src/crypto/aes-unwrap.c

   1 /*
   2  * AES key unwrap (RFC3394)
   3  *
   4  * Copyright (c) 2003-2007, Jouni Malinen <j@w1.fi>
   5  *
   6  * This software may be distributed under the terms of the BSD license.
   7  * See README for more details.
   8  */
   9
  10 #include "includes.h"
  11
  12 #include "common.h"
  13 #include "aes.h"
  14 #include "aes_wrap.h"
  15
  16 /**
  17  * aes_unwrap - Unwrap key with AES Key Wrap Algorithm (RFC3394)
  18  * @kek: Key encryption key (KEK)
  19  * @kek_len: Length of KEK in octets
  20  * @n: Length of the plaintext key in 64-bit units; e.g., 2 = 128-bit = 16
  21  * bytes
  22  * @cipher: Wrapped key to be unwrapped, (n + 1) * 64 bits
  23  * @plain: Plaintext key, n * 64 bits
  24  * Returns: 0 on success, -1 on failure (e.g., integrity verification failed)
  25  */
  26 int aes_unwrap(const u8 *kek, size_t kek_len, int n, const u8 *cipher,
  27                u8 *plain)
  28 {
  29         u8 a[8], *r, b[AES_BLOCK_SIZE];
  30         int i, j;
  31         void *ctx;
  32         unsigned int t;
  33
  34         /* 1) Initialize variables. */
  35         os_memcpy(a, cipher, 8);
  36         r = plain;
  37         os_memcpy(r, cipher + 8, 8 * n);
  38
  39         ctx = aes_decrypt_init(kek, kek_len);
  40         if (ctx == NULL)
  41                 return -1;
  42
  43         /* 2) Compute intermediate values.
  44          * For j = 5 to 0
  45          *     For i = n to 1
  46          *         B = AES-1(K, (A ^ t) | R[i]) where t = n*j+i
  47          *         A = MSB(64, B)
  48          *         R[i] = LSB(64, B)
  49          */
  50         for (j = 5; j >= 0; j--) {
  51                 r = plain + (n - 1) * 8;
  52                 for (i = n; i >= 1; i--) {
  53                         os_memcpy(b, a, 8);
  54                         t = n * j + i;
  55                         b[7] ^= t;
  56                         b[6] ^= t >> 8;
  57                         b[5] ^= t >> 16;
  58                         b[4] ^= t >> 24;
  59
  60                         os_memcpy(b + 8, r, 8);
  61                         aes_decrypt(ctx, b, b);
  62                         os_memcpy(a, b, 8);
  63                         os_memcpy(r, b + 8, 8);
  64                         r -= 8;
  65                 }
  66         }
  67         aes_decrypt_deinit(ctx);
  68
  69         /* 3) Output results.
  70          *
  71          * These are already in @plain due to the location of temporary
  72          * variables. Just verify that the IV matches with the expected value.
  73          */
  74         for (i = 0; i < 8; i++) {
  75                 if (a[i] != 0xa6)
  76                         return -1;
  77         }
  78
  79         return 0;
  80 }