(Objective-C) 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.

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#import <CkoJsonObject.h>
#import <CkoJwt.h>
#import <NSString.h>
#import <CkoCrypt2.h>
#import <CkoBinData.h>

// 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"
// }

CkoJsonObject *jsonToken = [[CkoJsonObject alloc] init];
BOOL success = [jsonToken LoadFile: @"qa_data/tokens/google_sample_id_token.json"];
if (success == NO) {
    NSLog(@"%@",@"Failed to load the JSON file...");
    return;
}

// Use Chilkat's JWT API to examine the id_token..
CkoJwt *jwt = [[CkoJwt alloc] init];
NSString *idToken = [jsonToken StringOf: @"id_token"];

// Extract the JOSE header..
NSString *jose = [jwt GetHeader: idToken];

CkoJsonObject *jsonHeader = [[CkoJsonObject alloc] init];
[jsonHeader Load: jose];
jsonHeader.EmitCompact = NO;
NSLog(@"%@",[jsonHeader Emit]);

// The JOSE header looks like this:

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

NSString *claims = [jwt GetPayload: idToken];

CkoJsonObject *jsonClaims = [[CkoJsonObject alloc] init];
[jsonClaims Load: claims];
jsonClaims.EmitCompact = NO;
NSLog(@"%@",[jsonClaims Emit]);

// 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.

NSString *token_to_hash = [jsonToken StringOf: @"access_token"];
NSString *token_hash_expected = [jsonClaims StringOf: @"at_hash"];

// Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
CkoCrypt2 *crypt = [[CkoCrypt2 alloc] init];
CkoBinData *bdHash = [[CkoBinData alloc] init];

crypt.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.EncodingMode = @"hex";

success = [bdHash AppendEncoded: [crypt HashStringENC: token_to_hash] encoding: @"hex"];
int sz = [bdHash.NumBytes intValue];

NSString *token_hash_computed = [bdHash GetEncodedChunk: [NSNumber numberWithInt: 0] numBytes: [NSNumber numberWithInt: (sz / 2)] encoding: @"base64url"];

// If the hashes are identical, then the access_token as issued for the given id_token.
NSLog(@"%@%@",@"token_hash_expected: ",token_hash_expected);
NSLog(@"%@%@",@"token_hash_computed: ",token_hash_computed);