| Internet-Draft | ApertoID-Signature | August 2026 |
| Ferro | Expires 10 February 2027 | [Page] |
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.¶
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Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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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.¶
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).¶
HTTP Message Signatures [RFC9421] provides a general-purpose framework for signing HTTP messages. ApertoID-Signature does not use RFC 9421 for the following reasons:¶
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.¶
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.¶
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-86chars>¶
The example above is shown wrapped across multiple lines with leading indentation solely for readability. On the wire the ApertoID-Signature header field value is a single line: obsolete line folding (obs-fold) MUST NOT be used, and the whitespace surrounding each ";" separator is optional whitespace (OWS), not a mandatory single space.¶
apertoid-sig-hdr = "ApertoID-Signature" ":" OWS sig-value OWS
sig-value = domain-tag OWS ";" OWS
selector-tag OWS ";" OWS
timestamp-tag OWS ";" OWS
nonce-tag OWS ";" OWS
signature-tag
domain-tag = "d=" domain-name
selector-tag = "s=" selector
timestamp-tag = "t=" 1*20DIGIT
nonce-tag = "n=" 8*32LHEXDIG
signature-tag = "sig=" base64-ed25519-sig
; Selector and domain-name are defined identically to [APERTOID-DNS]
; Section 5 / Section 7.1; the two documents MUST agree, because s= and
; d= locate the DNS Agent Declaration Record at <s>._apertoid.<d>.
domain-name = label *("." label)
label = ldh-label / underscore-label
ldh-label = alnum *(alnum / "-") ; RFC 1035 preferred; LDH,
; no leading/trailing hyphen
underscore-label = "_" 1*(alnum / "-") ; RFC 8552 scoped
selector = ldh-label ; DNS label: 1-63 chars, LDH,
; no leading/trailing hyphen,
; case-insensitive (see below)
alnum = ALPHA / DIGIT
; Signature: raw 64-byte Ed25519 signature per [RFC8032], standard
; Base64 [RFC4648] Section 4, UNPADDED (no "="). 64 bytes -> 86 chars.
; Identical to base64-ed25519-sig in [APERTOID-DNS] Section 5.
base64-ed25519-sig = 86*86(BASE64CHAR)
BASE64CHAR = ALPHA / DIGIT / "+" / "/"
LHEXDIG = DIGIT / "a" / "b" / "c" / "d" / "e" / "f"
OWS = *( SP / HTAB )
¶
The ldh-label and selector productions above express the letter-digit-hyphen (LDH) form but do not, by themselves, encode the "no leading or trailing hyphen", "at least one character", and "at most 63 characters" constraints; those constraints are normative and are stated in [RFC1035] and reproduced here: a selector and each label MUST be 1 to 63 characters, MUST consist only of alphanumerics and hyphens, and MUST NOT begin or end with a hyphen. Selectors and labels are compared case-insensitively. A leading digit is permitted (e.g., "42agents"). These rules are identical to [APERTOID-DNS] so that any selector or domain valid in one document is valid in the other.¶
The signing input is a byte string constructed by concatenating the following seven components in order, each immediately followed by ("terminated by") a single newline character (0x0A, "LF"). Because every component including the last is terminated by LF, the signing input ALWAYS ends with a trailing 0x0A. There are exactly seven LF bytes. Signers and verifiers MUST construct byte-identical signing input, including this trailing LF; a one-byte difference causes every signature to fail verification.¶
signing_input = d_value LF
s_value LF
t_value LF
n_value LF
method LF
target LF
body_hash LF
¶
Where:¶
