73 lines
2.0 KiB
C
73 lines
2.0 KiB
C
/* LibTomCrypt, modular cryptographic library -- Tom St Denis
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*
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* LibTomCrypt is a library that provides various cryptographic
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* algorithms in a highly modular and flexible manner.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*/
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/**
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@file ocb3_int_calc_offset_zero.c
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OCB implementation, INTERNAL ONLY helper, by Karel Miko
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*/
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#include "tomcrypt.h"
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#ifdef LTC_OCB3_MODE
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/**
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Sets 'ocb->Offset_current' to 'Offset_0' value (internal function)
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@param ocb The OCB state
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@param nonce The session nonce
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@param noncelen The length of the session nonce (octets)
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*/
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void ocb3_int_calc_offset_zero(ocb3_state *ocb, const unsigned char *nonce, unsigned long noncelen)
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{
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int x, y, bottom;
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int idx, shift;
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unsigned char iNonce[MAXBLOCKSIZE];
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unsigned char iKtop[MAXBLOCKSIZE];
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unsigned char iStretch[MAXBLOCKSIZE+8];
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/* Nonce = zeros(127-bitlen(N)) || 1 || N */
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zeromem(iNonce, sizeof(iNonce));
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for (x = ocb->block_len-1, y=0; y<(int)noncelen; x--, y++) {
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iNonce[x] = nonce[noncelen-y-1];
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}
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iNonce[x] = 0x01;
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/* bottom = str2num(Nonce[123..128]) */
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bottom = iNonce[ocb->block_len-1] & 0x3F;
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/* Ktop = ENCIPHER(K, Nonce[1..122] || zeros(6)) */
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iNonce[ocb->block_len-1] = iNonce[ocb->block_len-1] & 0xC0;
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if ((cipher_descriptor[ocb->cipher].ecb_encrypt(iNonce, iKtop, &ocb->key)) != CRYPT_OK) {
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zeromem(ocb->Offset_current, ocb->block_len);
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return;
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}
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/* Stretch = Ktop || (Ktop[1..64] xor Ktop[9..72]) */
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for (x = 0; x < ocb->block_len; x++) {
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iStretch[x] = iKtop[x];
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}
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for (y = 0; y < 8; y++) {
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iStretch[x+y] = iKtop[y] ^ iKtop[y+1];
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}
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/* Offset_0 = Stretch[1+bottom..128+bottom] */
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idx = bottom / 8;
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shift = (bottom % 8);
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for (x = 0; x < ocb->block_len; x++) {
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ocb->Offset_current[x] = iStretch[idx+x] << shift;
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if (shift > 0) {
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ocb->Offset_current[x] |= iStretch[idx+x+1] >> (8-shift);
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}
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}
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}
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#endif
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/* $Source$ */
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/* $Revision$ */
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/* $Date$ */
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