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 (at) 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 */ 27 int 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
