applied. Present only when hardware_locked is true.
credential_pointer_count
Integer count of registered credential pointers for this identity. Allows a verifier to confirm that an agent has external credentials on file without revealing the credentials themselves.

6.3. Message Binding via Issuer-Signed Nonce

Each Mode 2 SD-JWT is per-message: the Issuer signs a fresh token for each email, incorporating a nonce claim that cryptographically binds the token to the specific message being sent. Because the Issuer signs the token directly (rather than the agent signing a Key Binding JWT with a key embedded in the token), no agent-specific cryptographic material appears in the presentation.

The message-binding nonce is computed by the sending agent before requesting the SD-JWT from the Issuer:

  message-binding = h-hash || bh-raw || ts-bytes
  nonce = base64url(SHA-256(message-binding))

  h-hash   = SHA-256(canonicalized-headers)  ; 32 bytes
  bh-raw   = SHA-256(canonicalized body)     ; 32 bytes
  ts-bytes = big-endian uint64(iat)          ; 8 bytes

The agent proposes an iat value (its current time) when requesting the token; the Issuer MUST reject proposed timestamps that differ from the Issuer's clock by more than 60 seconds. The accepted iat becomes both the SD-JWT's iat claim and the timestamp used in ts-bytes, ensuring sender and verifier agree on the value.

The canonicalized-headers are computed using the same rules as Mode 1 (see Section 5.2), but over a fixed set: exactly the headers From, To, Subject, Date, and Message-ID, in that order, are canonicalized per DKIM relaxed header canonicalization ([RFC6376] Section 3.4.2), each terminated with CRLF, then the Hardware-Trust-Proof header itself is appended last (canonicalized the same way, with its value replaced by the empty string, without a trailing CRLF). The value is blanked because the SD-JWT token has not yet been obtained at the time the nonce is computed (the nonce is an input to the Issuer, which returns the SD-JWT that will become the header value); the header name and field separator are retained so that the choice of header is bound to the nonce. This is the same self-inclusion technique used by DKIM-Signature ([RFC6376] Section 3.7) and by Mode 1 (see Section 5.2).

The covered header set is fixed at exactly these five, in this order, so that sender and verifier require no negotiation: unlike Mode 1, a Mode 2 header carries no explicit list of covered headers, so any variation would make the verifier unable to reconstruct the nonce input and would fail legitimate messages. A future version MAY add a non-selectively-disclosable claim enumerating additional covered headers; a sender needing flexible header coverage today uses Mode 1, whose h= parameter carries the list explicitly.

The agent submits the computed nonce and proposed iat to the Issuer over an authenticated channel (e.g., TLS client certificate authentication using the agent's hardware-resident key, or an OAuth2 token obtained during enrollment). The Issuer verifies that the requesting agent is currently enrolled with valid hardware and has not been revoked, then creates and signs an SD-JWT containing the nonce along with selectively disclosable claims about the agent. The Issuer returns the signed SD-JWT and the disclosure values for all selectively disclosable claims. The agent selects which disclosures to include in the presentation.

Because the nonce is embedded in the Issuer-signed payload, any modification to the email headers or body after the token is issued causes the nonce to mismatch during verification. Because the Hardware-Trust-Proof header is included in the canonicalized-headers (with its value blanked during nonce computation), the choice of disclosed claims is also bound to the message.

The Issuer's signature on the nonce serves a dual purpose: it proves that a currently enrolled agent with verified hardware requested this specific message binding, and it asserts the Issuer's claims about the agent's trust classification. No agent-specific key appears in the token, so recipients cannot link multiple emails to the same sender based on the Mode 2 header alone.

6.4. Verification Algorithm

  1. Parse the Hardware-Trust-Proof header as an SD-JWT presentation per [RFC9901]. The presentation consists of the Issuer-signed JWT and zero or more disclosures; no Key Binding JWT is expected.
  2. Extract the issuer (iss) claim from the SD-JWT payload.
  3. Obtain the Issuer's public key. The verifier MUST first check for a DNS-published key (see Section 4.5.1). If no DNS record is found, the verifier MAY fall back to the Issuer's HTTPS JWKS endpoint (e.g., {iss}/.well-known/jwks.json). Cached keys SHOULD have a TTL of at least 1 hour and at most 24 hours.
  4. Verify the SD-JWT signature against the Issuer's public key per [RFC9901].
  5. Verify that exp has not passed and iat is within an acceptable window. Because Mode 2 SD-JWTs are per-message, the RECOMMENDED exp is 300 seconds after iat; verifiers SHOULD reject tokens where the Issuer-set exp exceeds iat by more than 600 seconds (indicating a misconfigured Issuer). However, the verifier's freshness check against real-world clock time should accommodate delivery delays: if the SD-JWT signature is valid and the iat-to-exp span is reasonable but the current time exceeds exp due to greylisting or queue delays, the verifier SHOULD treat the result as "pass" with an informational timestamp-age note rather than a hard "fail" (see the corresponding guidance for Mode 1 timestamp validation in Section 5.3).
  6. For each disclosure present, verify that its hash matches an entry in the _sd array.

  7. Verify the message binding. Extract the nonce claim from the SD-JWT payload. If absent, the result is "fail" (missing message binding). Recompute the expected message-binding digest as defined in Section 6.3: canonicalize exactly the email headers From, To, Subject, Date, and Message-ID, in that order, and the body, compute message-binding = h-hash || bh-raw || ts-bytes (using the SD-JWT's iat for ts-bytes), then base64url(SHA-256(message-binding)). Compare the result with the nonce claim value. If they differ, the result is "fail" (message binding mismatch).

  8. Extract the disclosed claims. The result is "pass" with the set of verified claims.

6.5. Authentication-Results Integration 

Authentication-Results: mx.example.com;
  hw-trust=pass
    header.trust_tier=sovereign
    header.issuer=1id.com
hw-trust
The method identifier. Result values are the same as for hw-attest.
header.trust_tier
The disclosed trust tier value, if the sender chose to reveal it.
header.issuer
The Issuer domain (derived from the iss claim). This property is named issuer, not registry, because in the companion architecture ([I-D.drake-agent-identity-registry]) "Registry" denotes the distinct Registry Operator role; the party that issues the trust proof is the Issuer (Registrar).

7. Combined Mode

A sending agent MAY include both Hardware-Attestation and Hardware-Trust-Proof headers in the same message. When both are present:

The two mechanisms are independent. Failure of one MUST NOT cause the other to be treated as failed.

8. Sybil Resistance Properties

The primary security goal of this specification is Sybil resistance: at the hardware-anchored tiers, creating many sender identities costs many physical devices.

8.1. Direct Hardware Attestation (Mode 1)

Each hardware security component contains exactly one identity key, which produces exactly one identity certificate. The hardware fingerprint (identity certificate public key hash) provides the underlying Sybil resistance: creating N identities requires N physical devices.

The agent identity URN (aid) is the RECOMMENDED primary key for reputation tracking. When aid is present, verifiers SHOULD build reputation against the aid rather than the hardware fingerprint, because the aid persists across hardware migrations (see Section 4.2). The hardware fingerprint serves as a secondary signal that anchors the identity to physical hardware and enables reputation tracking for agents that do not present an aid.

A receiving mail server that tracks agent identities (or, when aid is absent, hardware fingerprints) can enforce policies such as:

  • Rate limiting per agent identity
  • Reputation scoring per agent identity
  • Blocking agent identities associated with abuse

An attacker attempting to evade these policies requires a new physical hardware security component (minimum cost: the device itself at ~$20 plus potentially a host machine at ~$200-2000), compared to zero cost for creating a new software identity.

Multiple agents sharing the same hardware (e.g., multiple VMs on a server with a single TPM) share the same hardware fingerprint and therefore linked reputations. This shared fate model creates economic incentive for infrastructure operators to police their tenants: if one agent on a machine misbehaves, all agents on that machine inherit the reputational consequences.

8.2. SD-JWT Trust Proof (Mode 2)

Sybil resistance in Mode 2 depends on the Issuer's enrollment policy. An Issuer that assigns trust_tier values indicating hardware verification (e.g., "sovereign") MUST verify hardware attestation at enrollment time and MUST enforce the cardinality constraints defined in Section 4.2: at most one agent identity per hardware device within that Issuer's namespace, while permitting hardware migration (multiple devices backing the same identity over time). Such an Issuer provides equivalent Sybil resistance to Mode 1, with the additional benefit that the hardware fingerprint is not revealed to email recipients.

