(Ruby) 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 Ruby Downloads install from rubygems.org gem install chilkat or download... Ruby Library for Windows, MacOS, Linux, Alpine Linux

require 'chilkat'

# 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 = Chilkat::CkJsonObject.new()
success = jsonToken.LoadFile("qa_data/tokens/google_sample_id_token.json")
if (success == false)
    print "Failed to load the JSON file..." + "\n";
    exit
end

# Use Chilkat's JWT API to examine the id_token..
jwt = Chilkat::CkJwt.new()
idToken = jsonToken.stringOf("id_token")

# Extract the JOSE header..
jose = jwt.getHeader(idToken)

jsonHeader = Chilkat::CkJsonObject.new()
jsonHeader.Load(jose)
jsonHeader.put_EmitCompact(false)
print jsonHeader.emit() + "\n";

# The JOSE header looks like this:

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

claims = jwt.getPayload(idToken)

jsonClaims = Chilkat::CkJsonObject.new()
jsonClaims.Load(claims)
jsonClaims.put_EmitCompact(false)
print jsonClaims.emit() + "\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.

token_to_hash = jsonToken.stringOf("access_token")
token_hash_expected = jsonClaims.stringOf("at_hash")

# Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
crypt = Chilkat::CkCrypt2.new()
bdHash = Chilkat::CkBinData.new()

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")
sz = bdHash.get_NumBytes()

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.
print "token_hash_expected: " + token_hash_expected + "\n";
print "token_hash_computed: " + token_hash_computed + "\n";