Authorization Code Grant

Client Authentication - Client Assertion JWT

Assertion JWT Structure

The RP is required to generate an assertion JWT that has the following header and claims, and is signed with the JWK that was provided during onboarding.

JWT Header

The header must include alg and typ.

The supported alg types are:

• ES256

• ES384

• ES512.

This must match the alg value in the signing key used to sign the assertion (if the signing JWK specifies alg explicitly).

The header should also include kid of the signing key to help identify which of the RP’s signing keys was used, though this is not mandatory. If omitted, we will test against all known signing keys when attempting to verify the signature.

example

{
  "typ" : "JWT",
  "alg" : "ES256",
  "kid" : "rp_key_01"
}

JWT Claims

Path	Type	Description
sub	String	This should be client_id of the registered client.
aud	String	The recipient that the JWT is intended for. This must match the issuer field in the response of the OpenID discovery endpoint (https://id.singpass.gov.sg/.well-known/openid-configuration) e.g. https://id.singpass.gov.sg.
iss	String	This should be client_id of the registered client.
iat	Number	The time at which the JWT was issued. https://tools.ietf.org/html/rfc7519#section-4.1.6
exp	Number	The expiration time on or after which the JWT MUST NOT be accepted by Singpass for processing. Additionally, Singpass will not accept tokens with an exp longer than 2 minutes since iat. https://tools.ietf.org/html/rfc7519#section-4.1.4
code	String	(Optional) This should be the auth code which is used to exchange for an ID token. It should be identical to the code form param sent outside the client_assertion. This enables increased security by signing the code so that the client_assertion can only be used once for a specific request.

Request / Response Structure

Curl request

$ curl 'https://stg-id.singpass.gov.sg/token' -i -X POST \
    -H 'Content-Type: application/x-www-form-urlencoded; charset=ISO-8859-1' \
    -d 'grant_type=authorization_code&redirect_uri=http%3A%2F%2Fexample.com&client_assertion_type=urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer&client_assertion=eyJraWQiOiJycF9rZXlfMDEiLCJ0eXAiOiJKV1QiLCJhbGciOiJFUzM4NCJ9.eyJzdWIiOiJnblk2RXJpY2hwYjV0NE5GUlA5UjRMN2FFQzlOMEZRSCIsImF1ZCI6Imh0dHBzOi8vc3RnLWlkLnNpbmdwYXNzLmdvdi5zZyIsImNvZGUiOiJuMGVzYzNOUnplN0xUQ3U3aVl6UzZhNWFjYzNmMG9ncDQiLCJpc3MiOiJodHRwOi8vZXhhbXBsZS5jb20iLCJleHAiOjE3MjczMjIwNjUsImlhdCI6MTcyNzMyMTk0NX0.AAAAAAAAAAAAAAAAAAAAAIvHG8uapa4w10pPn_S_7uyYPXNJWU7389sTLNHXrCKhAAAAAAAAAAAAAAAAAAAAAIxyKbbZfLSkduU6WIPrrPe1kGvmHmzXL4PHpgAJ0g0d&code=n0esc3NRze7LTCu7iYzS6a5acc3f0ogp4&client_id=gnY6Erichpb5t4NFRP9R4L7aEC9N0FQH&code_verifier=mN7szCmmIc6Z2Vg-iaX7f7RDVsKAhY5GG-r7Crq0jxTdxY0xyPKnsAEWtEMdZ3D8QW5rs-C824W3Jwntcw'

Form parameters

Parameter	Description
client_id	The Client Identifier registered.
redirect_uri	The redirect URI being used in this auth session.
grant_type	The type of grant being requested. This must be set to authorization_code.
code	The code issued earlier in the auth session.
scope	(Optional) If no value is provided, it defaults to openid. If provided, then only openid is allowed.
client_assertion_type	This MUST be set to urn:ietf:params:oauth:client-assertion-type:jwt-bearer.
client_assertion	A JWT identifying the client.
code_verifier	(Mandatory) This is the session-based, unique, and non-guessable value that the RP had used to generate the code_challenge. Must match regexp pattern of [a-zA-Z0-9_\-.~]+ minimum length of 43 characters and a maximum length of 128 characters.

