| Internet-Draft | did:x509 | October 2025 |
| Riechert, et al. | Expires 23 April 2026 | [Page] |
Some abstract¶
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This document aims to define an interoperable and flexible issuer identifier format for COSE messages that transport or refer to X.509 certificates using [RFC9360]. The did:x509 identifier format implements a direct, resolvable binding between a certificate chain and a compact issuer string. It can be used in a COSE Header CWT Claims map as defined in [RFC9597]. This issuer identifier is convenient for references and policy evaluation, for example in the context of transparency ledgers.¶
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.¶
In this document, CDDL ([RFC8610], [RFC9165]) is used to describe the data formats, and ABNF (defined in [RFC5234]) to describe identifiers.¶
The reader is assumed to be familiar with the vocabulary and concepts defined in [I-D.ietf-scitt-architecture].¶
The did:x509 ABNF definition defined below uses the syntax defined in [RFC5234] and the corresponding definitions for ALPHA and DIGIT.
The [DIDV1] contains the definition for idchar.¶
did-x509 = "did:" method-name ":" method-specific-id
method-name = "x509"
method-specific-id = version ":" ca-fingerprint-alg ":" ca-fingerprint 1*("::" policy-name ":" policy-value)
version = 1*DIGIT
ca-fingerprint-alg = "sha256" / "sha384" / "sha512"
ca-fingerprint = base64url
policy-name = 1*ALPHA
policy-value = *(1*idchar ":") 1*idchar
base64url = 1*(ALPHA / DIGIT / "-" / "_")
¶
In this draft, version is 0.¶
ca-fingerprint-alg is one of sha256, sha384, or sha512.
ca-fingerprint is chain[i].fingerprint[ca-fingerprint-alg] with i > 0, that is, either an intermediate or root CA certificate.
policy-name is a policy name and policy-value is a policy-specific value.
:: is used to separate multiple policies from each other.¶
The following sections define the policies and their policy-specific syntax.¶
Validation of policies is formally defined using [REGO] policies, though there is no expectation that implementations use Rego.¶
The input to the Rego engine is the JSON document {"did": "<DID>", "chain": <CertificateChain>}.¶
Core Rego policy:¶
parse_did(did) := [ca_fingerprint_alg, ca_fingerprint, policies] if {
prefix := "did:x509:0:"
startswith(did, prefix) == true
rest := trim_prefix(did, prefix)
parts := split(rest, "::")
[ca_fingerprint_alg, ca_fingerprint] := split(parts[0], ":")
policies_raw := array.slice(parts, 1, count(parts))
policies := [y |
some i
s := policies_raw[i]
j := indexof(s, ":")
y := [substring(s, 0, j), substring(s, j+1, -1)]
]
}
valid if {
[ca_fingerprint_alg, ca_fingerprint, policies] := parse_did(input.did)
ca := [c | some i; i != 0; c := input.chain[i]]
ca[_].fingerprint[ca_fingerprint_alg] == ca_fingerprint
valid_policies := [i |
some i
[name, value] := policies[i]
validate_policy(name, value)
]
count(valid_policies) == count(policies)
}
¶
The overall Rego policy is assembled by concatenating the core Rego policy with the Rego policy fragments in the following sections, each one defining a validate_policy function.¶
Some of the policies that are defined in subsequent sections require values to be percent-encoded. Percent-encoding is specified in Section 2.1 of [RFC3986]. All characters that are not in the allowed set defined below must be percent-encoded:¶
allowed = ALPHA / DIGIT / "-" / "." / "_"¶
