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