aes-unwrap.c

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/*
 * AES key unwrap (128-bit KEK, RFC3394)
 *
 * Copyright (c) 2003-2007, Jouni Malinen <j@w1.fi>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 * Alternatively, this software may be distributed under the terms of BSD
 * license.
 *
 * See README and COPYING for more details.
 */

#include "includes.h"

#include "common.h"
#include "aes.h"
#include "aes_wrap.h"

int aes_unwrap(const u8 *kek, int n, const u8 *cipher, u8 *plain)
{
        u8 a[8], *r, b[16];
        int i, j;
        void *ctx;

        /* 1) Initialize variables. */
        os_memcpy(a, cipher, 8);
        r = plain;
        os_memcpy(r, cipher + 8, 8 * n);

        ctx = aes_decrypt_init(kek, 16);
        if (ctx == NULL)
                return -1;

        /* 2) Compute intermediate values.
         * For j = 5 to 0
         *     For i = n to 1
         *         B = AES-1(K, (A ^ t) | R[i]) where t = n*j+i
         *         A = MSB(64, B)
         *         R[i] = LSB(64, B)
         */
        for (j = 5; j >= 0; j--) {
                r = plain + (n - 1) * 8;
                for (i = n; i >= 1; i--) {
                        os_memcpy(b, a, 8);
                        b[7] ^= n * j + i;

                        os_memcpy(b + 8, r, 8);
                        aes_decrypt(ctx, b, b);
                        os_memcpy(a, b, 8);
                        os_memcpy(r, b + 8, 8);
                        r -= 8;
                }
        }
        aes_decrypt_deinit(ctx);

        /* 3) Output results.
         *
         * These are already in @plain due to the location of temporary
         * variables. Just verify that the IV matches with the expected value.
         */
        for (i = 0; i < 8; i++) {
                if (a[i] != 0xa6)
                        return -1;
        }

        return 0;
}