HTTP request

POST /token HTTP/1.1
Content-Type: application/x-www-form-urlencoded; charset=ISO-8859-1
Host: stg-id.singpass.gov.sg
Content-Length: 788

grant_type=authorization_code&redirect_uri=http%3A%2F%2Fexample.com&client_assertion_type=urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer&client_assertion=eyJraWQiOiJycF9rZXlfMDEiLCJ0eXAiOiJKV1QiLCJhbGciOiJFUzM4NCJ9.eyJzdWIiOiJnblk2RXJpY2hwYjV0NE5GUlA5UjRMN2FFQzlOMEZRSCIsImF1ZCI6Imh0dHBzOi8vc3RnLWlkLnNpbmdwYXNzLmdvdi5zZyIsImNvZGUiOiJuMGVzYzNOUnplN0xUQ3U3aVl6UzZhNWFjYzNmMG9ncDQiLCJpc3MiOiJodHRwOi8vZXhhbXBsZS5jb20iLCJleHAiOjE3MjczMjIwNjUsImlhdCI6MTcyNzMyMTk0NX0.AAAAAAAAAAAAAAAAAAAAAIvHG8uapa4w10pPn_S_7uyYPXNJWU7389sTLNHXrCKhAAAAAAAAAAAAAAAAAAAAAIxyKbbZfLSkduU6WIPrrPe1kGvmHmzXL4PHpgAJ0g0d&code=n0esc3NRze7LTCu7iYzS6a5acc3f0ogp4&client_id=gnY6Erichpb5t4NFRP9R4L7aEC9N0FQH&code_verifier=mN7szCmmIc6Z2Vg-iaX7f7RDVsKAhY5GG-r7Crq0jxTdxY0xyPKnsAEWtEMdZ3D8QW5rs-C824W3Jwntcw

HTTP response

HTTP/1.1 200 OK
Cache-Control: no-cache, no-store, must-revalidate
x-ndi-response-category: client-assertion-auth-code
X-XSS-Protection: 0
X-Frame-Options: DENY
Date: Thu, 26 Sep 2024 03:39:05 GMT
Connection: keep-alive
Vary: Origin
Vary: Access-Control-Request-Method
Vary: Access-Control-Request-Headers
X-Content-Type-Options: nosniff
Transfer-Encoding: chunked
Content-Type: application/json
Content-Length: 597

{
  "access_token" : "Ul5JqK8vJX8Jfgo7UjpiIigF7DciW7YGnUACLN1/T80=",
  "token_type" : "Bearer",
  "id_token" : "eyJraWQiOiJuZGlfc3RnXzAxIiwidHlwIjoiSldUIiwiYWxnIjoiRVMyNTYifQ.eyJhdWQiOiJnblk2RXJpY2hwYjV0NE5GUlA5UjRMN2FFQzlOMEZRSCIsInN1YiI6InM9Uzg4MjkzMTRCLHU9MWMwY2VlMzgtM2E4Zi00ZjhhLTgzYmMtN2EwZTRjNTlkNmE5IiwiYW1yIjpbInB3ZCIsInN3ayJdLCJpc3MiOiJodHRwczovL3N0Zy1pZC5zaW5ncGFzcy5nb3Yuc2ciLCJleHAiOjE3MjczMjI1NDUsImlhdCI6MTcyNzMyMTk0NSwibm9uY2UiOiJMNW5tUWZjZXREREllaW5jb3F2Q3JGeUd2K25Ib2JrdjRYb2NOWVBDWGFRPSJ9.bD1osnkuL-hXCXY9L_LbveANix_tBsjk8872bL04tTPFGMkzMjE85W8fTwQXrPSFnwQtgzmLB2__i0vdGYnfgA"
}

