Tcl

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. Tcl

load ./chilkat.dll

set success 0

# 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"
# }

set jsonToken [new_CkJsonObject]

set success [CkJsonObject_LoadFile $jsonToken "qa_data/tokens/google_sample_id_token.json"]
if {$success == 0} then {
    puts "Failed to load the JSON file..."
    delete_CkJsonObject $jsonToken
    exit
}

# Use Chilkat's JWT API to examine the id_token..
set jwt [new_CkJwt]

set idToken [CkJsonObject_stringOf $jsonToken "id_token"]

# Extract the JOSE header..
set jose [CkJwt_getHeader $jwt $idToken]

set jsonHeader [new_CkJsonObject]

CkJsonObject_Load $jsonHeader $jose
CkJsonObject_put_EmitCompact $jsonHeader 0
puts [CkJsonObject_emit $jsonHeader]

# The JOSE header looks like this:

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

set claims [CkJwt_getPayload $jwt $idToken]

set jsonClaims [new_CkJsonObject]

CkJsonObject_Load $jsonClaims $claims
CkJsonObject_put_EmitCompact $jsonClaims 0
puts [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.

set token_to_hash [CkJsonObject_stringOf $jsonToken "access_token"]
set 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`).
set crypt [new_CkCrypt2]

set bdHash [new_CkBinData]

CkCrypt2_put_HashAlgorithm $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_put_EncodingMode $crypt "hex"

set success [CkBinData_AppendEncoded $bdHash [CkCrypt2_hashStringENC $crypt $token_to_hash] "hex"]
set sz [CkBinData_get_NumBytes $bdHash]

set token_hash_computed [CkBinData_getEncodedChunk $bdHash 0 [expr $sz / 2] "base64url"]

# If the hashes are identical, then the access_token as issued for the given id_token.
puts "token_hash_expected: $token_hash_expected"
puts "token_hash_computed: $token_hash_computed"

delete_CkJsonObject $jsonToken
delete_CkJwt $jwt
delete_CkJsonObject $jsonHeader
delete_CkJsonObject $jsonClaims
delete_CkCrypt2 $crypt
delete_CkBinData $bdHash