Note that most libraries implement percent-encoding in the context of URLs and do NOT encode ~ (%7E).¶
policy-name = "subject"
policy-value = key ":" value *(":" key ":" value)
key = label / oid
value = 1*idchar
label = "CN" / "L" / "ST" / "O" / "OU" / "C" / "STREET"
oid = 1*DIGIT *("." 1*DIGIT)
¶
<key>:<value> are the subject name fields in chain[0].subject in any order. Field repetitions are not allowed. Values must be percent-encoded.¶
Example:¶
did:x509:0:sha256:WE4P5dd8DnLHSkyHaIjhp4udlkF9LqoKwCvu9gl38jk::subject:C:US:ST:California:L:San%20Francisco:O:GitHub%2C%20Inc.¶
Rego policy:¶
validate_policy(name, value) := true if {
name == "subject"
items := split(value, ":")
count(items) % 2 == 0
subject := {k: v |
some i
i % 2 == 0
k := items[i]
v := urlquery.decode(items[i+1])
}
count(subject) >= 1
object.subset(input.chain[0].subject, subject) == true
}
¶
policy-name = "san" policy-value = san-type ":" san-value san-type = "email" / "dns" / "uri" san-value = 1*idchar¶
san-type is the SAN type and must be one of email, dns, or uri. Note that dn is not supported.¶
san-value is the SAN value, percent-encoded.¶
The pair [<san_type>, <san_value>] is one of the items in chain[0].extensions.san.¶
Example:¶
did:x509:0:sha256:WE4P5dd8DnLHSkyHaIjhp4udlkF9LqoKwCvu9gl38jk::san:email:bob%40example.com¶
Rego policy:¶
validate_policy(name, value) := true if {
name == "san"
[san_type, san_value_encoded] := split(value, ":")
san_value := urlquery.decode(san_value_encoded)
[san_type, san_value] == input.chain[0].extensions.san[_]
}
¶
policy-name = "eku"
policy-value = eku
eku = oid
oid = 1*DIGIT *("." 1*DIGIT)
¶
eku is one of the OIDs within chain[0].extensions.eku.¶
Example:¶
did:x509:0:sha256:WE4P5dd8DnLHSkyHaIjhp4udlkF9LqoKwCvu9gl38jk::eku:1.3.6.1.4.1.311.10.3.13¶
Rego policy:¶
validate_policy(name, value) := true if {
name == "eku"
value == input.chain[0].extensions.eku[_]
}
¶
policy-name = "fulcio-issuer" policy-value = fulcio-issuer fulcio-issuer = 1*idchar¶
fulcio-issuer is chain[0].extensions.fulcio_issuer without leading https://, percent-encoded.¶
Example:¶
did:x509:0:sha256:WE4P5dd8DnLHSkyHaIjhp4udlkF9LqoKwCvu9gl38jk::fulcio-issuer:accounts.google.com::san:email:bob%40example.com¶
Example 2:¶
did:x509:0:sha256:WE4P5dd8DnLHSkyHaIjhp4udlkF9LqoKwCvu9gl38jk::fulcio-issuer:token.actions.githubusercontent.com::san:uri:https%3A%2F%2Fgithub.com%2Focto-org%2Focto-automation%2F.github%2Fworkflows%2Foidc.yml%40refs%2Fheads%2Fmain¶
Rego policy:¶
validate_policy(name, value) := true if {
name == "fulcio-issuer"
suffix := urlquery.decode(value)
concat("", ["https://", suffix]) == input.chain[0].extensions.fulcio_issuer
}
¶
The illustrates what a typical Controller document can look like once resolved:¶
{
"@context": "https://www.w3.org/ns/did/v1",
"id": "did:x509:0:sha256:hH32p4SXlD8n_HLrk_mmNzIKArVh0KkbCeh6eAftfGE::subject:CN:Microsoft%20Corporation",
"verificationMethod": [
{
"id": "did:x509:0:sha256:hH32p4SXlD8n_HLrk_mmNzIKArVh0KkbCeh6eAftfGE::subject:CN:Microsoft%20Corporation#key-1",
"type": "JsonWebKey2020",
"controller": "did:x509:0:sha256:hH32p4SXlD8n_HLrk_mmNzIKArVh0KkbCeh6eAftfGE::subject:CN:Microsoft%20Corporation",
"publicKeyJwk": {
"kty": "RSA",
"n": "s9HduD2rvmO-SGksB4HR-qvSK379St8NnUZBH8xBiQvt2zONOLUHWQibeBW4NLUfHfzMaOM77RhNlqPNiDRKhChlG1aHqEHSAaQBGrmr0ULGIzq-1YvqQufMGYBFfq0sc10UdvWqT0RjwkPQTu4bjg37zSYF9OcGxS9uGnPMdWRM0ThOsYUcDmMoCaJRebsLUBpMmYXkcUYXJrcSGAaUNd0wjhwIpEogOD-AbWW_7TPZOl-JciMj40a78EEXIc2p06lWHfe5hegQ7uGIlSAPG6zDzjhjNkzE63_-GoqJU-6QLazbL5_y27ZDUAEYJokbb305A-dOp930CjTar3BvWQ",
