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61 lines
2.1 KiB
C
61 lines
2.1 KiB
C
/* SPDX-License-Identifier: LGPL-2.1-or-later */
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#include <assert.h>
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#include <string.h>
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#include "hmac.h"
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#include "sha256.h"
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#define HMAC_BLOCK_SIZE 64
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#define INNER_PADDING_BYTE 0x36
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#define OUTER_PADDING_BYTE 0x5c
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void hmac_sha256(const void *key,
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size_t key_size,
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const void *input,
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size_t input_size,
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uint8_t res[static SHA256_DIGEST_SIZE]) {
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uint8_t inner_padding[HMAC_BLOCK_SIZE] = { };
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uint8_t outer_padding[HMAC_BLOCK_SIZE] = { };
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uint8_t replacement_key[SHA256_DIGEST_SIZE];
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struct sha256_ctx hash;
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assert(key);
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assert(key_size > 0);
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assert(res);
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/* Implement algorithm as described by FIPS 198. */
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/* The key needs to be block size length or less, hash it if it's longer. */
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if (key_size > HMAC_BLOCK_SIZE) {
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sha256_direct(key, key_size, replacement_key);
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key = replacement_key;
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key_size = SHA256_DIGEST_SIZE;
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}
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/* First, copy the key into the padding arrays. If it's shorter than
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* the block size, the arrays are already initialized to 0. */
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memcpy(inner_padding, key, key_size);
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memcpy(outer_padding, key, key_size);
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/* Then, XOR the provided key and any padding leftovers with the fixed
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* padding bytes as defined in FIPS 198. */
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for (size_t i = 0; i < HMAC_BLOCK_SIZE; i++) {
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inner_padding[i] ^= INNER_PADDING_BYTE;
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outer_padding[i] ^= OUTER_PADDING_BYTE;
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}
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/* First pass: hash the inner padding array and the input. */
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sha256_init_ctx(&hash);
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sha256_process_bytes(inner_padding, HMAC_BLOCK_SIZE, &hash);
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sha256_process_bytes(input, input_size, &hash);
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sha256_finish_ctx(&hash, res);
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/* Second pass: hash the outer padding array and the result of the first pass. */
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sha256_init_ctx(&hash);
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sha256_process_bytes(outer_padding, HMAC_BLOCK_SIZE, &hash);
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sha256_process_bytes(res, SHA256_DIGEST_SIZE, &hash);
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sha256_finish_ctx(&hash, res);
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}
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