(PureBasic) Validate the at_hash Claim of an ID Token

Demonstrates how to hash an access token to compare it with the at_hash claim of an ID token.

Chilkat PureBasic Module Download Chilkat PureBasic Module

IncludeFile "CkBinData.pb"
IncludeFile "CkJwt.pb"
IncludeFile "CkJsonObject.pb"
IncludeFile "CkCrypt2.pb"

Procedure ChilkatExample()

    ; This example requires the Chilkat API to have been previously unlocked.
    ; See Global Unlock Sample for sample code.

    ; This example uses a Google access_token + id_token that looks like this:

    ;  {
    ;   "access_token": "ya29.a0...0f",
    ;   "expires_in": 3599,
    ;   "scope": "openid https://www.googleapis.com/auth/userinfo.email",
    ;   "token_type": "Bearer",
    ;   "id_token": "eyJhb...o5nQ"
    ; }

    jsonToken.i = CkJsonObject::ckCreate()
    If jsonToken.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success.i = CkJsonObject::ckLoadFile(jsonToken,"qa_data/tokens/google_sample_id_token.json")
    If success = 0
        Debug "Failed to load the JSON file..."
        CkJsonObject::ckDispose(jsonToken)
        ProcedureReturn
    EndIf

    ; Use Chilkat's JWT API to examine the id_token..
    jwt.i = CkJwt::ckCreate()
    If jwt.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    idToken.s = CkJsonObject::ckStringOf(jsonToken,"id_token")

    ; Extract the JOSE header..
    jose.s = CkJwt::ckGetHeader(jwt,idToken)

    jsonHeader.i = CkJsonObject::ckCreate()
    If jsonHeader.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    CkJsonObject::ckLoad(jsonHeader,jose)
    CkJsonObject::setCkEmitCompact(jsonHeader, 0)
    Debug CkJsonObject::ckEmit(jsonHeader)

    ; The JOSE header looks like this:

    ; {
    ;   "alg": "RS256",
    ;   "kid": "e8799db06287515556213c80acbcfd022fb302a9",
    ;   "typ": "JWT"
    ; }

    claims.s = CkJwt::ckGetPayload(jwt,idToken)

    jsonClaims.i = CkJsonObject::ckCreate()
    If jsonClaims.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    CkJsonObject::ckLoad(jsonClaims,claims)
    CkJsonObject::setCkEmitCompact(jsonClaims, 0)
    Debug CkJsonObject::ckEmit(jsonClaims)

    ; The claims look like this:

    ; {
    ;   "iss": "https://accounts.google.com",
    ;   "azp": "258999997753-5ni8lu5f15r7mno97d82f5lir9i9f6i1.apps.googleusercontent.com",
    ;   "aud": "258999997753-5ni8lu5f15r7mno97d82f5lir9i9f6i1.apps.googleusercontent.com",
    ;   "sub": "111787341816486547572",
    ;   "email": "somebody@gmail.com",
    ;   "email_verified": true,
    ;   "at_hash": "HYJZImlW3mUK-UfjRfXjKw",
    ;   "iat": 1615315968,
    ;   "exp": 1615319568
    ; }

    ; The at_hash is the Access Token hash value. Its value is the base64url encoding of the
    ; left-most half of the hash of the octets of the ASCII representation of the access_token value,
    ; where the hash algorithm used is the hash algorithm used in the alg Header Parameter of the
    ; ID Token's JOSE Header. For instance, if the alg is RS256, hash the access_token value with SHA-256,
    ; then take the left-most 128 bits and base64url encode them. The at_hash value is a case sensitive string.

    token_to_hash.s = CkJsonObject::ckStringOf(jsonToken,"access_token")
    token_hash_expected.s = CkJsonObject::ckStringOf(jsonClaims,"at_hash")

    ; Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
    crypt.i = CkCrypt2::ckCreate()
    If crypt.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    bdHash.i = CkBinData::ckCreate()
    If bdHash.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    CkCrypt2::setCkHashAlgorithm(crypt, "sha256")
    ; This encoding mode must match the encoding mode passed in the 2nd arg to AppendEncoded.
    ; The encoding mode can be anything, as long as they are the same in both places.
    CkCrypt2::setCkEncodingMode(crypt, "hex")

    success = CkBinData::ckAppendEncoded(bdHash,CkCrypt2::ckHashStringENC(crypt,token_to_hash),"hex")
    sz.i = CkBinData::ckNumBytes(bdHash)

    token_hash_computed.s = CkBinData::ckGetEncodedChunk(bdHash,0,sz / 2,"base64url")

    ; If the hashes are identical, then the access_token as issued for the given id_token.
    Debug "token_hash_expected: " + token_hash_expected
    Debug "token_hash_computed: " + token_hash_computed


    CkJsonObject::ckDispose(jsonToken)
    CkJwt::ckDispose(jwt)
    CkJsonObject::ckDispose(jsonHeader)
    CkJsonObject::ckDispose(jsonClaims)
    CkCrypt2::ckDispose(crypt)
    CkBinData::ckDispose(bdHash)


    ProcedureReturn
EndProcedure