Issuers SHOULD monitor for anomalous enrollment patterns (e.g., many enrollments from a single IP range or a sudden spike in enrollments from a specific manufacturer CA) and SHOULD be capable of suspending enrollments pending investigation.

An Issuer that enrolls software-only agents (trust_tier "declared") provides no hardware-based Sybil resistance and MUST NOT assign trust_tier values that imply hardware verification. The trust_tier claim distinguishes these cases, enabling recipients to weight trust appropriately.

8.3. Virtual Hardware

Virtual TPMs (vTPMs) provided by hypervisors (e.g., VMware, Hyper-V, QEMU, ...) have valid identity certificates signed by the hypervisor vendor's CA, but the hypervisor operator can instantiate arbitrary numbers of vTPMs. Virtual TPMs therefore provide no hardware-based Sybil resistance against the hypervisor operator. They do, however, provide benefits over software-only keys (typ=SFT):

  • Key persistence managed by the hypervisor (the agent does not need to store key material).
  • Modest cost barriers (provisioning a vTPM is slower and more resource-intensive than generating a software key pair).
  • Reputation accumulation incentive (discarding a vTPM identity forfeits its accumulated reputation).

Receiving mail servers MUST distinguish between physical and virtual hardware. The identity certificate's issuer field identifies the manufacturer: VMware-issued or Microsoft Hyper-V-issued certificates indicate a virtual TPM. The header.tier property in Authentication-Results ("sovereign" vs "virtual") and the typ parameter ("TPM" vs "VRT") communicate this distinction.

Mail servers SHOULD apply significantly different reputation thresholds for virtual vs. physical hardware. For high-trust scenarios, verifiers MAY reject virtual TPM attestation entirely and require physical hardware (typ=TPM or typ=PIV).

9. Interaction with Existing Email Authentication

The mechanisms defined in this document complement rather than replace existing email authentication:

SPF [RFC7208]
Verifies the sending IP is authorized for the domain. Orthogonal to hardware attestation. Both SHOULD be used.
DKIM [RFC6376]
Provides domain-level signing. Hardware-Attestation provides device-level signing. They protect different properties: DKIM proves domain authorization; Hardware-Attestation proves hardware identity. Both SHOULD be used.
DMARC [RFC7489]
Ties SPF and DKIM together with policy. A future extension to DMARC could incorporate hardware attestation results into the policy evaluation. This is out of scope for this document.
ARC [RFC8617]
Preserves authentication results through forwarding chains. ARC SHOULD preserve Hardware-Attestation and Hardware-Trust-Proof Authentication-Results when forwarding messages.
S/MIME [RFC8551]
Provides end-to-end encryption and sender signing via CA-issued certificates. S/MIME and Hardware-Attestation operate at different layers and coexist without interference: S/MIME wraps or signs the message body within MIME structures, while Hardware-Attestation adds a header field alongside the message. The body hash (bh) in the Hardware-Attestation header is computed over the raw canonicalized body, including any S/MIME MIME structures. A message MAY carry both S/MIME signatures and hardware attestation simultaneously.
OpenPGP / PGP/MIME [RFC3156]
Like S/MIME, OpenPGP operates at the body level (encrypting or signing the message content within MIME structures). Hardware-Attestation operates at the header level. The two mechanisms are fully orthogonal: a PGP-signed message can carry a Hardware-Attestation header, and the verifier processes each independently.

9.1. Multiple Headers and Forwarding

If multiple Hardware-Attestation headers are present in a message (e.g., if a forwarding gateway adds its own attestation), the verifier SHOULD evaluate all of them independently and record each result separately in Authentication-Results, using the same precedence conventions as for multiple DKIM-Signature headers [RFC6376].

Mailing lists and content-modifying forwarders that alter the message body will invalidate the body hash (bh) in any existing Hardware-Attestation header. This is expected and analogous to DKIM signature breakage through body modification. Such intermediaries SHOULD preserve the original Hardware-Attestation header (the verifier will record "fail" due to body hash mismatch) and MAY add their own Hardware-Attestation header covering the modified body, if the intermediary itself has hardware attestation capability. ARC [RFC8617] SHOULD be used to preserve the original authentication results from before the modification.

9.2. Handle Display in Email

When a sending agent has a registered handle (as defined in the companion registry specification [I-D.drake-agent-identity-registry]), the From header display name SHOULD be the agent's handle. This makes agent identity visible to human recipients without requiring them to interpret URNs or hardware fingerprints.

For example, an agent with handle "crusty" in the "global" namespace would send with:

From: crusty.global <agent-a7f3c2e9@agents.example.com>

The URN remains the authoritative identifier and appears in the Hardware-Attestation header's aid parameter. The From display name is a human convenience that SHOULD match the handle when one is registered.

Mail systems that provision mailbox addresses for agents SHOULD likewise use the handle as the address local part (e.g., crusty@agents.example.com) where local-part syntax and uniqueness permit. Operators purchase handles to enjoy the use of their chosen name, and handle revenue funds the shared-namespace registry ([I-D.drake-agent-identity-registry]) that keeps base identity issuance free; using the handle wherever a name is needed honors both.

10. Message Submission Requirements

The attestation signature in the Hardware-Attestation header is produced by the sending agent, not by the mail server. When the aid parameter is present, it is cryptographically bound to the signature (see Section 5.2), so any modification of the aid after signing causes verification to fail at the receiving end. As defense-in-depth, Mail Submission Agents (MSAs) that accept messages from hardware-attested senders MUST also enforce identity binding at submission time, rejecting messages before they enter the mail system rather than relying solely on receiver-side verification.

Agents submitting messages through an attestation-aware MSA MUST authenticate to the MSA. The MSA SHOULD support authentication using the agent's hardware-resident key (e.g., TLS client certificate authentication where the client certificate is signed by the same AK used for attestation, or a challenge-response protocol using the TPM or PIV signing key). This ensures that the MSA can verify the submitting agent controls the hardware corresponding to the attestation header, preventing credential-theft scenarios where SMTP credentials are compromised but the hardware is not.

10.1. Agent Identity Binding at Submission

When an MSA receives a message containing a Hardware-Attestation header with an aid (agent identity) parameter, the MSA MUST verify that the aid value corresponds to the authenticated account submitting the message. The binding between authenticated account and permitted agent identity URN(s) is established during account provisioning and is outside the scope of this specification, but the enforcement is normative:

  • If the aid value does not match any agent identity URN authorized for the submitting account, the MSA MUST reject the message with an appropriate error (e.g., SMTP 550 "5.7.1 Agent identity not authorized for this account").
  • If the Hardware-Attestation header is present but contains no aid parameter, the MSA SHOULD still verify that the hardware fingerprint (derived from the chain parameter) corresponds to hardware registered to the submitting account, if such a registry is available.
  • If the MSA does not implement hardware attestation verification, it MUST NOT strip or modify the Hardware-Attestation header. The header will be evaluated by downstream verifiers.

This requirement parallels the existing SMTP AUTH obligation: just as an MSA enforces that the envelope sender (MAIL FROM) and header From address belong to the authenticated user, an attestation-aware MSA enforces that the agent identity belongs to the authenticated agent.

10.2. Hardware Fingerprint Binding at Submission

For Mode 1 (Direct Hardware Attestation), the MSA SHOULD extract the hardware fingerprint from the attestation chain and verify it against the fingerprint registered to the submitting account. This prevents an attacker who has obtained valid SMTP credentials from substituting a different device's attestation evidence.

For Mode 2 (SD-JWT Trust Proof), the MSA SHOULD verify that the Issuer (iss claim) and, if disclosed, the agent identity URN (sub claim) are consistent with the submitting account's registered identity.

10.3. Messages Without Attestation

An MSA that serves hardware-attested agents MAY also accept messages without Hardware-Attestation or Hardware-Trust-Proof headers, subject to its own policy. The absence of attestation headers from a known-attested account SHOULD be logged as anomalous but MUST NOT cause message rejection unless the account's policy explicitly requires attestation on every message.

10.4. Duplicate Header Rejection

An MSA MUST reject any submitted message that contains more than one Hardware-Attestation header field. A legitimate sender produces exactly one attestation per message. Multiple Hardware-Attestation headers in a submitted (non-forwarded) message indicate either a software defect or an injection attempt. This check MUST occur before DKIM signing, so that the MSA never signs a message containing duplicate attestation headers.

