S_hash is the OAuth state parameter
hash value and is the base64url
encoding of the left-most half of the hash
of the octets of the ASCII
representation of the state value, where the hash
algorithm used is id_token
's alg JOSE Header Parameter
S_hash value is a Case-sensitive string.
For example, if the alg is HS512, hash the code value with SHA-512, then take the left-most 256 bits and base64url encode them.
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