________________________________________________________________________

This file is part of Logtalk <https://logtalk.org/>  
SPDX-FileCopyrightText: 1998-2026 Paulo Moura <pmoura@logtalk.org>  
SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
________________________________________________________________________


`hmac`
======

The `hmac` library provides a portable implementation of HMAC
(Keyed-Hashing for Message Authentication) as specified in RFC 2104:

https://datatracker.ietf.org/doc/html/rfc2104.html

The library exports a single object, `hmac`, implementing the
`hmac_protocol` protocol with the predicates:

- `digest/4`
- `hex_digest/4`
- `digest/5`
- `hex_digest/5`

The first argument is a hash object implementing the `hash_digest_protocol`
protocol from the `hashes` library. Currently supported hash objects are:

- `md5`
- `sha1`
- `sha224`
- `sha256`
- `sha384`
- `sha512`
- `sha512_256`
- `sha3_224`
- `sha3_256`
- `sha3_384`
- `sha3_512`

On backend Prolog compilers supporting only bounded integer arithmetic,
only `md5` is available. On backends supporting unbounded integer
arithmetic, all the listed hash objects are available.

The `digest/5` and `hex_digest/5` predicates implement the truncation rule
described in RFC 2104 and RFC 2202 by returning the requested number of
leftmost digest bytes.


API documentation
-----------------

Open the [../../apis/library_index.html#hmac](../../apis/library_index.html#hmac)
link in a web browser.


Loading
-------

To load all entities in this library, load the `loader.lgt` file:

	| ?- logtalk_load(hmac(loader)).


Testing
-------

To test this library predicates, load the `tester.lgt` file:

	| ?- logtalk_load(hmac(tester)).


Examples
--------

Compute the HMAC-SHA-256 digest for a text message and return it as a
hexadecimal atom:

	| ?- atom_codes('Jefe', Key),
	     atom_codes('what do ya want for nothing?', Message),
	     hmac::hex_digest(sha256, Key, Message, Digest).
	Key = [74,101,102,101],
	Message = [119,104,97,116,32,100,111,32,121,97,32,119,97,110,116,32,102,111,114,32,110,111,116,104,105,110,103,63],
	Digest = '5bdcc146bf60754e6a042426089575c75a003f089d2739839dec58b964ec3843'
	yes

Compute a truncated 16-byte HMAC-SHA-256 digest:

	| ?- atom_codes('Jefe', Key),
	     atom_codes('what do ya want for nothing?', Message),
	     hmac::hex_digest(sha256, Key, Message, 16, Digest).
	Key = [74,101,102,101],
	Message = [119,104,97,116,32,100,111,32,121,97,32,119,97,110,116,32,102,111,114,32,110,111,116,104,105,110,103,63],
	Digest = '5bdcc146bf60754e6a042426089575c7'
	yes