Request body

grant_type=authorization_code&redirect_uri=http%3A%2F%2Fexample.com&client_assertion_type=urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer&client_assertion=eyJraWQiOiJycF9rZXlfMDEiLCJ0eXAiOiJKV1QiLCJhbGciOiJFUzM4NCJ9.eyJzdWIiOiJnblk2RXJpY2hwYjV0NE5GUlA5UjRMN2FFQzlOMEZRSCIsImF1ZCI6Imh0dHBzOi8vc3RnLWlkLnNpbmdwYXNzLmdvdi5zZyIsImNvZGUiOiJuMGVzYzNOUnplN0xUQ3U3aVl6UzZhNWFjYzNmMG9ncDQiLCJpc3MiOiJodHRwOi8vZXhhbXBsZS5jb20iLCJleHAiOjE3MjczMjIwNjUsImlhdCI6MTcyNzMyMTk0NX0.AAAAAAAAAAAAAAAAAAAAAIvHG8uapa4w10pPn_S_7uyYPXNJWU7389sTLNHXrCKhAAAAAAAAAAAAAAAAAAAAAIxyKbbZfLSkduU6WIPrrPe1kGvmHmzXL4PHpgAJ0g0d&code=n0esc3NRze7LTCu7iYzS6a5acc3f0ogp4&client_id=gnY6Erichpb5t4NFRP9R4L7aEC9N0FQH&code_verifier=mN7szCmmIc6Z2Vg-iaX7f7RDVsKAhY5GG-r7Crq0jxTdxY0xyPKnsAEWtEMdZ3D8QW5rs-C824W3Jwntcw

Response body

{
  "access_token" : "Ul5JqK8vJX8Jfgo7UjpiIigF7DciW7YGnUACLN1/T80=",
  "token_type" : "Bearer",
  "id_token" : "eyJraWQiOiJuZGlfc3RnXzAxIiwidHlwIjoiSldUIiwiYWxnIjoiRVMyNTYifQ.eyJhdWQiOiJnblk2RXJpY2hwYjV0NE5GUlA5UjRMN2FFQzlOMEZRSCIsInN1YiI6InM9Uzg4MjkzMTRCLHU9MWMwY2VlMzgtM2E4Zi00ZjhhLTgzYmMtN2EwZTRjNTlkNmE5IiwiYW1yIjpbInB3ZCIsInN3ayJdLCJpc3MiOiJodHRwczovL3N0Zy1pZC5zaW5ncGFzcy5nb3Yuc2ciLCJleHAiOjE3MjczMjI1NDUsImlhdCI6MTcyNzMyMTk0NSwibm9uY2UiOiJMNW5tUWZjZXREREllaW5jb3F2Q3JGeUd2K25Ib2JrdjRYb2NOWVBDWGFRPSJ9.bD1osnkuL-hXCXY9L_LbveANix_tBsjk8872bL04tTPFGMkzMjE85W8fTwQXrPSFnwQtgzmLB2__i0vdGYnfgA"
}

Response fields

Path	Type	Description
access_token	String	For Login flows, the access token will be a random string that is not to be used. For Myinfo flows, the token will be an encoded JSON Web Token (JWT). This token is to be used to exchange for payload at UserInfo Endpoint.
token_type	String	The type of token being requested, Bearer only so far.
id_token	String	The ID token with relevant claims in JWT format signed by the ASP. Note that the example response body shows a JWS (3-part structure separated by dots), but the format will differ for a JWS in JWE (5-part structure). Refer here for more details about the ID token structure.

Error Response

Singpass generally follows OIDC error response specifications. For more information, please refer to Token Error Response specifications.

ID Token Structure

The format and structure of the issued ID Token will vary depending on the client’s profile as specified in this table below:

Client Profile	sub Claim Content	ID token format
direct	UUID only (eg. u=32af8b7d-ad1d-4c25-8dc7-0a981b533000)	JWS
direct_pii_allowed	Regular NRIC holders: NRIC and UUID (eg. s=S1234567A,u=32af8b7d-ad1d-4c25-8dc7-0a981b533000) Singpass Foreign Account (SFA) holders: Singpass User ID (UID), Foreigner ID (FID), Country-of-Issuance (COI) and UUID (eg. s=Y7613265T,fid=G730Z-H5P96,coi=DE,u=e2af740e-25b4-4b19-b527-494670952cb0) This class of users were previously known as "Foreign Unique Account" or "Singpass Foreign Unique Account" users. Only designated relying parties are able to have SFA users authenticate & complete token exchange.	JWS in JWE (encrypted with client’s JWK) See section below for more details about the JWE format.
bridge (special case internal use only)	NRIC and UUID (eg. s=S1234567A,u=32af8b7d-ad1d-4c25-8dc7-0a981b533000)	JWS

