ApertoID-Signature: HTTP Request Signing for AI Agent Identity
draft-ferro-httpbis-apertoid-sig-00
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| Document | Type | Active Internet-Draft (individual) | |
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| Author | Andrea Ferro | ||
| Last updated | 2026-03-24 | ||
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draft-ferro-httpbis-apertoid-sig-00
HTTPBIS A. Ferro
Internet-Draft ApertoID
Intended status: Standards Track 22 March 2026
Expires: 23 September 2026
ApertoID-Signature: HTTP Request Signing for AI Agent Identity
draft-ferro-httpbis-apertoid-sig-00
Abstract
This document defines the ApertoID-Signature HTTP header field, which
enables AI agents to cryptographically prove their identity on each
HTTP request. The agent signs the request method, target URL, body
hash, and identity metadata using an Ed25519 private key whose
corresponding public key is published in DNS via the ApertoID
protocol [APERTOID-DNS]. The mechanism provides request-level
identity verification, action binding (the signature is tied to the
specific method and URL), and replay protection via timestamps and
nonces.
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
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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 23 September 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Relationship to HTTP Message Signatures . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. The ApertoID-Signature Header Field . . . . . . . . . . . . . 4
2.1. Header Syntax . . . . . . . . . . . . . . . . . . . . . . 4
2.2. ABNF Definition . . . . . . . . . . . . . . . . . . . . . 4
2.3. Header Tags . . . . . . . . . . . . . . . . . . . . . . . 4
3. Signing Procedure . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Signing Input Construction . . . . . . . . . . . . . . . 5
3.2. Producing the Signature . . . . . . . . . . . . . . . . . 6
3.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Verification Procedure . . . . . . . . . . . . . . . . . . . 7
4.1. Result Values . . . . . . . . . . . . . . . . . . . . . . 8
5. Replay Protection . . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6.1. Action Binding Scope . . . . . . . . . . . . . . . . . . 9
6.2. HTTP Headers Not Signed . . . . . . . . . . . . . . . . . 9
6.3. Clock Synchronization . . . . . . . . . . . . . . . . . . 10
6.4. Nonce Cache Requirements . . . . . . . . . . . . . . . . 10
6.5. Private Key Protection . . . . . . . . . . . . . . . . . 10
6.6. Signature Stripping . . . . . . . . . . . . . . . . . . . 10
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8.1. HTTP Header Field Registration . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Full Request/Response Example . . . . . . . . . . . 12
Appendix B. Implementation Guidance . . . . . . . . . . . . . . 14
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
The ApertoID protocol [APERTOID-DNS] enables domain owners to declare
authorized AI agents in DNS, including publishing Ed25519 public keys
for agent identity verification. However, publishing a key in DNS
only establishes which key belongs to which agent — it does not prove
that a particular HTTP request was made by the holder of that key,
nor does it bind the signature to the specific action being
performed.
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This document defines the ApertoID-Signature HTTP header field, which
closes both gaps. When an agent makes an HTTP request (e.g., to an
MCP server, an API, or any HTTP service), it includes this header
containing an Ed25519 signature over the request method, target URL,
body hash, and identity metadata. The receiving service can then
verify the signature against the public key published in the agent's
ApertoID DNS record, confirming both that the request originates from
the authorized agent AND that the signature applies to this specific
request — not a different endpoint, not a different method, not a
different body.
This mechanism is analogous to DKIM signatures for email: DKIM key
records are published in DNS, and DKIM signatures are attached to
email messages. Similarly, ApertoID key records are published in DNS
(per [APERTOID-DNS]), and ApertoID-Signature headers are attached to
HTTP requests (per this document).
1.1. Relationship to HTTP Message Signatures
HTTP Message Signatures [RFC9421] provides a general-purpose
framework for signing HTTP messages. ApertoID-Signature does not use
RFC 9421 for the following reasons:
* RFC 9421 requires structured headers (RFC 8941) support, component
identifiers, algorithm negotiation, and signature metadata — all
of which add implementation complexity that is unnecessary for the
single-purpose case of agent identity verification with a fixed
algorithm.
* ApertoID-Signature uses DNS-based key discovery (the public key is
in the agent's DNS TXT record), which does not map to RFC 9421's
key resolution model.