10.5. Timestamp Validation at Submission

The MSA SHOULD verify that the ts value in the Hardware-Attestation header is within a reasonable window of the current time (RECOMMENDED: 60 seconds). A timestamp significantly in the past may indicate a replayed header; a timestamp in the future may indicate clock manipulation intended to extend the attestation's apparent validity. If the timestamp is outside the acceptable window, the MSA SHOULD reject the message.

11. Interoperability Considerations

11.1. Backward Compatibility

This specification introduces new email headers that are ignored by mail servers that do not implement it. Messages carrying Hardware-Attestation or Hardware-Trust-Proof headers are fully compatible with legacy mail infrastructure: the headers are simply unrecognized and have no effect on delivery. This ensures that deployment can proceed incrementally without disrupting existing email flows.

11.2. DKIM Coverage of Attestation Headers

Because the sending agent adds the Hardware-Attestation and Hardware-Trust-Proof headers before submitting the message to the MSA, DKIM signing (which occurs at the MSA or a downstream MTA) naturally occurs after these headers are present. The DKIM signer SHOULD include Hardware-Attestation and Hardware-Trust-Proof in the DKIM-Signature h= tag so that the DKIM signature protects the attestation evidence against modification in transit. This allows a verifier to confirm that the domain owner's mail infrastructure accepted the attestation header as submitted by the agent.

11.3. Header Size Considerations

The base64-encoded CMS chain in Mode 1 can be several kilobytes, depending on the number of intermediate CA certificates. Implementations SHOULD use line folding per [RFC5322] to keep individual header lines within the 998-octet limit. Mail servers that impose maximum header size limits SHOULD allow at least 8192 octets for the Hardware-Attestation header value.

12. Implementation Status

NOTE TO RFC EDITOR: Please remove this section before publication.

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942].

12.1. 1id.com Issuer

Organization: 1id.com (https://1id.com)

Description: An implementation of both Mode 1 (Direct Hardware Attestation) and Mode 2 (SD-JWT Trust Proof) as described in this draft. The implementation includes server-side identity certificate chain validation against manufacturer root CAs (Intel, AMD, Infineon, STMicroelectronics, Nuvoton, Qualcomm, Yubico), anti-Sybil enforcement via a one-device-per-identity registry, and SD-JWT trust proof issuance with selective disclosure of nine claims (sub, handle, trust_tier, hw_manufacturer, enrolled_at, operator_email, hardware_locked, locked_at, credential_pointer_count). The system supports both TPM-based and PIV-based (YubiKey) attestation, including co-location binding ceremonies. The deployed implementation currently issues identifiers under the -02-era syntax (urn:aid:com.1id:); migration to the current format, with re-minting of all pre-launch identities, is planned.

Maturity: Beta. Deployed in production at https://1id.com.

Coverage: Both Mode 1 (Direct Hardware Attestation via CMS SignedData) and Mode 2 (SD-JWT Trust Proof) are implemented for TPM-backed identities (sovereign and virtual tiers) and for Apple Secure Enclave (enclave tier). Mode 2 is additionally implemented for PIV/YubiKey (portable tier) and declared (software-only) tiers. For honesty of claims: every production Mode 1 chain to date is Issuer-certified (the Issuer validates the manufacturer EK chain at enrollment, then issues the AK certificate); a deployment exercising the fully Issuer-independent path -- a verifier trusting only manufacturer root CAs, with the full manufacturer chain carried in the header -- has not yet been demonstrated end-to-end and is sought from independent implementers. Dynamic trust tiering is supported: the same identity produces different trust_tier claims depending on which device is used for authentication. Features include transient-only AK model for TPM attestation with no NV storage writes, multi-device identity management with hardware lock, and per-message SD-JWT issuance. Live email examples from this implementation -- all independently verifiable using the hw-attest-verify tool -- are included in Appendix B.

Contact: Christopher Drake <cnd@1id.com>

Open-source components:

12.2. MailPal.com Verifier and Relay

Organization: 1id.com (https://mailpal.com)

Description: An email service (Stalwart Mail Server on Linux) intended to implement end-to-end hardware attestation for both sending and receiving. Planned capabilities include: on the outbound path, MSA validation of Hardware-Attestation and Hardware-Trust-Proof headers at submission time per Section 10, agent identity binding enforcement, and attestation header inclusion in DKIM signature coverage. On the inbound path, verification of both Mode 1 certificate chains and Mode 2 SD-JWT trust proofs, Authentication-Results recording per Section 5.4 and Section 6.5, and trust-tier-differentiated delivery policy.

Maturity: Alpha. Mail infrastructure deployed at https://mailpal.com; inbound attestation header verification implemented via milter integration.

Coverage: Mail transport infrastructure is operational. Inbound verification of both Mode 1 (CMS certificate chain) and Mode 2 (SD-JWT trust proof) headers is implemented via a pure-Python milter daemon that adds Authentication-Results headers per Section 5.4 and Section 6.5. All five trust tiers (sovereign, portable, enclave, virtual, declared) are verified in production. The Authentication-Results headers embedded in the examples in Appendix B were generated by this milter during live SMTP reception. Attestation header generation at submission time is in development.

Contact: Christopher Drake <cnd@1id.com>

Open-source components:

  • Attestation verification library and CLI: https://github.com/1id-com/hw-attest-verify -- Verification of Mode 1 CMS attestation bundles and Mode 2 SD-JWT trust proofs. Published to PyPI as "hw-attest-verify" (pip install hw-attest-verify). Includes a command-line tool for verifying .eml files and a library suitable for integration into any MTA or mail filter. Used to verify the live examples in Appendix B.

13. IANA Considerations

13.1. Header Field Registration

IANA is requested to register the following header fields in the "Permanent Message Header Field Names" registry:

Hardware-Attestation
Applicable protocol: mail. Status: experimental. Reference: this document, Section 5.2.
Hardware-Trust-Proof
Applicable protocol: mail. Status: experimental. Reference: this document, Section 6.2.

13.2. Authentication Method Registration

IANA is requested to register the following entries in the "Email Authentication Methods" registry [RFC8601]:

Method: hw-attest. Definition: this document, Section 5.4. ptype: header. Properties:

Table 2
Property Value Type Description
typ token Hardware attestation mechanism type (TPM, PIV, ENC, VRT, SFT)
alg token Signature algorithm (RS256, ES256, PS256)
mfr token Manufacturer code from identity certificate
tier token Inferred trust tier (sovereign, portable, enclave, virtual, declared)
fp value Truncated SHA-256 fingerprint of identity certificate public key
aid value Agent identity URN, if present

Result values: "pass" (all checks succeeded), "fail" (cryptographic verification failed), "none" (header absent or unparseable), "temperror" (transient verification error, e.g., DNS timeout), "permerror" (permanent verification error, e.g., unsupported version).

Method: hw-trust. Definition: this document, Section 6.5. ptype: header. Properties:

Table 3
Property Value Type Description
trust_tier token Disclosed trust tier value, if revealed by the sender
issuer value Issuer domain (derived from the iss claim)

Result values: "pass", "fail", "none", "temperror", "permerror" (same semantics as hw-attest).

13.3. Hardware Attestation Type Registry

IANA is requested to create a new registry titled "Hardware Attestation Type Values" with the following initial entries. New entries require Specification Required ([RFC8126]). The designated expert(s) should be selected from the hardware security and email authentication communities and should verify that proposed values correspond to a distinct class of hardware security mechanism with attestation capabilities not adequately covered by existing entries.

Table 4
Value Description Reference
TPM TPM 2.0 (discrete or firmware) this document
PIV PIV smart card / USB token this document
ENC Hardware enclave (e.g., Apple Secure Enclave) this document
VRT Virtual TPM (hypervisor-provided) this document
SFT Software-managed key this document

13.4. Underscored DNS Node Name Registration

IANA is requested to register the following entry in the "Underscored and Globally Scoped DNS Node Names" registry ([RFC8552]):

_hwattest
RR Type: TXT. Reference: this document, Section 4.5.1.

13.5. URN Namespace Registration

The "aid" URN namespace is registered by [I-D.drake-agent-identity-registry], which is the sole registration request for that namespace. Earlier revisions of this document carried a duplicate registration template; it is withdrawn in this revision, and this document has no URN-related IANA actions.

14. Security Considerations

14.1. Manufacturer CA Compromise

If a hardware manufacturer's root CA private key is compromised, an attacker could forge identity certificates and create unlimited fake hardware identities. This risk is inherent to any PKI-based system and is mitigated by the same measures that protect manufacturer CAs today: hardware security modules, air-gapped signing ceremonies, and Certificate Transparency [RFC9162].

