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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.
*
* Tom St Denis, tomstdenis@gmail.com, http://libtom.org
*/
#include "tomcrypt.h"
/**
@file der_length_integer.c
ASN.1 DER, get length of encoding, Tom St Denis
*/
#ifdef LTC_DER
/**
Gets length of DER encoding of num
@param num The int to get the size of
@param outlen [out] The length of the DER encoding for the given integer
@return CRYPT_OK if successful
*/
int der_length_integer(void *num, unsigned long *outlen)
{
unsigned long z, len;
int leading_zero;
LTC_ARGCHK(num != NULL);
LTC_ARGCHK(outlen != NULL);
if (mp_cmp_d(num, 0) != LTC_MP_LT) {
/* positive */
/* we only need a leading zero if the msb of the first byte is one */
if ((mp_count_bits(num) & 7) == 0 || mp_iszero(num) == LTC_MP_YES) {
leading_zero = 1;
} else {
leading_zero = 0;
}
/* size for bignum */
z = len = leading_zero + mp_unsigned_bin_size(num);
} else {
/* it's negative */
/* find power of 2 that is a multiple of eight and greater than count bits */
leading_zero = 0;
z = mp_count_bits(num);
z = z + (8 - (z & 7));
if (((mp_cnt_lsb(num)+1)==mp_count_bits(num)) && ((mp_count_bits(num)&7)==0)) --z;
len = z = z >> 3;
}
/* now we need a length */
if (z < 128) {
/* short form */
++len;
} else {
/* long form (relies on z != 0), assumes length bytes < 128 */
++len;
while (z) {
++len;
z >>= 8;
}
}
/* we need a 0x02 to indicate it's INTEGER */
++len;
/* return length */
*outlen = len;
return CRYPT_OK;
}
#endif
/* $Source$ */
/* $Revision$ */
/* $Date$ */
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