At_hash is an OPTIONAL Access Token hash value.

At_hash 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 Identity Token's JOSE Header.

For Example, if the alg is RS256, hash the access_token value with SHA-256, then take the left-most 128 bits and base64url encode them.

At_hash value is a case-sensitive string.

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