hmac.c

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/*
 * Copyright (C) 2007 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

FILE_LICENCE ( GPL2_OR_LATER );

/**
 * @file
 *
 * Keyed-Hashing for Message Authentication
 */

#include <string.h>
#include <assert.h>
#include <gpxe/crypto.h>
#include <gpxe/hmac.h>

/**
 * Reduce HMAC key length
 *
 * @v digest            Digest algorithm to use
 * @v digest_ctx        Digest context
 * @v key               Key
 * @v key_len           Length of key
 */
static void hmac_reduce_key ( struct digest_algorithm *digest,
                              void *key, size_t *key_len ) {
        uint8_t digest_ctx[digest->ctxsize];

        digest_init ( digest, digest_ctx );
        digest_update ( digest, digest_ctx, key, *key_len );
        digest_final ( digest, digest_ctx, key );
        *key_len = digest->digestsize;
}

/**
 * Initialise HMAC
 *
 * @v digest            Digest algorithm to use
 * @v digest_ctx        Digest context
 * @v key               Key
 * @v key_len           Length of key
 *
 * The length of the key should be less than the block size of the
 * digest algorithm being used.  (If the key length is greater, it
 * will be replaced with its own digest, and key_len will be updated
 * accordingly).
 */
void hmac_init ( struct digest_algorithm *digest, void *digest_ctx,
                 void *key, size_t *key_len ) {
        unsigned char k_ipad[digest->blocksize];
        unsigned int i;

        /* Reduce key if necessary */
        if ( *key_len > sizeof ( k_ipad ) )
                hmac_reduce_key ( digest, key, key_len );

        /* Construct input pad */
        memset ( k_ipad, 0, sizeof ( k_ipad ) );
        memcpy ( k_ipad, key, *key_len );
        for ( i = 0 ; i < sizeof ( k_ipad ) ; i++ ) {
                k_ipad[i] ^= 0x36;
        }
        
        /* Start inner hash */
        digest_init ( digest, digest_ctx );
        digest_update ( digest, digest_ctx, k_ipad, sizeof ( k_ipad ) );
}

/**
 * Finalise HMAC
 *
 * @v digest            Digest algorithm to use
 * @v digest_ctx        Digest context
 * @v key               Key
 * @v key_len           Length of key
 * @v hmac              HMAC digest to fill in
 *
 * The length of the key should be less than the block size of the
 * digest algorithm being used.  (If the key length is greater, it
 * will be replaced with its own digest, and key_len will be updated
 * accordingly).
 */
void hmac_final ( struct digest_algorithm *digest, void *digest_ctx,
                  void *key, size_t *key_len, void *hmac ) {
        unsigned char k_opad[digest->blocksize];
        unsigned int i;

        /* Reduce key if necessary */
        if ( *key_len > sizeof ( k_opad ) )
                hmac_reduce_key ( digest, key, key_len );

        /* Construct output pad */
        memset ( k_opad, 0, sizeof ( k_opad ) );
        memcpy ( k_opad, key, *key_len );
        for ( i = 0 ; i < sizeof ( k_opad ) ; i++ ) {
                k_opad[i] ^= 0x5c;
        }
        
        /* Finish inner hash */
        digest_final ( digest, digest_ctx, hmac );

        /* Perform outer hash */
        digest_init ( digest, digest_ctx );
        digest_update ( digest, digest_ctx, k_opad, sizeof ( k_opad ) );
        digest_update ( digest, digest_ctx, hmac, digest->digestsize );
        digest_final ( digest, digest_ctx, hmac );
}