C

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. C

#include <C_CkJsonObject.h>
#include <C_CkJwt.h>
#include <C_CkCrypt2.h>
#include <C_CkBinData.h>

void ChilkatSample(void)
    {
    BOOL success;
    HCkJsonObject jsonToken;
    HCkJwt jwt;
    const char *idToken;
    const char *jose;
    HCkJsonObject jsonHeader;
    const char *claims;
    HCkJsonObject jsonClaims;
    const char *token_to_hash;
    const char *token_hash_expected;
    HCkCrypt2 crypt;
    HCkBinData bdHash;
    int sz;
    const char *token_hash_computed;

    success = FALSE;

    // 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 = CkJsonObject_Create();
    success = CkJsonObject_LoadFile(jsonToken,"qa_data/tokens/google_sample_id_token.json");
    if (success == FALSE) {
        printf("Failed to load the JSON file...\n");
        CkJsonObject_Dispose(jsonToken);
        return;
    }

    // Use Chilkat's JWT API to examine the id_token..
    jwt = CkJwt_Create();
    idToken = CkJsonObject_stringOf(jsonToken,"id_token");

    // Extract the JOSE header..
    jose = CkJwt_getHeader(jwt,idToken);

    jsonHeader = CkJsonObject_Create();
    CkJsonObject_Load(jsonHeader,jose);
    CkJsonObject_putEmitCompact(jsonHeader,FALSE);
    printf("%s\n",CkJsonObject_emit(jsonHeader));

    // The JOSE header looks like this:

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

    claims = CkJwt_getPayload(jwt,idToken);

    jsonClaims = CkJsonObject_Create();
    CkJsonObject_Load(jsonClaims,claims);
    CkJsonObject_putEmitCompact(jsonClaims,FALSE);
    printf("%s\n",CkJsonObject_emit(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 = CkJsonObject_stringOf(jsonToken,"access_token");
    token_hash_expected = CkJsonObject_stringOf(jsonClaims,"at_hash");

    // Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
    crypt = CkCrypt2_Create();
    bdHash = CkBinData_Create();

    CkCrypt2_putHashAlgorithm(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_putEncodingMode(crypt,"hex");

    success = CkBinData_AppendEncoded(bdHash,CkCrypt2_hashStringENC(crypt,token_to_hash),"hex");
    sz = CkBinData_getNumBytes(bdHash);

    token_hash_computed = CkBinData_getEncodedChunk(bdHash,0,sz / 2,"base64url");

    // If the hashes are identical, then the access_token as issued for the given id_token.
    printf("token_hash_expected: %s\n",token_hash_expected);
    printf("token_hash_computed: %s\n",token_hash_computed);


    CkJsonObject_Dispose(jsonToken);
    CkJwt_Dispose(jwt);
    CkJsonObject_Dispose(jsonHeader);
    CkJsonObject_Dispose(jsonClaims);
    CkCrypt2_Dispose(crypt);
    CkBinData_Dispose(bdHash);

    }