The origin-form request target, defined independently of HTTP version as path [ "?" query ]: the absolute path and, if present, the query string, without the scheme, authority (host/port), or fragment (e.g., "/mcp/tools/search?limit=10"). In HTTP/1.1 [RFC9112] this is the request-line target for origin-form requests; in HTTP/2 [RFC9113] and HTTP/3 [RFC9114] it is the value of the ":path" pseudo-header field. The two MUST yield the same byte string.¶
The target is taken exactly as sent on the wire, with NO normalization: percent-encoding MUST be preserved exactly as transmitted (it MUST NOT be percent-decoded or re-encoded), dot-segments MUST NOT be removed ("/a/../b" is signed as "/a/../b", not "/b"), and case MUST NOT be changed. This binds the signature to the specific endpoint. A signature created for "/mcp/search" MUST NOT be valid for "/mcp/delete". Intermediaries that normalize the request target will invalidate the signature; deployments in which such intermediaries are present are out of scope.¶
If there is no query component, only the path is included (e.g., "/mcp/tools/search"); a present-but-empty query ("/x?") is signed with the trailing "?" exactly as sent. This mechanism is defined for origin-form request targets only. The asterisk-form target of "OPTIONS *" MUST be signed as the single character "*". The authority-form target of a CONNECT request is not a path and is NOT supported; ApertoID-Signature MUST NOT be applied to CONNECT requests.¶
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.¶
The agent produces the signature as follows:¶
An agent "leadhunter" acting for "example.com" sends:¶
POST /mcp/tools/search HTTP/1.1
Host: api.example.com
Content-Type: application/json
{"query": "find leads in tech sector", "limit": 10}
¶
The body is hashed exactly as transmitted on the wire; no JSON or other canonicalization is applied. The SHA-256 of the 51-byte body shown above is the hex string below. The signing input is the seven components, each terminated by LF (note the trailing LF after body_hash):¶
example.com leadhunter 1711100000 a1b2c3d4e5f6 POST /mcp/tools/search 628e22adadb97ae8d0de9bbf50b3556d252763f2d5710c2c6b342173c1aa4675¶
The agent signs this input with its Ed25519 private key and attaches the header below. The signature is a raw 64-byte Ed25519 signature, unpadded Base64, 86 characters. The values in this example are real and reproducible: the private key is the Ed25519 key whose seed is SHA-256("apertoid-sig-example:leadhunter"), and the corresponding raw 32-byte public key (43-char unpadded Base64, per [APERTOID-DNS]) is pk=ZgUmBeB/kgMsrD8+qlFCfJ7KeRse6RSJnvlL4qQnyGE. Verifying the signature below against that public key and the signing input above succeeds.¶
ApertoID-Signature: d=example.com; s=leadhunter;
t=1711100000; n=a1b2c3d4e5f6;
sig=w2nU1SptFk15VYlB8WUC3fV3CT5URCYFOYoRrt3W0Fx+Fq81sy
kTOCgtcjU5mdFDTLgEkGXmjRfhaIAkj3unDQ
¶
The sig value above is a single 86-character token; it is shown wrapped only to fit the page and contains no whitespace on the wire.¶
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:¶
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
(two-sided; rejects both past and future skew;
window defaults to 300s, configurable 60-600 per
Section 5):
If |current_time - t| > window: Return "timestamp_invalid"
4. Check nonce n= against nonce cache (READ ONLY here;
do NOT insert yet -- see step 9a):
If n= is in cache: Return "nonce_reused"
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 (origin-form, unnormalized),
SHA-256(request.body)
(seven LF-terminated components; see Section 3.1)
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"
9a. Signature is valid: NOW insert n= into the nonce
cache with expiry = t + window (the same window as
step 3, so the nonce is retained for exactly as long
as the signature remains valid). The nonce MUST be
cached only after the signature verifies, so that
requests bearing a bad signature cannot burn a
victim's nonce or flood the cache (an unauthenticated
request never mutates verifier state).
