Validating the token hashes (at_hash, c_hash)

We now have the modulus (n) and the exponent (e) of the public key. This can be used to create the public key and validate the signature.

In specific client profiles, a specific hash is included in the id_token to use to verify that the associated token was issued along with the id_token. For example, when using the implicit client profile, an at_hash value is included in the id_token that provides a means to verify that the access_token was issued along with the id_token.

The following example uses the id_token above and associated access_token to verify the at_hash id_token claim:

1. Hash the octets of the ASCII representation of the access token (using the hash algorithm specified in the JWT header (i.e. for this example, RS256 uses a SHA-256 hash)): SHA256HASH("dNZX1hEZ9wBCzNL40Upu646bdzQA") = c1f82f98 4f55c630 2e76c97d 95ce93a8 9a5d61f7 dc99b9ad 37dc12b3 7231ff9d
2. Take the left-most half of the hashed access token and Base64url encode the value. Left-most half: c1f82f98 4f55c630 2e76c97d 95ce93a8 Base64urlencode([0xC1, 0xF8, 0x2F, 0x98, 0x4F, 0x55, 0xC6, 0x30, 0x2E, 0x76, 0xC9, 0x7D, 0x95, 0xCE, 0x93, 0xA8]) = "wfgvmE9VxjAudsl9lc6TqA"
3. Compare the at_hash value to the base64 URL encoded left-most half of the access token hash bytes.