Note
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The format identifier hash is represented by the bits 0…31 of keyed-hash message authentication code (HMAC) (as specified in [RFC4634]) based on the SHA-256 algorithm, as specified in [SHA256], over the format identifier string, as specified in [RFC4634].
Note Sample code for the calculation of the HMAC is as specified in [RFC4634].
The key used is the "NULL" key (NULL pointer to the authentication key, and zero length authentication key per the source code in [RFC4634]).
The characters and spaces of the format identifier string are encoded as Unicode UTF-16 using little-endian representation.
The hash of the format identifier is transmitted beginning with the first octet of the octet array.
For sample format identifier strings and their corresponding hashes, see section 4.