"e": "AQAB"
}
}
],
"assertionMethod": [
"did:x509:0:sha256:hH32p4SXlD8n_HLrk_mmNzIKArVh0KkbCeh6eAftfGE::subject:CN:Microsoft%20Corporation#key-1"
],
"keyAgreement": [
"did:x509:0:sha256:hH32p4SXlD8n_HLrk_mmNzIKArVh0KkbCeh6eAftfGE::subject:CN:Microsoft%20Corporation#key-1"
]
}
CertificateChain = [2*Certificate] ; leaf is first
Certificate = {
fingerprint: {
; base64url-encoded hashes of the DER-encoded certificate
sha256: base64url, ; FIPS 180-4, SHA-256
sha384: base64url, ; FIPS 180-4, SHA-384
sha512: base64url ; FIPS 180-4, SHA-512
},
issuer: Name, ; RFC 5280, Section 4.1.2.4
subject: Name, ; RFC 5280, Section 4.1.2.6
extensions: {
? eku: [+OID], ; RFC 5280, Section 4.2.1.12
? san: [+SAN], ; RFC 5280, Section 4.2.1.6
? fulcio_issuer: tstr ; http://oid-info.com/get/1.3.6.1.4.1.57264.1.1
}
}
; X.509 Name as an object of attributes
; Repeated attribute types are not supported
; Common attribute types have human-readable labels (see below)
; Other attribute types use dotted OIDs
; Values are converted to UTF-8
Name = {
; See RFC 4514, Section 3, for meaning of common attribute types
? CN: tstr,
? L: tstr,
? ST: tstr,
? O: tstr,
? OU: tstr,
? C: tstr,
? STREET: tstr,
* OID => tstr
}
; base64url-encoded data, see RFC 4648, Section 5
base64url = tstr
; ASN.1 Object Identifier
; Dotted string, for example "1.2.3"
OID = tstr
; X.509 Subject Alternative Name
; Strings are converted to UTF-8
SAN = rfc822Name / DNSName / URI / DirectoryName
rfc822Name = ["email", tstr] ; Example: ["email", "bill@microsoft.com"]
DNSName = ["dns", tstr] ; Example: ["dns", "microsoft.com"]
URI = ["uri", tstr] ; Example: ["uri", "https://microsoft.com"]
DirectoryName = ["dn", Name] ; Example: ["dn", {CN: "Microsoft"}]
Some considerations¶
This DID method maps characteristics of X.509 certificate chains to identifiers. It allows a single identifier to map to multiple certificate chains, giving the identifier stability across the expiry of individual chains. However, if the policies used in the identifier are chosen too loosely, the identifier may match too wide a set of certificate chains. This may have security implications as it may authorize an identity for actions it was not meant to be authorized for.¶
To mitigate this issue, the certificate authority should publish their expected usage of certificate fields and indicate which ones constitute a unique identity, versus any additional fields that may be of an informational nature. This will help users create an appropriate did:x509 as well as consumers of signed content to decide whether it is appropriate to trust a given did:x509.¶
Typically, a verifier trusts an X.509 certificate by applying chain validation defined in Section 6 of [RFC5280] using a set of certificate authority (CA) certificates as trust store, together with additional application-specific policies.¶
This DID method does not require an X.509 trust anchor store but rather relies on verifiers either trusting an individual DID directly or using third-party endorsements for a given DID, like [VC], to establish trust.¶
By layering this DID method on top of X.509, verifiers are free to use traditional chain validation (for example, verifiers unaware of DID), or rely on DID as an ecosystem to establish trust.¶
RFC Editor: Please replace "RFCthis" with the RFC number assigned to this document.¶
RFC Editor: Some considerations¶
The authors would like to thank list for their reviews and suggestions.¶