60 lines
1.3 KiB
C
60 lines
1.3 KiB
C
/* LibTomCrypt, modular cryptographic library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/**
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@file ocb_decrypt.c
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OCB implementation, decrypt data, by Tom St Denis
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*/
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#include "tomcrypt_private.h"
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#ifdef LTC_OCB_MODE
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/**
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Decrypt a block with OCB.
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@param ocb The OCB state
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@param ct The ciphertext (length of the block size of the block cipher)
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@param pt [out] The plaintext (length of ct)
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@return CRYPT_OK if successful
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*/
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int ocb_decrypt(ocb_state *ocb, const unsigned char *ct, unsigned char *pt)
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{
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unsigned char Z[MAXBLOCKSIZE], tmp[MAXBLOCKSIZE];
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int err, x;
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LTC_ARGCHK(ocb != NULL);
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LTC_ARGCHK(pt != NULL);
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LTC_ARGCHK(ct != NULL);
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/* can't use a encrypt-only descriptor */
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LTC_ARGCHK(cipher_descriptor[ocb->key.cipher].ecb_decrypt != NULL);
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/* Get Z[i] value */
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ocb_shift_xor(ocb, Z);
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/* xor ct in, encrypt, xor Z out */
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for (x = 0; x < ocb->block_len; x++) {
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tmp[x] = ct[x] ^ Z[x];
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}
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if ((err = ecb_decrypt_block(tmp, pt, &ocb->key)) != CRYPT_OK) {
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return err;
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}
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for (x = 0; x < ocb->block_len; x++) {
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pt[x] ^= Z[x];
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}
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/* compute checksum */
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for (x = 0; x < ocb->block_len; x++) {
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ocb->checksum[x] ^= pt[x];
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}
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#ifdef LTC_CLEAN_STACK
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zeromem(Z, sizeof(Z));
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zeromem(tmp, sizeof(tmp));
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#endif
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return CRYPT_OK;
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}
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#endif
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