DataFlex

Validate the at_hash Claim of an ID Token

See more JSON Web Token (JWT) Examples

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

Use ChilkatAx-win32.pkg

Procedure Test
    Boolean iSuccess
    Handle hoJsonToken
    Handle hoJwt
    String sIdToken
    String sJose
    Handle hoJsonHeader
    String sClaims
    Handle hoJsonClaims
    String sToken_to_hash
    String sToken_hash_expected
    Handle hoCrypt
    Handle hoBdHash
    Integer iSz
    String sToken_hash_computed
    String sTemp1

    Move False To iSuccess

    // 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"
    // }

    Get Create (RefClass(cComChilkatJsonObject)) To hoJsonToken
    If (Not(IsComObjectCreated(hoJsonToken))) Begin
        Send CreateComObject of hoJsonToken
    End
    Get ComLoadFile Of hoJsonToken "qa_data/tokens/google_sample_id_token.json" To iSuccess
    If (iSuccess = False) Begin
        Showln "Failed to load the JSON file..."
        Procedure_Return
    End

    // Use Chilkat's JWT API to examine the id_token..
    Get Create (RefClass(cComChilkatJwt)) To hoJwt
    If (Not(IsComObjectCreated(hoJwt))) Begin
        Send CreateComObject of hoJwt
    End
    Get ComStringOf Of hoJsonToken "id_token" To sIdToken

    // Extract the JOSE header..
    Get ComGetHeader Of hoJwt sIdToken To sJose

    Get Create (RefClass(cComChilkatJsonObject)) To hoJsonHeader
    If (Not(IsComObjectCreated(hoJsonHeader))) Begin
        Send CreateComObject of hoJsonHeader
    End
    Get ComLoad Of hoJsonHeader sJose To iSuccess
    Set ComEmitCompact Of hoJsonHeader To False
    Get ComEmit Of hoJsonHeader To sTemp1
    Showln sTemp1

    // The JOSE header looks like this:

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

    Get ComGetPayload Of hoJwt sIdToken To sClaims

    Get Create (RefClass(cComChilkatJsonObject)) To hoJsonClaims
    If (Not(IsComObjectCreated(hoJsonClaims))) Begin
        Send CreateComObject of hoJsonClaims
    End
    Get ComLoad Of hoJsonClaims sClaims To iSuccess
    Set ComEmitCompact Of hoJsonClaims To False
    Get ComEmit Of hoJsonClaims To sTemp1
    Showln sTemp1

    // 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.

    Get ComStringOf Of hoJsonToken "access_token" To sToken_to_hash
    Get ComStringOf Of hoJsonClaims "at_hash" To sToken_hash_expected

    // Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
    Get Create (RefClass(cComChilkatCrypt2)) To hoCrypt
    If (Not(IsComObjectCreated(hoCrypt))) Begin
        Send CreateComObject of hoCrypt
    End
    Get Create (RefClass(cComChilkatBinData)) To hoBdHash
    If (Not(IsComObjectCreated(hoBdHash))) Begin
        Send CreateComObject of hoBdHash
    End

    Set ComHashAlgorithm Of hoCrypt To "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.
    Set ComEncodingMode Of hoCrypt To "hex"

    Get ComHashStringENC Of hoCrypt sToken_to_hash To sTemp1
    Get ComAppendEncoded Of hoBdHash sTemp1 "hex" To iSuccess
    Get ComNumBytes Of hoBdHash To iSz

    Get ComGetEncodedChunk Of hoBdHash 0 (iSz / 2) "base64url" To sToken_hash_computed

    // If the hashes are identical, then the access_token as issued for the given id_token.
    Showln "token_hash_expected: " sToken_hash_expected
    Showln "token_hash_computed: " sToken_hash_computed


End_Procedure