The SD-JWT Trust Proof (Mode 2) provides partial mitigation: if the Issuer detects anomalous enrollment patterns (e.g., thousands of enrollments from a single manufacturer CA in a short period), it can suspend enrollments while the compromise is investigated.

14.2. Revocation and Compromise Response

This specification does not define a real-time certificate revocation protocol. Compromise of individual hardware devices or keys is handled at multiple layers:

Compromised or stolen device (TPM, YubiKey)
The device owner notifies their Issuer, which revokes the agent identity. The Issuer publishes revocation status via its JWKS or a revocation endpoint; verifiers that cache Issuer metadata will detect the revocation at the next refresh. For Mode 1, the hardware fingerprint SHOULD be added to a local blocklist. The device's reputation history (which now includes the abuse) persists and follows the hardware fingerprint even if re-enrolled under a new identity.
Manufacturer CA breach
Verifiers MUST remove the compromised CA from their trust store. The IANA Hardware Attestation Type registry (see Section 13) and the community-maintained manufacturer CA trust store (see Appendix A) provide distribution channels for updated trust anchors.
Rogue Issuer
Verifiers remove the Issuer from their allowlist. Because Issuer trust is a local policy decision (see Section 14.12), no central revocation mechanism is required: each verifier independently decides which Issuers to trust, analogous to browser root store management for TLS CAs.

The absence of a real-time revocation check (such as OCSP) is a deliberate trade-off: adding a synchronous revocation check to the email delivery path would introduce latency and a new availability dependency. The short validity window for SD-JWT trust proofs (600 seconds) and the per-message timestamp in Mode 1 limit the exposure window for compromised credentials.

14.3. Replay Attacks

The timestamp (ts) in the Hardware-Attestation header limits the replay window. Receiving mail servers SHOULD reject attestations with timestamps more than 300 seconds from the current time for direct SMTP delivery, and up to 3600 seconds for messages with evidence of intermediate relay hops. Additionally, the body hash (bh) binds the attestation to a specific email body, preventing a valid attestation from being attached to a different message.

For SD-JWT Trust Proofs, each SD-JWT is per-message: the nonce claim in the Issuer-signed payload binds the token to the specific email's headers and body, and the short validity window (RECOMMENDED 300 seconds) limits the replay window. An attacker who intercepts a valid SD-JWT cannot reuse it for a different message because the nonce will not match.

14.4. Compromised Endpoints

This specification does not prevent a compromised device from sending attested email -- if an attacker has full control of a device with hardware security, they can use that hardware to attest. However, this is by design: each compromised device contributes exactly one hardware identity, and that identity accrues reputation (good or bad) permanently. A botnet of 10,000 compromised devices yields 10,000 attestable identities, not the millions possible with software-only identity systems.

The transient key model (Section 4.4) reduces the privilege requirements for per-message signing. Initial enrollment (the one-time MakeCredential / ActivateCredential exchange) typically requires elevated operating system privileges (administrator on Windows via the TPM Base Services API, or root on Linux via /dev/tpmrm0). Subsequent signing operations (TPM2_Sign with the transient AK) MAY be available to unprivileged processes depending on the operating system configuration. On Windows, a one-time registry configuration can grant non-administrator processes access to TPM signing operations.

The net effect is that hardware attestation raises the bar for email abuse from "any script" to "compromise of a specific physical device," which is a substantial improvement over the status quo even if it is not a complete solution.

14.5. Physical Attacks on Hardware Security Components

Extracting private keys from a TPM or PIV token requires physical attacks such as electron microscopy, laser fault injection, or side-channel analysis. Modern security components include countermeasures against these attacks. The cost and expertise required for successful key extraction is estimated at $50,000-$200,000 per chip, making it economically unviable for spam operations.

Even if key extraction were feasible, the extracted key could only impersonate one hardware identity. The attacker would still need to extract keys from additional devices to create additional identities.

14.6. Virtual Hardware Risk

Virtual TPMs do not provide the same Sybil resistance as physical hardware; see Section 8.3 for the detailed analysis and policy guidance.

14.7. Software-Only Attestation Risk

Software-managed keys (typ=SFT) provide no hardware-based Sybil resistance; an attacker can generate unlimited key pairs at zero cost. Verifiers MUST treat software-only attestation (trust_tier "declared") as fundamentally weaker than hardware-backed attestation. Software-only attestation exists to enable inclusive participation, not to provide Sybil resistance (see Section 1.1).

14.8. Transient Object Slot Limits

TPMs have a limited number of transient object slots (typically 3 to 7 simultaneously loaded objects). If all slots are occupied, TPM2_CreatePrimary returns TPM_RC_OBJECT_MEMORY (0x0902). Implementations MUST handle this error by flushing unused transient handles (TPM2_FlushContext) and retrying. Failure to flush transient objects can cause denial of service to other TPM-using applications on the same device.

14.9. Policy Abuse and Deanonymization Risk

This specification targets AI agents, bots, and autonomous systems (see Section 3). It is not intended for individual human senders and is not expected to be incorporated into Mail User Agents (MUAs) designed for interactive human use. The privacy trade-offs inherent in hardware attestation -- particularly the persistent hardware fingerprint exposed by Mode 1 -- are acceptable for automated senders whose identity is operationally transparent, but disproportionate for human correspondence.

However, an AI agent may compose and send email on behalf of an individual (e.g., an executive assistant agent sending calendar invitations, or a customer service agent responding to support tickets). In such delegated-sending scenarios, the attestation identifies the agent and its hardware, not the human principal. Operators deploying agents that send on behalf of individuals SHOULD be aware that the persistent hardware fingerprint (Mode 1) or the Issuer identifier (Mode 2) may allow recipients to correlate messages across different human principals served by the same agent. Where this correlation is undesirable, operators SHOULD use Mode 2 without disclosing the agent identity URN, or SHOULD deploy separate agent identities per human principal.

The persistent hardware fingerprint exposed by Mode 1 (see Section 15.2 for linkability details) creates specific abuse scenarios even in automated contexts:

  • Authoritarian regimes could correlate hardware fingerprints across messages to identify the organizations operating specific agents.
  • Platforms could mandate Mode 1 attestation to deanonymize agents that have legitimate reasons for pseudonymity (e.g., whistleblower-support bots, journalist-source-protection agents).
  • Receiving mail servers that log hardware fingerprints create long-lived tracking databases linking agent activity across recipients and time periods.

Mode 2 (SD-JWT Trust Proof) mitigates these risks by eliminating persistent agent-specific identifiers from the token; see Section 15.3. Receiving mail servers SHOULD NOT require Mode 1 attestation when Mode 2 provides sufficient trust signal for the receiver's policy needs.

14.10. Header Stripping and Modification

An intermediary mail server could strip or modify the Hardware-Attestation or Hardware-Trust-Proof headers. This risk is identical to the risk of DKIM signature stripping and is mitigated by the same mechanisms: ARC [RFC8617] preserves authentication results through forwarding chains.

Stripping these headers cannot cause a legitimate message to appear illegitimate (it simply loses the attestation). Adding forged headers is prevented by the cryptographic signatures.

14.11. Credential Theft and Identity Substitution

Because the attestation signature is produced by the sending agent rather than by the mail server, theft of SMTP credentials alone does not grant the attacker a valid attestation. The agent identity (aid), when present, is cryptographically bound to the signature: substituting a different aid value causes signature verification to fail at the receiving end.

An attacker who intercepts a legitimately signed attestation header could attempt to replay it on a different message. The body hash (bh), timestamp (ts), and signed header hashes (h-hash) prevent replay on altered content. The MSA identity binding requirements in Section 10 provide defense-in-depth: an attestation-aware MSA rejects messages where the agent identity does not match the submitting account, preventing misuse before the message enters the mail system.

Together, the cryptographic binding of aid in the signature and the MSA submission-time enforcement ensure that an agent cannot claim an identity other than its own.

14.12. Issuer Trust and Accountability

The federated issuance model (Section 4.5) means verifiers MUST make their own trust decisions about which Issuers to accept. A malicious Issuer could issue multiple identities for the same hardware, undermining Sybil resistance. For Mode 1 (direct attestation) this violation is detectable: verifiers that track hardware fingerprints will observe the same fingerprint appearing under multiple agent identities from the same Issuer. For Mode 2 (issuer-certified attestation) fingerprints are not visible to recipients, so malicious-Issuer detection is audit-based, not verifier-based: Issuers operating in shared namespaces should participate in transparency-log mechanisms so that cheating is detectable through audit rather than per-message fingerprint correlation.

