1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis */
2 /* SPDX-License-Identifier: Unlicense */
3
4 /**
5 @file ocb_decrypt.c
6 OCB implementation, decrypt data, by Tom St Denis
7 */
8 #include "tomcrypt_private.h"
9
10 #ifdef LTC_OCB_MODE
11
12 /**
13 Decrypt a block with OCB.
14 @param ocb The OCB state
15 @param ct The ciphertext (length of the block size of the block cipher)
16 @param pt [out] The plaintext (length of ct)
17 @return CRYPT_OK if successful
18 */
ocb_decrypt(ocb_state * ocb,const unsigned char * ct,unsigned char * pt)19 int ocb_decrypt(ocb_state *ocb, const unsigned char *ct, unsigned char *pt)
20 {
21 unsigned char Z[MAXBLOCKSIZE], tmp[MAXBLOCKSIZE];
22 int err, x;
23
24 LTC_ARGCHK(ocb != NULL);
25 LTC_ARGCHK(pt != NULL);
26 LTC_ARGCHK(ct != NULL);
27
28 /* check if valid cipher */
29 if ((err = cipher_is_valid(ocb->cipher)) != CRYPT_OK) {
30 return err;
31 }
32 LTC_ARGCHK(cipher_descriptor[ocb->cipher]->ecb_decrypt != NULL);
33
34 /* check length */
35 if (ocb->block_len != cipher_descriptor[ocb->cipher]->block_length) {
36 return CRYPT_INVALID_ARG;
37 }
38
39 /* Get Z[i] value */
40 ocb_shift_xor(ocb, Z);
41
42 /* xor ct in, encrypt, xor Z out */
43 for (x = 0; x < ocb->block_len; x++) {
44 tmp[x] = ct[x] ^ Z[x];
45 }
46 if ((err = cipher_descriptor[ocb->cipher]->ecb_decrypt(tmp, pt, &ocb->key)) != CRYPT_OK) {
47 return err;
48 }
49 for (x = 0; x < ocb->block_len; x++) {
50 pt[x] ^= Z[x];
51 }
52
53 /* compute checksum */
54 for (x = 0; x < ocb->block_len; x++) {
55 ocb->checksum[x] ^= pt[x];
56 }
57
58
59 #ifdef LTC_CLEAN_STACK
60 zeromem(Z, sizeof(Z));
61 zeromem(tmp, sizeof(tmp));
62 #endif
63 return CRYPT_OK;
64 }
65
66 #endif
67
68