hmac_sha256.c

#include <stdio.h>
#include <stddef.h>
#include <stdint.h>

//#include "hmac_sha256.h"
//#include "sha256.h"
#include "sha256.c"

#define B 64
#define HMAC_SHA256_DIGEST_SIZE 32  /* Same as SHA-256's output size. */
#define SHA256_DIGEST_SIZE 32    
#define L (SHA256_DIGEST_SIZE)
#define K (SHA256_DIGEST_SIZE * 2)


//typedef unsigned char uint8_t;

#define I_PAD 0x36
#define O_PAD 0x5C

void
hmac_sha256 (uint8_t out[HMAC_SHA256_DIGEST_SIZE],
             const uint8_t *data, size_t data_len,
             const uint8_t *key, size_t key_len)
{
//    api_check_return (out);
//    api_check_return (data);
//    api_check_return (key);
//    api_check_return (key_len <= B);

    SHA256_CTX ss;
    uint8_t kh[SHA256_DIGEST_SIZE];

    /*
     * If the key length is bigger than the buffer size B, apply the hash
     * function to it first and use the result instead.
     */
    if (key_len > B) {
        sha256_init (&ss);
        sha256_update (&ss, key, key_len);
        sha256_final (&ss, kh);
        key_len = SHA256_DIGEST_SIZE;
        key = kh;
    }

    /*
     * (1) append zeros to the end of K to create a B byte string
     *     (e.g., if K is of length 20 bytes and B=64, then K will be
     *     appended with 44 zero bytes 0x00)
     * (2) XOR (bitwise exclusive-OR) the B byte string computed in step
     *     (1) with ipad
     */
    uint8_t kx[B];
    size_t i=0;
    for (i = 0; i < key_len; i++) kx[i] = I_PAD ^ key[i];
    for (i = key_len; i < B; i++) kx[i] = I_PAD ^ 0;

    /*
     * (3) append the stream of data 'text' to the B byte string resulting
     *     from step (2)
     * (4) apply H to the stream generated in step (3)
     */
    sha256_init (&ss);
    sha256_update (&ss, kx, B);
    sha256_update (&ss, data, data_len);
    sha256_final (&ss, out);

    /*
     * (5) XOR (bitwise exclusive-OR) the B byte string computed in
     *     step (1) with opad
     *
     * NOTE: The "kx" variable is reused.
     */
    for (i = 0; i < key_len; i++) kx[i] = O_PAD ^ key[i];
    for (i = key_len; i < B; i++) kx[i] = O_PAD ^ 0;

    /*
     * (6) append the H result from step (4) to the B byte string
     *     resulting from step (5)
     * (7) apply H to the stream generated in step (6) and output
     *     the result
     */
    sha256_init (&ss);
    sha256_update (&ss, kx, B);
    sha256_update (&ss, out, SHA256_DIGEST_SIZE);
    sha256_final (&ss, out);
}