* ApertoID-Signature is designed to be implementable as a drop-in
middleware in under 100 lines of code in any language, without
requiring an RFC 9421 library.
However, ApertoID-Signature follows RFC 9421's principle of binding
signatures to specific request components. The signing input
includes the HTTP method and request target (path + query), ensuring
that a signature is valid only for the specific action it was created
for.
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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.
2. The ApertoID-Signature Header Field
2.1. Header Syntax
The ApertoID-Signature header field contains semicolon-separated tag-
value pairs. The formal grammar uses ABNF [RFC5234]:
ApertoID-Signature: d=example.com; s=leadhunter;
t=1711100000; n=a1b2c3d4e5f6;
sig=<base64-ed25519-signature-88chars>
2.2. ABNF Definition
apertoid-sig-hdr = "ApertoID-Signature" ":" OWS sig-value OWS
sig-value = domain-tag ";" SP selector-tag ";"
SP timestamp-tag ";" SP nonce-tag ";"
SP signature-tag
domain-tag = "d=" domain-name
selector-tag = "s=" selector
timestamp-tag = "t=" 1*DIGIT
nonce-tag = "n=" 1*16HEXDIG
signature-tag = "sig=" base64url
domain-name = label *("." label)
label = ALPHA *(ALPHA / DIGIT / "-")
selector = ALPHA *(ALPHA / DIGIT / "-")
base64url = 1*( ALPHA / DIGIT / "+" / "/" / "=" )
OWS = *( SP / HTAB )
2.3. Header Tags
d (REQUIRED) The domain the agent claims to represent. The verifier
uses this to locate the ApertoID Policy Record at "_apertoid.<d>".
s (REQUIRED) The agent selector. Combined with the domain, this
identifies the Agent Declaration Record at "<s>._apertoid.<d>"
where the public key is published.
t (REQUIRED) The signature timestamp as a Unix timestamp (seconds
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since 1970-01-01T00:00:00Z). MUST be within the validity window
(default: 300 seconds / 5 minutes) of the verifier's current time.
Requests with timestamps outside this window MUST be rejected.
n (REQUIRED) A nonce: a unique, non-repeating value for this
request, encoded as 1-16 hexadecimal characters (lowercase). The
nonce provides replay protection within the timestamp validity
window. Verifiers MUST maintain a nonce cache for the duration of
the validity window and MUST reject requests with previously seen
nonces.
sig (REQUIRED) The Ed25519 signature over the signing input
(Section 3.1), encoded as unpadded Base64 per [RFC4648] Section 4
(88 characters for 64 bytes).
3. Signing Procedure
3.1. Signing Input Construction
The signing input is a byte string constructed by concatenating the
following components, each terminated by a newline character (0x0A):
signing_input = d_value LF
s_value LF
t_value LF
n_value LF
method LF
target LF
body_hash LF
Where:
d_value The value of the d= tag (the domain name, lowercase).
s_value The value of the s= tag (the selector, lowercase).
t_value The decimal string representation of the t= tag (the
timestamp).
n_value The value of the n= tag (the nonce, lowercase hex).
method The HTTP request method, uppercase (e.g., "GET", "POST",
"DELETE"). This binds the signature to the specific HTTP action.
A signature created for a POST request MUST NOT be valid for a GET
or DELETE request.
target The request target as sent in the HTTP request line: the path
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and query string, without the scheme, host, or fragment (e.g.,
"/mcp/tools/search?limit=10"). If there is no query string, only
the path is included (e.g., "/mcp/tools/search"). This binds the
signature to the specific endpoint. A signature created for /mcp/
search MUST NOT be valid for /mcp/delete.
body_hash The lowercase hexadecimal SHA-256 hash of the raw HTTP
request body. This binds the signature to the specific request
content. If the request has no body (e.g., GET, HEAD, DELETE
without body), the SHA-256 hash of the empty string MUST be used:
e3b0c44298fc1c149afbf4c8996fb924
27ae41e4649b934ca495991b7852b855
All components MUST be encoded as UTF-8. The signing input MUST be
deterministic: the same input parameters MUST always produce the same
signing input byte string.
3.2. Producing the Signature
The agent produces the signature as follows:
1. Determine the request method (e.g., "POST") and request target
(e.g., "/mcp/tools/search").