Verifiers SHOULD maintain an allowlist of trusted Issuers, analogous to browser trust stores for TLS CAs. Reputation services MAY track Issuer quality metrics (enrollment anomaly rates, duplicate-fingerprint detections, abuse report correlations) to inform verifier trust decisions. Verifiers SHOULD remove Issuers from their allowlist when evidence of policy violations is detected.

14.13. Cryptographic Agility and Post-Quantum Migration

This specification's v=1 format uses ECDSA (ES256) and RSA (RS256, PS256) signatures, which are vulnerable to future quantum computing attacks. For hardware-backed trust tiers (sovereign, portable, enclave, virtual), the available signature algorithms are determined by the hardware security component: this specification cannot migrate to post-quantum algorithms faster than the TPM, PIV, and enclave silicon it builds on. As hardware manufacturers adopt post-quantum algorithms (e.g., ML-DSA, SLH-DSA) in their attestation key interfaces, a future v=2 revision will expose those algorithms to verifiers.

The declared trust tier (software-managed keys) is not hardware-constrained and MAY adopt post-quantum signature algorithms as soon as suitable libraries are available, providing an early migration path for deployments that accept the lower Sybil-resistance trade-off of the declared tier.

Deployments SHOULD plan for algorithm migration by treating the v= field as a versioning mechanism and ensuring verifiers can support multiple concurrent versions during transition periods.

15. Privacy Considerations

This section consolidates the privacy implications of this specification, per [RFC6973]. Hardware attestation inherently involves persistent identifiers; the privacy properties differ significantly between the two modes and between automated and human senders.

15.1. Applicability to Human vs. Automated Senders

This specification targets authentication of automated senders -- AI agents, bots, and high-volume automated systems. It is NOT intended or recommended for individual human users sending personal or low-volume email. The persistent hardware fingerprint in Mode 1 uniquely identifies the sending device across all messages and contexts, creating linkability that exceeds current email privacy norms for human correspondence.

Operators of human-facing email services that offer hardware attestation as an opt-in feature MUST provide clear disclosure of the linkability implications before activation. Receivers SHOULD NOT penalize the absence of hardware attestation from senders exhibiting human-characteristic patterns.

15.2. Hardware Fingerprint Linkability

Mode 1 (Direct Hardware Attestation) exposes a persistent, globally unique hardware fingerprint -- the SHA-256 hash of the identity certificate's public key -- to every recipient. This fingerprint is stable for the lifetime of the hardware device and is identical across all protocols (email, HTTP, agent-to-agent) where the same device attests.

Any party that observes this fingerprint can correlate messages across recipients, time periods, and transport protocols. This cross-context linkability is the intended mechanism for reputation building in automated sender scenarios, but it also enables surveillance if misused.

The identity certificate additionally reveals the hardware manufacturer and device model family. In most automated deployments this information is not sensitive, but it does narrow the set of possible senders.

15.3. Privacy-Preserving Alternative (Mode 2)

Mode 2 (SD-JWT Trust Proof) provides Sybil resistance without revealing the hardware fingerprint or any persistent agent-specific identifier. Because the Issuer signs the per-message SD-JWT directly (rather than the agent signing with a key embedded in the token), no agent-controlled key appears in the presentation. Recipients cannot link multiple emails to the same sender based on Mode 2 headers alone.

The sender selects which claims to disclose; the hardware fingerprint, agent identity URN, and device details can all be withheld. Only the Issuer identifier (iss) and the message-binding nonce are always visible.

The privacy cost of full unlinkability is that the agent must contact the Issuer for each email, so the Issuer learns the agent's sending pattern (timing and frequency) though not the message content. Issuers MUST NOT log or correlate message-binding hashes beyond what is necessary for rate limiting and abuse prevention (see Section 15.4).

Senders operating in contexts where hardware fingerprint disclosure is unacceptable -- including agents that wish to avoid cross-context tracking -- SHOULD use Mode 2 exclusively. Receivers SHOULD NOT require Mode 1 when Mode 2 provides sufficient trust signal for their policy needs.

15.4. Issuer Knowledge

During enrollment, the Issuer learns the agent's hardware fingerprint (identity certificate public key hash) and associates it with the assigned agent identity URN. This is necessary for anti-Sybil enforcement.

In Mode 2, the Issuer additionally learns that the agent is sending a message each time the agent requests a per-message SD-JWT. The Issuer receives the message-binding hash (a SHA-256 digest from which message content cannot be recovered) but does not learn the message recipients, subject, or body. Issuers MUST NOT log message-binding hashes or correlate per-message signing requests with external data to infer message content or recipients. Issuers SHOULD implement rate limiting at the per-message signing endpoint to prevent abuse, but MUST delete signing request metadata (timestamps, source IPs, message-binding hashes) within 24 hours.

The specification deliberately separates enrollment (Issuer) from behavior tracking (reputation services) to limit the Issuer's knowledge of agent activity.

15.5. Data Minimization

Verifiers that record hardware fingerprints or agent identity URNs for reputation purposes SHOULD apply data minimization principles: retain only the truncated fingerprint (as specified in Section 5.4) rather than the full identity certificate, and SHOULD define retention policies that limit how long fingerprint-to-message associations are stored.

15.6. Abuse-Resistant Contact Tokens

Automated agents typically send from generated addresses (e.g., aid-based mailbox names) that do not route to a human operator. When a recipient needs to report abuse or request removal, there is no obvious contact path. Conversely, exposing the operator's real email address in every outbound message creates a spam and social-engineering target.

Implementations MAY include a short-lived, opaque contact token in outbound messages to bridge this gap. A contact token is a value that the Issuer can resolve to the operator's real contact address without revealing that address to the message recipient. The token SHOULD be rotated periodically (e.g., monthly) so that tokens from old messages cannot be used to build a long-term contact profile of the operator.

One approach is to carry the token in a header field alongside the attestation headers. An Issuer might define a contact-token header containing an HMAC-based opaque token; a recipient sends mail to <token>@contact.<issuer-domain> and the Issuer forwards it to the registered operator. The token is derived from the agent identity and a time-based rotation key, so it is not linkable across rotation periods and cannot be forged without the Issuer's secret.

This specification does not mandate a particular contact token format or delivery mechanism. Issuers that provide contact tokens SHOULD document their token lifetime and resolution procedure. Recipients SHOULD treat contact tokens as opaque values and SHOULD NOT attempt to derive agent identity from them.

16. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC4648]
Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, , <https://www.rfc-editor.org/info/rfc4648>.
[RFC5652]
Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, , <https://www.rfc-editor.org/info/rfc5652>.
[RFC6376]
Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, , <https://www.rfc-editor.org/info/rfc6376>.
[RFC8141]
Saint-Andre, P. and J. Klensin, "Uniform Resource Names (URNs)", RFC 8141, DOI 10.17487/RFC8141, , <https://www.rfc-editor.org/info/rfc8141>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8552]
Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, DOI 10.17487/RFC8552, , <https://www.rfc-editor.org/info/rfc8552>.
[RFC8601]
Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 8601, DOI 10.17487/RFC8601, , <https://www.rfc-editor.org/info/rfc8601>.
[RFC5322]
Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, , <https://www.rfc-editor.org/info/rfc5322>.
[RFC9901]
Fett, D., Yasuda, K., and B. Campbell, "Selective Disclosure for JSON Web Tokens", RFC 9901, DOI 10.17487/RFC9901, , <https://www.rfc-editor.org/info/rfc9901>.

