1/* LibTomCrypt, modular cryptographic library -- Tom St Denis
2 *
3 * LibTomCrypt is a library that provides various cryptographic
4 * algorithms in a highly modular and flexible manner.
5 *
6 * The library is free for all purposes without any express
7 * guarantee it works.
8 *
9 * Tom St Denis, tomstdenis@gmail.com, http://libtomcrypt.com
10 */
11
12/**
13   @file ocb_decrypt.c
14   OCB implementation, decrypt data, by Tom St Denis
15*/
16#include "tomcrypt.h"
17
18#ifdef OCB_MODE
19
20/**
21  Decrypt a block with OCB.
22  @param ocb    The OCB state
23  @param ct     The ciphertext (length of the block size of the block cipher)
24  @param pt     [out] The plaintext (length of ct)
25  @return CRYPT_OK if successful
26*/
27int ocb_decrypt(ocb_state *ocb, const unsigned char *ct, unsigned char *pt)
28{
29   unsigned char Z[MAXBLOCKSIZE], tmp[MAXBLOCKSIZE];
30   int err, x;
31
32   LTC_ARGCHK(ocb != NULL);
33   LTC_ARGCHK(pt  != NULL);
34   LTC_ARGCHK(ct  != NULL);
35
36   /* check if valid cipher */
37   if ((err = cipher_is_valid(ocb->cipher)) != CRYPT_OK) {
38      return err;
39   }
40   LTC_ARGCHK(cipher_descriptor[ocb->cipher].ecb_decrypt != NULL);
41
42   /* check length */
43   if (ocb->block_len != cipher_descriptor[ocb->cipher].block_length) {
44      return CRYPT_INVALID_ARG;
45   }
46
47   /* Get Z[i] value */
48   ocb_shift_xor(ocb, Z);
49
50   /* xor ct in, encrypt, xor Z out */
51   for (x = 0; x < ocb->block_len; x++) {
52       tmp[x] = ct[x] ^ Z[x];
53   }
54   if ((err = cipher_descriptor[ocb->cipher].ecb_decrypt(tmp, pt, &ocb->key)) != CRYPT_OK) {
55      return err;
56   }
57   for (x = 0; x < ocb->block_len; x++) {
58       pt[x] ^= Z[x];
59   }
60
61   /* compute checksum */
62   for (x = 0; x < ocb->block_len; x++) {
63       ocb->checksum[x] ^= pt[x];
64   }
65
66
67#ifdef LTC_CLEAN_STACK
68   zeromem(Z, sizeof(Z));
69   zeromem(tmp, sizeof(tmp));
70#endif
71   return CRYPT_OK;
72}
73
74#endif
75
76
77/* $Source: /cvs/libtom/libtomcrypt/src/encauth/ocb/ocb_decrypt.c,v $ */
78/* $Revision: 1.5 $ */
79/* $Date: 2006/03/31 14:15:35 $ */
80