2. Compute the SHA-256 hash of the request body (or the empty-body
hash for bodyless requests).
3. Generate a unique nonce (RECOMMENDED: 8-16 random hex
characters).
4. Record the current Unix timestamp.
5. Construct the signing input as defined in Section 3.1.
6. Sign the signing input using the agent's Ed25519 private key per
[RFC8032], producing a 64-byte signature.
7. Encode the signature as unpadded Base64 per [RFC4648] Section 4.
8. Construct the ApertoID-Signature header with all required tags.
9. Attach the header to the outgoing HTTP request.
3.3. Example
An agent "leadhunter" acting for "example.com" sends:
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POST /mcp/tools/search HTTP/1.1
Host: api.target.com
Content-Type: application/json
{"query": "find leads in tech sector", "limit": 10}
The signing input (each line terminated by LF):
example.com
leadhunter
1711100000
a1b2c3d4e5f6
POST
/mcp/tools/search
7d5e4a8b... (SHA-256 of the JSON body)
The agent signs this input with its Ed25519 private key and attaches:
ApertoID-Signature: d=example.com; s=leadhunter;
t=1711100000; n=a1b2c3d4e5f6;
sig=MEUCIQDx4f... (88 base64 characters)
4. Verification Procedure
Services that have deployed ApertoID SHOULD inspect incoming HTTP
requests for the ApertoID-Signature header. If the header is
present, the service SHOULD verify it per this specification. If the
header is absent but the agent's domain publishes an ApertoID policy
with "p=reject", the service MAY reject the unsigned request.
When a service receives a request with an ApertoID-Signature header,
it performs the following verification:
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VERIFY_APERTOID_SIGNATURE(request):
1. Extract ApertoID-Signature header from request
2. Parse d=, s=, t=, n=, sig= tags
If any required tag is missing: Return "malformed"
3. Check timestamp t= is within validity window:
If |current_time - t| > 300: Return "timestamp_invalid"
4. Check nonce n= against nonce cache:
If n= is in cache: Return "nonce_reused"
Add n= to cache with expiry = t + 300
5. Perform DNS verification per [APERTOID-DNS]:
Query "_apertoid.<d>" for policy record
Query "<s>._apertoid.<d>" for agent declaration
Extract pk= (public key) and check exp=
6. If DNS verification fails:
Apply policy p= from policy record
Return DNS verification result
7. Reconstruct signing_input from:
d, s, t, n,
request.method (uppercase),
request.target (path + query),
SHA-256(request.body)
8. Verify Ed25519 signature sig= against signing_input
using public key pk= from DNS record
9. If signature is invalid:
Apply policy p= from policy record
Return "sig_invalid"
10. Return "pass"
4.1. Result Values
pass The signature is valid, the agent is authorized, and the
signature matches the specific request method, target, and body.
malformed The ApertoID-Signature header is present but cannot be
parsed.
timestamp_invalid The timestamp is outside the validity window.
nonce_reused The nonce was already seen within the validity window.
sig_invalid The Ed25519 signature does not match the signing input
and public key.
DNS-level results (none, revoked, expired, url_mismatch,
key_mismatch, permerror, temperror) are as defined in [APERTOID-DNS].
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5. Replay Protection
ApertoID-Signature provides three layers of replay protection:
Timestamp window Signatures are valid for at most 300 seconds (5
minutes). Requests with timestamps outside this window are
rejected. This limits the useful lifetime of any intercepted
signature.
Nonce uniqueness Within the timestamp window, each nonce may be used
only once. Verifiers MUST maintain a nonce cache and reject
duplicate nonces. The cache need only retain entries for the
duration of the validity window; older entries can be safely
evicted.
Action binding The signing input includes the HTTP method and
request target. A valid signature for "POST /mcp/search" cannot
be replayed against "DELETE /mcp/data" or "POST /mcp/export" —
even within the timestamp window and with a fresh nonce, because
the signing input would differ.
Verifiers SHOULD use a validity window of 300 seconds (5 minutes).
Shorter windows reduce the replay surface but increase sensitivity to
clock skew. Verifiers MAY allow configuration of the validity window
within the range of 60 to 600 seconds.