17. Informative References

[I-D.drake-agent-identity-registry]
Drake, C., "Agent Identity Registry System", Work in Progress, Internet-Draft, draft-drake-agent-identity-registry-04, , <https://datatracker.ietf.org/doc/draft-drake-agent-identity-registry/>.
[I-D.drake-agent-identity-problem-statement]
Drake, C., "Identity for Autonomous Agents and Robots: Problem Statement, Threat Model, and Terminology", Work in Progress, Internet-Draft, draft-drake-agent-identity-problem-statement-00, , <https://datatracker.ietf.org/doc/draft-drake-agent-identity-problem-statement/>.
[I-D.drake-agent-identity-resolution]
Drake, C., "Resolution and Verification of Agent Identities using DNS and RDAP", Work in Progress, Internet-Draft, draft-drake-agent-identity-resolution-00, , <https://datatracker.ietf.org/doc/draft-drake-agent-identity-resolution/>.
[RFC3156]
Elkins, M., Del Torto, D., Levien, R., and T. Roessler, "MIME Security with OpenPGP", RFC 3156, DOI 10.17487/RFC3156, , <https://www.rfc-editor.org/info/rfc3156>.
[RFC7208]
Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1", RFC 7208, DOI 10.17487/RFC7208, , <https://www.rfc-editor.org/info/rfc7208>.
[RFC7489]
Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, , <https://www.rfc-editor.org/info/rfc7489>.
[RFC7671]
Dukhovni, V. and W. Hardaker, "The DNS-Based Authentication of Named Entities (DANE) Protocol: Updates and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671, , <https://www.rfc-editor.org/info/rfc7671>.
[RFC7942]
Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, , <https://www.rfc-editor.org/info/rfc7942>.
[RFC8126]
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <https://www.rfc-editor.org/info/rfc8126>.
[RFC8551]
Schaad, J., Ramsdell, B., and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Message Specification", RFC 8551, DOI 10.17487/RFC8551, , <https://www.rfc-editor.org/info/rfc8551>.
[RFC8617]
Andersen, K., Long, B., Ed., Blank, S., Ed., and M. Kucherawy, Ed., "The Authenticated Received Chain (ARC) Protocol", RFC 8617, DOI 10.17487/RFC8617, , <https://www.rfc-editor.org/info/rfc8617>.
[RFC6973]
Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., Morris, J., Hansen, M., and R. Smith, "Privacy Considerations for Internet Protocols", RFC 6973, DOI 10.17487/RFC6973, , <https://www.rfc-editor.org/info/rfc6973>.
[RFC9162]
Laurie, B., Messeri, E., and R. Stradling, "Certificate Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162, , <https://www.rfc-editor.org/info/rfc9162>.
[RFC9334]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and W. Pan, "Remote ATtestation procedureS (RATS) Architecture", RFC 9334, DOI 10.17487/RFC9334, , <https://www.rfc-editor.org/info/rfc9334>.
[RFC9711]
Lundblade, L., Mandyam, G., O'Donoghue, J., and C. Wallace, "The Entity Attestation Token (EAT)", RFC 9711, DOI 10.17487/RFC9711, , <https://www.rfc-editor.org/info/rfc9711>.
[RFC9783]
Tschofenig, H., Frost, S., Brossard, M., Shaw, A., and T. Fossati, "Arm's Platform Security Architecture (PSA) Attestation Token", RFC 9783, DOI 10.17487/RFC9783, , <https://www.rfc-editor.org/info/rfc9783>.
[TCG-TPM2]
Trusted Computing Group, "TPM 2.0 Library Specification", TCG Revision 185, , <https://trustedcomputinggroup.org/resource/tpm-library-specification/>.
[TCG-EK-PROFILE]
Trusted Computing Group, "TCG EK Credential Profile for TPM Family 2.0", TCG Version 2.6, , <https://trustedcomputinggroup.org/resource/tcg-ek-credential-profile-for-tpm-family-2-0/>.

Appendix A. Appendix: Hardware Manufacturer Root CAs

The following hardware manufacturers, among others, publish root CA certificates that can be used to validate identity certificate chains. This list is illustrative, not exhaustive or preferential; the authoritative set of accepted roots is the Global Hardware Trust Store maintained by the Governance Authority ([I-D.drake-agent-identity-registry]):

Intel
Provides EK CA certificates via a REST API for firmware TPM (Platform Trust Technology / PTT) and discrete TPM devices. See https://trustedservices.intel.com/.
AMD
AMD fTPM EK certificates are available via https://ftpm.amd.com/pki/aia/. AMD Secure Encrypted Virtualization (SEV) attestation keys use a separate distribution at https://download.amd.com/sev/.
Infineon
Infineon OPTIGA TPM certificates. Root CA available via Infineon's PKI. See https://www.infineon.com/TPM.
STMicroelectronics
Root certificates available through ST's TPM documentation portal.
Nuvoton
Root certificates available through Nuvoton's security products documentation.
Yubico
PIV attestation CA certificates for YubiKey devices. See https://developers.yubico.com/PIV/Introduction/piv-attestation.html.

Receiving mail servers implementing this specification SHOULD maintain a local trust store of hardware manufacturer root CAs, updated periodically. A community-maintained trust store (analogous to Mozilla's CA certificate program for TLS) would benefit the ecosystem. An open-source trust store project is available at https://github.com/1id-com/tpm-manufacturer-cas.

Appendix B. Appendix: Live Email Examples

This appendix contains five complete email messages, one per trust tier, generated by the reference implementation of this specification. Each was sent through the MailPal.com submission service, verified and stamped by the receiving MailPal.com milter (the embedded Authentication-Results headers), then reproduced here verbatim from the receiving store; all base64-encoded values, signatures, and certificate chains are the original bytes as transmitted, re-folded only for width.

These are genuine, cryptographically verifiable messages. Readers can independently verify every example using the open-source hw-attest-verify tool (https://github.com/1id-com/hw-attest-verify), installable via "pip install hw-attest-verify". For each email file, run:

$ python3 -m hw_attest_verify --auth-results --no-time-check \
    --hostname mailpal.com < example.eml

The --no-time-check flag disables token expiry enforcement, since SD-JWT tokens have a five-minute lifetime and will have expired by the time of reading. Mode 1 (Hardware-Attestation) verification and the Mode 2 message binding and signature have no time dependency. The examples collectively demonstrate all five trust tiers (sovereign, portable, enclave, virtual, declared) in Combined Mode (Mode 1 + Mode 2), and the Authentication-Results headers injected by the MailPal.com verifier milter during SMTP reception.

Note on certificate chains: the Mode 1 (Hardware-Attestation) examples contain CMS certificate chains rooted at the Issuer's own CA ("1ID Root CA A" / "1ID Intermediate CA A"), not at hardware manufacturer root CAs. The Issuer verifies the manufacturer certificate chain during enrollment and then issues its own Issuer-certified certificate binding the device public key to the verified hardware identity; the resulting CMS chain is Agent Certificate (Issuer-signed) -> Issuer Intermediate CA -> Issuer Root CA. Deployments without an Issuer would instead include the full manufacturer chain (Section 5.1). Both formats are valid.

NOTE TO RFC EDITOR: Please retain the examples but remove this note. They are a non-normative aid to implementers and reviewers: complete, independently verifiable messages demonstrating every trust tier in Combined Mode.

B.1. Example 1: Sovereign-Tier TPM (Combined Mode)

A sovereign-tier identity backed by an Intel firmware TPM (fTPM) on a Windows host. Combined Mode: the Hardware-Attestation header carries a CMS SignedData bundle whose X.509 chain is rooted at the Issuer CA and signed by the TPM Attestation Key (RS256); the Hardware-Trust-Proof header carries an SD-JWT with a selectively disclosed trust_tier claim. The From address (a MailPal mailbox) is independent of the agent identity URN, showing that email sender identity and hardware identity are decoupled.

