Lua

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. Lua

local chilkat = 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"
-- }

local jsonToken = chilkat.JsonObject.new()
local success = jsonToken:load_file("qa_data/tokens/google_sample_id_token.json")
if not success then
    print("Failed to load the JSON file...")
    return
end

-- Use Chilkat's JWT API to examine the id_token..
local jwt = chilkat.Jwt.new()
local idToken = jsonToken:string_of("id_token")

-- Extract the JOSE header..
local jose = jwt:get_header(idToken)

local jsonHeader = chilkat.JsonObject.new()
jsonHeader:load(jose)
jsonHeader.emit_compact = false
print(jsonHeader:emit())

-- The JOSE header looks like this:

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

local claims = jwt:get_payload(idToken)

local jsonClaims = chilkat.JsonObject.new()
jsonClaims:load(claims)
jsonClaims.emit_compact = false
print(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.

local token_to_hash = jsonToken:string_of("access_token")
local token_hash_expected = jsonClaims:string_of("at_hash")

-- Step 1. hashes the access token using SHA-256 (Google uses `RS256` as the ID Token `alg`).
local crypt = chilkat.Crypt2.new()
local bdHash = chilkat.BinData.new()

crypt.hash_algorithm = "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.encoding_mode = "hex"

success = bdHash:append_encoded(crypt:hash_string_enc(token_to_hash), "hex")
local sz = bdHash.num_bytes

local token_hash_computed = bdHash:get_encoded_chunk(0, math.floor(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)
print("token_hash_computed: " .. token_hash_computed)