6. Security Considerations
6.1. Action Binding Scope
The signing input includes the HTTP method and request target (path +
query), preventing cross-endpoint and cross-method replay attacks.
However, it does not include the Host header or scheme. This means a
valid signature could theoretically be replayed against a different
host serving the same path, if the attacker can redirect the request.
In practice, this is mitigated by TLS: the agent establishes a TLS
connection to a specific host, and the signature is only transmitted
over that connection. Services MUST require HTTPS per [RFC9110];
HTTP connections MUST NOT be used with ApertoID-Signature.
6.2. HTTP Headers Not Signed
HTTP headers (other than the request method and target) are not
included in the signing input. This means headers such as Content-
Type, Authorization, and custom headers can be modified by an
intermediary without invalidating the signature. The rationale is
that ApertoID-Signature authenticates agent identity and binds it to
a specific action and payload — it is not a general-purpose message
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integrity mechanism. TLS provides full message integrity in transit.
Services requiring header integrity beyond what TLS provides SHOULD
use HTTP Message Signatures [RFC9421] in addition to ApertoID-
Signature.
6.3. Clock Synchronization
The timestamp-based validity window requires that agents and
verifiers maintain reasonably synchronized clocks. Agents and
verifiers SHOULD use NTP [RFC5905] or equivalent time
synchronization. Clock skew greater than the validity window will
cause all requests to fail verification.
6.4. Nonce Cache Requirements
Verifiers MUST maintain a nonce cache for the duration of the
timestamp validity window. The cache MUST be shared across all
verification instances if the service runs multiple processes or
nodes. Failure to maintain a shared nonce cache allows replay
attacks across processes. For services running on a single node, an
in-memory cache is sufficient. For distributed services, a shared
cache (e.g., Redis, Memcached) is RECOMMENDED.
6.5. Private Key Protection
The agent's Ed25519 private key MUST be protected with the same care
as any other signing key. It SHOULD be stored in a hardware security
module (HSM), trusted platform module (TPM), or at minimum in
encrypted storage with appropriate access controls. If the private
key is compromised, the domain owner MUST immediately revoke the
agent's DNS record per [APERTOID-DNS].
6.6. Signature Stripping
An attacker who can intercept and modify HTTP requests could strip
the ApertoID-Signature header entirely, causing the request to appear
unsigned. Verifiers SHOULD query the agent's ApertoID policy record
to determine whether the domain expects signed requests. If the
policy specifies "p=reject", the verifier SHOULD reject unsigned
requests from agents claiming to represent that domain.
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7. Privacy Considerations
The ApertoID-Signature header reveals the agent's domain (d=) and
selector (s=) to the receiving service and to any intermediary that
can observe HTTP headers. This is by design — the purpose of the
header is to declare agent identity. However, domain owners should
be aware that the same d= and s= values appear on all requests from
the same agent, creating a correlation identifier that enables
request tracking across time and endpoints.
Services that observe ApertoID-Signature headers learn which domains
are using AI agents and which specific agents are making requests.
This information is inherent to the protocol's purpose and cannot be
mitigated without defeating the protocol's goals. Domain owners who
wish to limit correlation SHOULD rotate selectors periodically,
though this requires publishing new DNS records.
8. IANA Considerations
8.1. HTTP Header Field Registration
This document requests registration of the following HTTP header
field in the "Hypertext Transfer Protocol (HTTP) Field Name Registry"
maintained at <https://www.iana.org/assignments/http-fields>:
Field Name: ApertoID-Signature
Status: permanent
Structured Type: N/A
Reference: [this document]
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5234] Crocker, D., "Augmented BNF for Syntax Specifications:
ABNF", RFC 5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
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[RFC8032] Josefsson, S., "Edwards-Curve Digital Signature Algorithm
(EdDSA)", RFC 8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", RFC 8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.
[RFC9110] Fielding, R., "HTTP Semantics", RFC 9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[APERTOID-DNS]
Ferro, A., "ApertoID: DNS-Based Agent Identity Declaration
Protocol", Work in Progress, Internet-Draft, draft-ferro-
dnsop-apertoid-00, March 2026,
<https://datatracker.ietf.org/doc/html/draft-ferro-dnsop-
apertoid-00>.