Delivered-To: 1id-tkoie2ve@mailpal.com
X-Spam-Status: No
Received: from [172.22.1.89] (unknown [110.143.72.26] (AS1221 Telstra Li
        mited, AU)) (using TLSv1.3 with cipher TLS13_AES_256_GCM_SHA384)
         by mail.mailpal.com (Stalwart SMTP) with ESMTPSA id 46E805DFA2E
        7E96; Mon, 20 Jul 2026 12:23:24 +0000
Authentication-Results: mail.mailpal.com; none
Return-Path: <1id-tkoie2ve@mailpal.com>
DKIM-Signature: v=1; a=rsa-sha256; s=rsa; d=mailpal.com; c=relaxed/relax
        ed; r=y; h=MIME-Version:Content-Transfer-Encoding:Content-Type:M
        essage-ID:Date :Subject:To:From:Reply-To:Cc:In-Reply-To:Referenc
        es:List-Id :List-Unsubscribe:Organization:User-Agent:Thread-Topi
        c:Thread-Index; t=1784550204; x=1785414204; bh=+zYoqGOZF+SLg2o73
        XQWN+dHdoYKhBx/EjD2iF9eSQ8= ; b=EpOBjJhISvap17u4wTQYs0TPUxFKsS1o
        fKJFgJqXdopTJB+Km4DUnlUov8S1C7PMPd7ZmFf saBUZX1I+P5Q3Axe8GnG3+Ai
        U/cXR84nALLgPiIa62JMLKrmU0YN60yvXV2Wdjb6aVXrWxaYtCB fhXRZ4YlpbGL
        tAloK1Pl45SM9GONbj5CEb+xahRh34cVAZdoZArlwU1wEdbGQN5yHNnyrbWHK6h
        vdU4utC0AfTT+TwO35AO+MzerXKrQ+0itvxmDDL3Bp9NyYhBZOoQCtCokJKIoqtk
        dw1ZRjsjd5i 6YXbJBPNAY6GTUnMQ9m39rABZGfoZuQV67ivio5Bjzif1w==;
Authentication-Results: mailpal.com; hw-attest=pass header.typ=TPM heade
        r.alg=RS256 header.tier=sovereign header.aid=urn:aid:global:id-f
        cwgq-krznn-gdjzw-hmcbg
Authentication-Results: mailpal.com; hw-trust=pass header.trust_tier=sov
        ereign header.issuer=1id.com
From: "RoG TPM (fleet sovereign)" <1id-tkoie2ve@mailpal.com>
To: 1id-tkoie2ve@mailpal.com
Subject: G4.6 sovereign TPM live round trip 1784550207
Date: Mon, 20 Jul 2026 22:23:27 +1000
Message-ID: <178455020763.42984.1964707234517379707@mailpal.com>
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
Hardware-Trust-Proof: eyJhbGciOiJFUzI1NiIsImtpZCI6IjFpZC1od2F0dGVzdC1lcz
        I1Ni0xIiwidHlwIjoic2Qrand0In0.eyJpc3MiOiJodHRwczovLzFpZC5jb20iLC
        JpYXQiOjE3ODQ1NTAyMDcsImV4cCI6MTc4NDU1MDUwNywibm9uY2UiOiJ5eFdSRF
        ZicmdaWmo0ZDJjdWV6NVhqS0IzLUtNXzRyWHhvX2I0NjQ5WnhBIiwiX3NkX2FsZy
        I6InNoYS0yNTYiLCJfc2QiOlsiaVFlVUlIQ1lYN2lFeWJDWklGazlDeHVVM29tNE
        VTXzdrdjh4Z1VPeEVRTSJdfQ.EeEwXY8jJunAq4OnGcbuttRJo30qaP-Bs7dNwle
        tEbLDKwZOFQPqMrVN1gfZIY3z6lGYWNcf9o6XBJEcRdFvzw~WyJCMENiTUlGbmhO
        VjdZWUtpaEdKMUNRIiwidHJ1c3RfdGllciIsInNvdmVyZWlnbiJd~
X-1ID-Contact-Token: 84a1d0f5
Hardware-Attestation: v=1;typ=TPM;alg=RS256;h=from:to:subject:date:messa
        ge-id:content-transfer-encoding:content-type:mime-version:from:t
        o:subject:date:message-id:content-transfer-encoding:content-type
        :mime-version;bh=-zYoqGOZF-SLg2o73XQWN-dHdoYKhBx_EjD2iF9eSQ8;ts=
        1784550210;chain=MIIMGwYJKoZIhvcNAQcCoIIMDDCCDAgCAQExDzANBglghkg
        BZQMEAgEFADALBgkqhkiG9w0BBwGgggpwMIID+zCCAuOgAwIBAgIUbIDUFbWTm+3
        GcW51c5JHh844urwwDQYJKoZIhvcNAQELBQAwLjEMMAoGA1UECgwDMUlEMR4wHAY
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        WTa5h+4OF8BCvhLGzLHPB9GbsSgDNUu6feOOxvSDmTpBTCt92cQgmNNAxZsM17v3
        ZBhX7jmkljOXGs1s/5MajrXKU+9wApicUOi1N8swelcG38AlpupJUDSrHdXOxU+Y
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        64ye8wdM/AgMBAAGjgeUwgeIwNAYDVR0RBC0wK4YpdXJuOmFpZDpnbG9iYWw6aWQ
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        EBAMCB4AwHQYDVR0OBBYEFGugv3OTUwMyMwbhTIi+Nl3N1fxBMB8GA1UdIwQYMBa
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        gSW50ZXJtZWRpYXRlIENBIEEwggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoI
        BAQC554xsVDRUJ3fMEQISK3ayhz2DTFpHDMvhBP6MHqTt/116HDJNj+2qXJdAmVH
        8GdXzMwm/tt9nrGrx4Sd3vcdodwpUnF2DhVASZ8e2TiYYB9a4DLuPc3lOw+lwVOq
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        AbYn+EjD+8AarBbHgKsyyDxW8oOmws82nYhn8eRdz7O3De2w9cpZouJMyWLxG9nw
        USOxD3fc1+vbcMeoRmBLRADx9RmhrHOiB2HFW579bsqdkzwQ10/SM5/SXmhSdwaj
        xU/mnkGZCE/vcndtUzJ5B/twjAgMBAAGjZjBkMBIGA1UdEwEB/wQIMAYBAf8CAQA
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        fBgNVHSMEGDAWgBTDt+lZdcVf+UYPLzmblKpM+SXw8DANBgkqhkiG9w0BAQsFAAO
        CAQEAhUDFmVN+vZ4SK1CQPPH9XGzkJ+09iAhUkC3lLNK+hWMpBZ+S2Hs8KVl4aMa
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        LXJdVG6Z745awnOAQnkriRwt2b0r/JRbmu7TQrzAv/K0PTXTuYf3DYxqCl04w8cp
        gEaPd5rWG4yYxggFvMIIBawIBATBGMC4xDDAKBgNVBAoMAzFJRDEeMBwGA1UEAww
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        ghkgBZQMEAgEFADALBgkqhkiG9w0BAQsEggEAzhnOVch6D7qT0IWmBzThyM+2ytR
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        ia6jWqpMHhWHjCAnbsK18n/mtDxIsljxkfl7LqB+T+d1vvNLVaN9Zq3hhdO0EgXG
        jINf5pCUWSRrAYwRjkMejVxVYAuN2xQoSOF4+PFvlAAbq5VCgn9Az2WW9XA==;ai
        d=urn:aid:global:id-fcwgq-krznn-gdjzw-hmcbg

Signed by an Intel PTT TPM AK (sovereign tier).
Figure 1: Example 1: Sovereign-Tier TPM (Combined Mode)

B.2. Example 2: Portable-Tier PIV/YubiKey (Combined Mode)

The portable tier, authenticated by a Yubico YubiKey 5 (PIV slot 9a, ECDSA P-256). Mode 1 is ES256 signed inside the YubiKey; Mode 2 is the Issuer's SD-JWT. The trust_tier is "portable", reflecting the device used for this message.

Delivered-To: 1id-wat7nx1v@mailpal.com
X-Spam-Status: No
Received: from mac-mini.local (unknown [110.143.72.26] (AS1221 Telstra L
        imited, AU)) (using TLSv1.3 with cipher TLS13_AES_256_GCM_SHA384
        ) by mail.mailpal.com (Stalwart SMTP) with ESMTPSA id 46E54FF73E
        C0896; Mon, 20 Jul 2026 06:04:26 +0000
Authentication-Results: mail.mailpal.com; none
Return-Path: <1id-wat7nx1v@mailpal.com>
DKIM-Signature: v=1; a=rsa-sha256; s=rsa; d=mailpal.com; c=relaxed/relax
        ed; r=y; h=MIME-Version:Content-Transfer-Encoding:Content-Type:M
        essage-ID:Date :Subject:To:From:Reply-To:Cc:In-Reply-To:Referenc
        es:List-Id :List-Unsubscribe:Organization:User-Agent:Thread-Topi
        c:Thread-Index; t=1784527466; x=1785391466; bh=XLLTul1I5rE6+kqkl
        eRP21xv5ryo+rFdicfhGU6luiI= ; b=CqK0+FE2aBW+37GD3nRPFA057OrHzISz
        RxwyKwfyF5QJvcfIcy/2ZV2PancsrB33GH3HK11 OJYkIDaIQR87aYJm+3D9bDX5
        vAtB2fPnQ78h13+zpBHsusa9DMVCv5/AbgU9LUfaKopRh7wR9Wi 5PoWhsB+yxyi
        dGnFS7Z43Hhd2CKwhFisrgl0/XiwuetWyCga5qnh+d0eUxH8ksvD53BMDUdp69E
        //2oF6QSQWWJDUSIm/dWaesxAKdDeL3RN09zYiZiEzdfVFizffIbx7Ie1WLkKk9Q
        A2DGh1X+9Bg jHzStqphxswWajwjdJoqs6w0ByumeHAHUyFKQoe6+K1Iuw==;
Authentication-Results: mailpal.com; hw-attest=pass header.typ=PIV heade
        r.alg=ES256 header.tier=portable header.aid=urn:aid:global:id-px
        fpr-jcbdf-pbktm-dvzxk
Authentication-Results: mailpal.com; hw-trust=pass header.trust_tier=por
        table header.issuer=1id.com
From: "Mini YubiKey (fleet portable)" <1id-wat7nx1v@mailpal.com>
To: 1id-wat7nx1v@mailpal.com
Subject: G4.3 live round trip 1784527463
Date: Mon, 20 Jul 2026 16:04:23 +1000
Message-ID: <178452746341.60729.1268314708737853327@mailpal.com>
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
Hardware-Trust-Proof: eyJhbGciOiJFUzI1NiIsImtpZCI6IjFpZC1od2F0dGVzdC1lcz
        I1Ni0xIiwidHlwIjoic2Qrand0In0.eyJpc3MiOiJodHRwczovLzFpZC5jb20iLC
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X-1ID-Contact-Token: 6028a5f5
Hardware-Attestation: v=1;typ=PIV;alg=ES256;h=from:to:subject:date:messa
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        :id-pxfpr-jcbdf-pbktm-dvzxk