10. Return "pass"
¶
DNS-level results (none, revoked, expired, url_mismatch, key_mismatch, permerror, temperror) are as defined in [APERTOID-DNS].¶
ApertoID-Signature provides three layers of replay protection:¶
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.¶
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 scheme, the authority (Host), or the port. A signature is therefore valid for the same method + path?query + body against ANY host that honors the same agent key. This has two consequences that implementers MUST understand:¶
In practice, cross-host replay is mitigated by TLS: the agent establishes a TLS connection to a specific host, and the signature is only transmitted over that connection, so an attacker cannot obtain the signature without compromising the destination or the transport. Services MUST require HTTPS per [RFC9110]; HTTP connections MUST NOT be used with ApertoID-Signature. Deployments that require binding a signature to a specific destination SHOULD NOT rely on the nonce cache for this and instead SHOULD consider an out-of-band recipient binding (for example, an audience identifier agreed between agent and service); a normative audience binding is deferred to a future revision.¶
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 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.¶
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.¶
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.¶
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].¶
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.¶
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.¶
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>:¶
All keys, hashes, and signatures in this example are real and mutually consistent. The agent's Ed25519 private key is the key whose 32-byte seed is SHA-256("apertoid-sig-example:leadhunter"); its raw public key is pk=ZgUmBeB/kgMsrD8+qlFCfJ7KeRse6RSJnvlL4qQnyGE (43-char unpadded Base64, the raw 32-byte key format of [APERTOID-DNS], NOT an SPKI wrapping). The body is the 51-byte string shown; its SHA-256 is the hex value in the signing input. The sig= value is the raw 64-byte Ed25519 signature over the signing input, unpadded Base64 (86 characters). It is shown wrapped for the page but is a single whitespace-free token on the wire.¶
=== Agent sends signed POST request ===
POST /mcp/tools/search HTTP/1.1
Host: api.example.com
Content-Type: application/json
ApertoID-Signature: d=example.com; s=leadhunter;
t=1711100000; n=a1b2c3d4e5f6;
sig=w2nU1SptFk15VYlB8WUC3fV3CT5URCYFOYoRrt3W0Fx+Fq
81sykTOCgtcjU5mdFDTLgEkGXmjRfhaIAkj3unDQ
{"query": "find leads in tech sector", "limit": 10}
=== Signing input that was signed ===
(seven components, each terminated by LF; note the
trailing LF after body_hash. Shown here with a literal
"\n" marking each 0x0A; the bytes contain no "\n" text.)
example.com\n
leadhunter\n
1711100000\n
a1b2c3d4e5f6\n
POST\n
/mcp/tools/search\n
628e22adadb97ae8d0de9bbf50b3556d252763f2d5710c2c6b34\
2173c1aa4675\n
(The body_hash line is the 64-hex-char SHA-256 of the
body, shown wrapped with a trailing "\" continuation;
it is one line followed by a single LF. The full signing
input is 135 bytes and ends with 0x0A.)
=== 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 (do NOT cache yet)
4. DNS: _apertoid.example.com -> policy p=reject
5. DNS: leadhunter._apertoid.example.com ->
pk=ZgUmBeB/kgMsrD8+qlFCfJ7KeRse6RSJnvlL4qQnyGE
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. Signature valid -> NOW insert nonce a1b2c3d4e5f6 into
the cache with expiry t+300
10. 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=w2nU1SptFk15VYlB8... (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
(nonce is NOT cached, because the signature did not verify)
¶
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
# target is the origin-form request target: path, plus "?"+query if a
# query is present (Section 3.1). Percent-encoding is preserved as sent.
def sign_request(method, target, body, domain, selector, key):
# Normalize per Section 3.1: d/s/n lowercase, method uppercase.
domain, selector = domain.lower(), selector.lower()
method = method.upper()
t = str(int(time.time())) # canonical decimal, no zero-pad
n = secrets.token_hex(8) # 16 lowercase hex chars
body_hash = hashlib.sha256(body).hexdigest()
signing_input = f"{domain}\n{selector}\n{t}\n{n}\n"
signing_input += f"{method}\n{target}\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 (read only), DNS lookup, then:
# Reconstruct the origin-form target = path [ "?" query ] exactly as
# sent; request.path alone DROPS the query and would break the sig.
target = request.path
if request.query_string:
target += "?" + request.query_string
body_hash = hashlib.sha256(request.body).hexdigest()
signing_input = (
f"{tags['d'].lower()}\n{tags['s'].lower()}\n"
f"{tags['t']}\n{tags['n'].lower()}\n"
f"{request.method.upper()}\n{target}\n{body_hash}\n"
)
pubkey = get_apertoid_pubkey(tags['d'], tags['s']) # DNS
ok = verify_ed25519(pubkey, signing_input, tags['sig'])
# Only now, after the signature verifies, insert n= into the nonce
# cache (Section 4, step 9a).
return ok
¶
Reference implementations in Python, Go, and JavaScript are maintained at https://github.com/ApertoID.¶
This appendix is non-normative and summarizes the changes made in this revision.¶
Without Valentina Tognizioli, none of this would exist, not for anything she did, but for who she is. She was where the ideas started, and the reason I didn't stop. Grazie, Tina.¶
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.¶