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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@iahu.ca, http://libtomcrypt.org
*/
#include "mycrypt.h"
/* Gets length of DER encoding of num */
int der_length_integer(mp_int *num, unsigned long *outlen)
{
unsigned long z, len;
int leading_zero;
_ARGCHK(num != NULL);
_ARGCHK(outlen != NULL);
/* we only need a leading zero if the msb of the first byte is one */
if ((mp_count_bits(num) & 7) == 7 || mp_iszero(num) == MP_YES) {
leading_zero = 1;
} else {
leading_zero = 0;
}
/* size for bignum */
z = len = leading_zero + mp_unsigned_bin_size(num);
/* we need a 0x02 */
++len;
/* now we need a length */
if (z < 128) {
/* short form */
++len;
} else {
/* long form (relies on z != 0) */
++len;
while (z) {
++len;
z >>= 8;
}
}
*outlen = len;
return CRYPT_OK;
}
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