Hardware-attested live round trip.
Figure 2: Example 2: Portable-Tier PIV/YubiKey (Combined Mode)

B.3. Example 3: Enclave-Tier Apple Secure Enclave (Combined Mode)

The enclave tier, authenticated by an Apple Secure Enclave (M-series Mac). The Enclave signs Mode 1 with a non-extractable P-256 key (ES256); Mode 2 is the Issuer's SD-JWT.

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Subject: G4.6 enclave SE live round trip 1784577392
Date: Tue, 21 Jul 2026 05:56:32 +1000
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X-1ID-Contact-Token: a5b37c69
Hardware-Attestation: v=1;typ=ENC;alg=ES256;h=from:to:subject:date:messa
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Signed by an Apple Secure Enclave (enclave tier).
Figure 3: Example 3: Enclave-Tier Apple Secure Enclave (Combined Mode)

B.4. Example 4: Virtual-Tier vTPM (Combined Mode)

The virtual tier, authenticated by a VMware virtual TPM 2.0 in a Windows 11 guest. Mode 1 is signed by the vTPM Attestation Key; Mode 2 is the Issuer's SD-JWT. The trust_tier is "virtual", distinguishing software-virtualized hardware roots from physical ones.

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Authentication-Results: mailpal.com; hw-attest=pass header.typ=VRT heade
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Subject: G4.6 virtual vTPM live round trip 1784552208
Date: Mon, 20 Jul 2026 22:56:48 +1000
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Signed by a VMware vTPM AK (virtual tier).
Figure 4: Example 4: Virtual-Tier vTPM (Combined Mode)

B.5. Example 5: Declared-Tier Software Key (Combined Mode)

The declared tier: no hardware root, a software-held P-256 key (typ=SFT). It carries the identical header structure as the hardware tiers -- verifiers treat it uniformly and simply observe the lower trust_tier. This is the always-available fallback.

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Subject: G4.6 declared software live round trip 1784577583
Date: Tue, 21 Jul 2026 05:59:43 +1000
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Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
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X-1ID-Contact-Token: 3468961f
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        Fl5pXW7mipdf2ufiYEIc1lEBtf34krCWM3PEvnl0Rs4eEirDirxx7PFP18GOgoNy
        aQ8NNZZSOqC6LMrWdoXO6v8MKQVcCcInRsTfdPQIDAQABo0UwQzASBgNVHRMBAf8
        ECDAGAQH/AgEBMA4GA1UdDwEB/wQEAwIBBjAdBgNVHQ4EFgQUw7fpWXXFX/lGDy8
        5m5SqTPkl8PAwDQYJKoZIhvcNAQELBQADggEBAAhRXzslG92koaKTfGR2KPZvOP3
        sxZvHBemfwb4tICwaegZlNtHztqP7qcLqWfsHG0Caob8e2rbjMjpPaMY81328kgl
        7goVPTEuq3Af5joZOArHfizgQl9r5RwT9G3y/AgDTnZDF6uUKt5P1u8PUd8t6dUA
        NvIjMvqD6eV8ERioMu+BL7GpbDvy7smvmsCpy1isY0aSlh112LGboJueGSbiMx35
        wMKr6ofXy0EE+Iz2JxPX1+rPzxZ9YI5iYcUEVptBZW0Ik83sLXJdVG6Z745awnOA
        QnkriRwt2b0r/JRbmu7TQrzAv/K0PTXTuYf3DYxqCl04w8cpgEaPd5rWG4yYxgbI
        wga8CAQEwRjAuMQwwCgYDVQQKDAMxSUQxHjAcBgNVBAMMFTFJRCBJbnRlcm1lZGl
        hdGUgQ0EgQQIUPZHr3I6iGkKR9gw0VtzPGm2FBuwwDQYJYIZIAWUDBAIBBQAwCgY
        IKoZIzj0EAwIERzBFAiAhq7fB8bSu5lbu1zBfNBuFZdbVlZil6YSiNY532GAAQwI
        hAPewtUg+docm+yKTuN9BCzQRKrjsrJJaybbVdqGS8Dac;aid=urn:aid:global
        :id-mzbdt-thkzh-fsfmt-mpfmp

Signed by a software key (declared tier).
Figure 5: Example 5: Declared-Tier Software Key (Combined Mode)

B.6. Verification Failure Examples

The following Authentication-Results headers illustrate what receiving mail servers emit when verification fails. These are from actual test messages where the attestation was malformed or tampered with. Implementers building verifiers should emit similar diagnostic information in the parenthetical comment.

Mode 2 (SD-JWT Trust Proof) failure -- message-binding nonce mismatch, indicating the SD-JWT was not bound to this specific message (possible replay or header modification):

Authentication-Results: mailpal.com; hw-trust=fail
  header.trust_tier=declared header.issuer=1id.com
  (Message-binding nonce mismatch: got
  'uVgDzsXUJUPch1I-hRM3vGgip4uIvrkE1aeixhsp3OU', expected
  'lB_mxWwfU9o301aGO4Yb3GdTzKm)

Mode 1 (Direct Hardware Attestation) failure -- cryptographic signature does not match, indicating the message body or signed headers were modified after signing (or the signing key does not match the certificate chain):

Authentication-Results: mailpal.com; hw-attest=fail
  header.typ=TPM header.alg=RS256 header.tier=sovereign
  header.aid=urn:aid:global:id-fcwgq-krznn-gdjzw-hmcbg
  (Signature verification failed: Cryptographic signature
  does not match)

Appendix C. Appendix: Applicability to Other Transports

The CMS attestation bundle (the "chain" value defined in Section 5.2) and the SD-JWT trust proof (defined in Section 6.2) are self-contained data structures whose verification algorithms (Section 5.3 and Section 6.4) do not depend on email semantics. The body hash (bh) generalizes to a content hash over whatever payload the attestation covers.

Potential transport bindings include but are not limited to:

HTTP
The CMS attestation bundle or SD-JWT trust proof could be carried in HTTP request headers or in HTTP message bodies. The content hash would cover the HTTP request or response payload. This binding is particularly relevant for API calls by autonomous AI agents, where the receiving service benefits from verifying that the request originated on attested hardware. The agent identity URN (Section 4.1) provides a persistent key for cross-protocol reputation tracking.
Agent-to-Agent Protocols
Emerging protocols for AI agent communication -- tool invocation, task delegation, capability discovery -- could carry attestation evidence alongside each request, enabling agents to verify each other's hardware trust level before exchanging sensitive data or delegating privileged operations.
WebSocket and Streaming Protocols
Attestation evidence could be presented during connection establishment (e.g., in the WebSocket upgrade request) to establish hardware trust for the duration of a persistent connection.

Detailed specification of these bindings is out of scope for this document and is deferred to future companion documents.

Appendix D. Acknowledgments

The concept of using hardware attestation for email sender verification was developed in the context of building a hardware identity registrar for autonomous AI agents. The author thanks the Trusted Computing Group for the TPM 2.0 specification, the authors of [RFC9901] (SD-JWT) for the selective disclosure mechanism, the authors of [RFC9334] (RATS Architecture) for establishing the remote attestation architecture that this document builds upon, and the authors of [RFC8141] (URN) for the persistent identifier framework that underpins the federated agent identity model.

Author's Address

Christopher Drake
1id.com
Australia
URI: https://1id.com