Overview of a JWS in JWE

An encrypted ID token will returned from the /token endpoint is a JWS in JWE that is in compact serialization form. It has the following structure:

• JWE Header

• Encrypted Key

• Initialization Vector

• Encrypted Payload (if decrypted, this would be the Base64 encoded form of a JWS ID Token)
• Authentication Tag

See https://datatracker.ietf.org/doc/html/rfc7516#section-3.1 for more details.

JWE Header

The JWE will contain these standard headers. Refer to https://tools.ietf.org/html/rfc7515#section-4 for more information about each header.

Note: Relying parties should use the kid field in the header to determine which key NDI used for encryption.

example

{
	"kid": "client_01",
	"cty": "JWT",
	"enc": "A256CBC-HS512",
	"alg": "ECDH-ES+A256KW"
}

Overview of JWS

The JWS ID token returned from the /token endpoint is in compact serialization form. A JWS has the following structure.

• JWS Header

• Payload (containing claims)

• Signature

JWS Header

The JWS ID token will contain these standard headers. Refer to https://tools.ietf.org/html/rfc7515#section-4 for more information about each header.

example

{
  "kid" : "ndi_stg_01",
  "typ" : "JWT",
  "alg" : "ES256"
}

JWS Claims

The JWS ID token will contain the following claims.

example

{
  "aud" : "gnY6Erichpb5t4NFRP9R4L7aEC9N0FQH",
  "sub" : "s=S8829314B,u=1c0cee38-3a8f-4f8a-83bc-7a0e4c59d6a9",
  "amr" : [ "pwd", "swk" ],
  "iss" : "https://stg-id.singpass.gov.sg",
  "exp" : 1727322545,
  "iat" : 1727321945,
  "nonce" : "L5nmQfcetDDIeincoqvCrFyGv+nHobkv4XocNYPCXaQ="
}

Table 1. Description of Claims

Path	Type	Description
sub	String	The principal that is the subject of the JWT. Contains a comma-separated, key=value mapping that identifies the user; possibly including multiple alternate IDs representing the user. The keys included vary by the profile of the OIDC client, and the user type, however the minimal format is u=<UUID> where UUID represents the user’s globally unique identifier. https://tools.ietf.org/html/rfc7519#section-4.1.2
aud	String	The client_id of the relying party. https://tools.ietf.org/html/rfc7519#section-4.1.3
iss	String	The principal that issued the JWT. https://tools.ietf.org/html/rfc7519#section-4.1.1
iat	Number	The time at which the JWT was issued. https://tools.ietf.org/html/rfc7519#section-4.1.6
exp	Number	The expiration time on or after which the JWT MUST NOT be accepted for processing. Defaults to 10 minutes since "iat". https://tools.ietf.org/html/rfc7519#section-4.1.4
amr	Array	Authentication method references. Example values are ["face"], ["fv"], ["fv-alt"], ["otp"], ["pwd","fv"], ["pwd","otp-email"], ["pwd","sms"], ["pwd","swk"], ["pwd"], ["sso"]. Note that this list is non-exhaustive, and NDI reserves the right to introduce new values without prior notice to RPs.
nonce	String	String value used to associate a Client session with an ID Token, and to mitigate replay attacks. The value is passed through unmodified from the Authentication Request to the ID Token. Clients MUST verify that the nonce Claim Value is equal to the value of the nonce parameter sent in the Authentication Request. https://openid.net/specs/openid-connect-core-1_0.html#IDToken
acr	String	(Optional) The Authentication Context Class Reference. The values are context-specific and agreed upon between NDI and relying parties when used.