Attesto.IDToken (Attesto v0.5.1)

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Mint and verify OpenID Connect ID Tokens (OpenID Connect Core 1.0 §2).

An ID Token is the JWT that asserts the authentication of an End-User to a Relying Party. It is a different artifact from the RFC 9068 access token Attesto.Token produces, with different semantics: its aud is the OAuth client_id rather than the protected-resource audience, it carries no scope claim, and its JOSE header typ is the generic JWT (NOT at+jwt, which is reserved for access tokens). The two are kept in separate modules rather than overloading one mint path.

Like Attesto.Token, the operations are pure: they read only the Attesto.Config passed in. Signing uses the same keystore/kid path and the same RS256 pinning, and every JOSE call funnels through JOSE.JWS / JOSE.JWT.verify_strict so the alg whitelist (no none, no HS256 confusion) lives in one place and fails closed.

Claims (OpenID Connect Core §2)

Every minted ID Token carries:

  • iss - the configured issuer.
  • sub - the subject identifier for the End-User.
  • aud - the OAuth client_id of the Relying Party. This is the client, NOT the Attesto.Config audience an access token uses.
  • exp / iat - expiry and issued-at, unix seconds.

Conditionally / optionally present:

  • nonce - the value from the Authentication Request. REQUIRED to be present and identical when the request carried one (OIDC Core §2, §3.1.3.7 item 11).
  • azp - the authorized party. REQUIRED when aud contains a value other than the client_id (OIDC Core §2); always safe to include.
  • auth_time - time of End-User authentication (OIDC Core §2).
  • acr - Authentication Context Class Reference (OIDC Core §2).
  • amr - Authentication Methods References, a JSON array (OIDC Core §2).
  • at_hash - Access Token hash (OIDC Core §3.1.3.6 / §3.3.2.11).
  • c_hash - Authorization Code hash (OIDC Core §3.3.2.11).

There is deliberately no scope claim: scope is a property of the authorization grant, not of the identity assertion.

Hash claims

at_hash and c_hash use the same construction (OIDC Core §3.1.3.6, §3.3.2.11): hash the ASCII octets of the access_token / code with the hash of the ID Token's signature algorithm (SHA-256 for RS256), take the left-most half of the digest, and base64url-encode it without padding.

Summary

Types

claims()
mint_error()
mint_opts()
verify_error()
verify_opts()

Functions

header_typ()

The JOSE header typ ID Tokens carry: "JWT" (never "at+jwt").

mint(config, subject, client_id, opts \\ [])

Mint a signed OpenID Connect ID Token for subject, addressed to the Relying Party identified by client_id.

signing_alg()

The JWS algorithm used to sign ID Tokens. Pinned; verifiers reject anything else.

verify(config, id_token, opts \\ [])

Verify and decode an ID Token previously minted under the same config.

Types

claims()

@type claims() :: %{optional(String.t()) => term()}

mint_error()

@type mint_error() ::
  :invalid_subject
  | :invalid_client_id
  | :invalid_extra_claims
  | :reserved_claim_conflict

mint_opts()

@type mint_opts() :: [
  now: DateTime.t() | non_neg_integer(),
  lifetime: pos_integer(),
  nonce: String.t(),
  azp: String.t(),
  auth_time: non_neg_integer(),
  acr: String.t(),
  amr: [String.t()],
  access_token: String.t(),
  code: String.t(),
  extra_claims: %{optional(String.t()) => term()}
]

verify_error()

@type verify_error() ::
  :invalid_token
  | :invalid_signature
  | :unsupported_critical_header
  | :unexpected_typ
  | :invalid_issuer
  | :invalid_audience
  | :invalid_azp
  | :expired
  | :not_yet_valid
  | :invalid_claims
  | :missing_client_id
  | :nonce_required
  | :nonce_mismatch

verify_opts()

@type verify_opts() :: [
  now: DateTime.t() | non_neg_integer(),
  client_id: String.t(),
  nonce: String.t()
]

Functions

header_typ()

@spec header_typ() :: String.t()

The JOSE header typ ID Tokens carry: "JWT" (never "at+jwt").

mint(config, subject, client_id, opts \\ [])

@spec mint(Attesto.Config.t(), String.t(), String.t(), mint_opts()) ::
  {:ok, String.t()} | {:error, mint_error()}

Mint a signed OpenID Connect ID Token for subject, addressed to the Relying Party identified by client_id.

client_id becomes the aud claim (OIDC Core §2), distinguishing the ID Token from a resource-addressed access token; config.audience is not used here.

Options:

  • :nonce - the Authentication Request nonce. When supplied it is placed in the nonce claim, and verify/3 then requires a match (OIDC Core §2). Omit only when the request carried no nonce.
  • :azp - the authorized party (OIDC Core §2). REQUIRED by the spec when aud has more than one audience.
  • :auth_time - unix time of End-User authentication (OIDC Core §2).
  • :acr - Authentication Context Class Reference (OIDC Core §2).
  • :amr - Authentication Methods References, a list (OIDC Core §2).
  • :access_token - when given, the at_hash claim is computed from it (OIDC Core §3.1.3.6).
  • :code - when given, the c_hash claim is computed from it (OIDC Core §3.3.2.11).
  • :extra_claims - a string-keyed map of additional claims (e.g. profile claims). MUST NOT collide with a reserved protocol claim (:reserved_claim_conflict) and MUST have string keys.
  • :now - DateTime or unix-seconds clock override. Defaults to now.
  • :lifetime - positive seconds; may only shorten the default (a larger value is capped to the default), so a miswired caller cannot mint a long-lived identity assertion.

Returns {:ok, id_token} (compact JWS) or {:error, reason}.

signing_alg()

@spec signing_alg() :: String.t()

The JWS algorithm used to sign ID Tokens. Pinned; verifiers reject anything else.

verify(config, id_token, opts \\ [])

@spec verify(Attesto.Config.t(), String.t(), verify_opts()) ::
  {:ok, claims()} | {:error, verify_error()}

Verify and decode an ID Token previously minted under the same config.

Mirrors Attesto.Token.verify/3 where the OIDC semantics line up. Runs, in order:

  1. Signature. The compact JWS is canonical - three base64url-no-pad segments - and its RS256 signature verifies against a keystore key selected by the JWS header kid. A kid naming a key we do not hold, or an alg other than RS256, fails as :invalid_signature (alg-confusion is impossible). A protected header carrying a crit parameter (RFC 7515 §4.1.11) is rejected with :unsupported_critical_header. The JOSE header typ, when present, MUST be "JWT"; an access-token header such as "at+jwt" is :unexpected_typ.
  2. iss equals the configured issuer (OIDC Core §3.1.3.7 item 1).
  3. aud contains the expected client_id (OIDC Core §3.1.3.7 item 3).
  4. azp - when present, equals the client_id (OIDC Core §3.1.3.7 item 4/5).
  5. Required claims are present and well-typed: sub a non-empty string, iat a non-negative integer.
  6. Temporal. exp is strictly greater than now (no skew leeway); an iat meaningfully in the future is :not_yet_valid.
  7. nonce - when a :nonce is supplied, the claim is present and identical (OIDC Core §3.1.3.7 item 11).

Options:

  • :client_id - the Relying Party client id to require in aud (REQUIRED; OIDC Core §3.1.3.7 item 3).
  • :nonce - the nonce sent in the Authentication Request. When supplied, the nonce claim MUST be present and equal.
  • :now - clock override.

Returns {:ok, claims} (string-keyed payload) or {:error, reason}.