9.2. Informative References
[RFC5905] Mills, D., "Network Time Protocol Version 4: Protocol and
Algorithms Specification", RFC 5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC6376] Crocker, D., "DomainKeys Identified Mail (DKIM)
Signatures", RFC 6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>.
[RFC9421] Backman, A., "HTTP Message Signatures", RFC 9421, February
2024, <https://www.rfc-editor.org/info/rfc9421>.
Appendix A. Full Request/Response Example
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=== Agent sends signed POST request ===
POST /mcp/tools/search HTTP/1.1
Host: api.targetservice.com
Content-Type: application/json
ApertoID-Signature: d=example.com; s=leadhunter;
t=1711100000; n=a1b2c3d4e5f6;
sig=MEUCIQDx4fakebase64signaturehere...88chars
{"query": "find leads in tech sector", "limit": 10}
=== Signing input that was signed ===
example.com\n
leadhunter\n
1711100000\n
a1b2c3d4e5f6\n
POST\n
/mcp/tools/search\n
<sha256-hex-of-body>\n
=== Verifier checks ===
1. Parse header: d=example.com, s=leadhunter
2. Timestamp 1711100000 within 300s of now: OK
3. Nonce a1b2c3d4e5f6 not in cache: OK, cache it
4. DNS: _apertoid.example.com -> policy p=reject
5. DNS: leadhunter._apertoid.example.com -> pk=MCow...
6. exp= not passed: OK
7. Reconstruct signing_input with method=POST,
target=/mcp/tools/search, body_hash=sha256(body)
8. Ed25519 verify sig against signing_input with pk: OK
9. Result: pass
=== Same signature replayed to DELETE endpoint ===
DELETE /mcp/data/all HTTP/1.1
ApertoID-Signature: d=example.com; s=leadhunter;
t=1711100000; n=a1b2c3d4e5f6;
sig=MEUCIQDx4f... (same signature)
Verification FAILS at step 8:
signing_input includes "DELETE" and "/mcp/data/all"
which differs from original "POST" and "/mcp/tools/search"
-> Ed25519 verify FAILS -> Result: sig_invalid
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Appendix B. Implementation Guidance
This appendix is non-normative.
To maximize adoption, implementations SHOULD provide middleware or
decorator patterns that require minimal code changes.
# Python: Agent side - sign outgoing requests
import hashlib, time, secrets, base64
from nacl.signing import SigningKey
def sign_request(method, url_path, body, domain, selector, key):
t = str(int(time.time()))
n = secrets.token_hex(8)
body_hash = hashlib.sha256(body).hexdigest()
signing_input = f"{domain}\n{selector}\n{t}\n{n}\n"
signing_input += f"{method}\n{url_path}\n{body_hash}\n"
sig = key.sign(signing_input.encode()).signature
sig_b64 = base64.b64encode(sig).decode().rstrip("=")
return {
"ApertoID-Signature":
f"d={domain}; s={selector}; t={t}; n={n}; sig={sig_b64}"
}
# Python: Verifier side - verify incoming requests
def verify_request(request):
header = request.headers.get("ApertoID-Signature")
if not header:
return "unsigned"
tags = parse_tags(header) # extract d, s, t, n, sig
# ... check timestamp, nonce, DNS lookup, then:
body_hash = hashlib.sha256(request.body).hexdigest()
signing_input = (
f"{tags['d']}\n{tags['s']}\n{tags['t']}\n{tags['n']}\n"
f"{request.method}\n{request.path}\n{body_hash}\n"
)
pubkey = get_apertoid_pubkey(tags['d'], tags['s']) # DNS
return verify_ed25519(pubkey, signing_input, tags['sig'])
Reference implementations in Python, Go, and JavaScript are
maintained at https://github.com/ApertoID.
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Acknowledgements
The signing mechanism in this document was inspired by the DKIM
signature scheme [RFC6376]. The principle of binding signatures to
specific request components follows the approach established by HTTP
Message Signatures [RFC9421], adapted for the single-purpose case of
AI agent identity verification with DNS-based key discovery.
Author's Address
Andrea Ferro
ApertoID
Verona
Italy
Email: irn@irn3.com
URI: https://github.com/ApertoID
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