(MFC) 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 C/C++ Library Downloads MS Visual C/C++ Libs

See Also: Using MFC CString in Chilkat

#include <CkJsonObject.h>
#include <CkJwt.h>
#include <CkCrypt2.h>
#include <CkBinData.h>

void ChilkatSample(void)
    {
    CkString strOut;

    // 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"
    // }

    CkJsonObject jsonToken;
    bool success = jsonToken.LoadFile("qa_data/tokens/google_sample_id_token.json");
    if (success == false) {
        strOut.append("Failed to load the JSON file...");
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    // Use Chilkat's JWT API to examine the id_token..
    CkJwt jwt;
    const char *idToken = jsonToken.stringOf("id_token");

    // Extract the JOSE header..
    const char *jose = jwt.getHeader(idToken);

    CkJsonObject jsonHeader;
    jsonHeader.Load(jose);
    jsonHeader.put_EmitCompact(false);
    strOut.append(jsonHeader.emit());
    strOut.append("\r\n");

    // The JOSE header looks like this:

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

    const char *claims = jwt.getPayload(idToken);

    CkJsonObject jsonClaims;
    jsonClaims.Load(claims);
    jsonClaims.put_EmitCompact(false);
    strOut.append(jsonClaims.emit());
    strOut.append("\r\n");

    // 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.

    const char *token_to_hash = jsonToken.stringOf("access_token");
    const char *token_hash_expected = jsonClaims.stringOf("at_hash");

    // Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
    CkCrypt2 crypt;
    CkBinData bdHash;

    crypt.put_HashAlgorithm("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.
    crypt.put_EncodingMode("hex");

    success = bdHash.AppendEncoded(crypt.hashStringENC(token_to_hash),"hex");
    int sz = bdHash.get_NumBytes();

    const char *token_hash_computed = bdHash.getEncodedChunk(0,sz / 2,"base64url");

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


    SetDlgItemText(IDC_EDIT1,strOut.getUnicode());

    }