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80 lines
1.9 KiB
C
80 lines
1.9 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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#include "tomcrypt.h"
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/**
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@file der_length_integer.c
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ASN.1 DER, get length of encoding, Tom St Denis
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*/
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#ifdef LTC_DER
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/**
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Gets length of DER encoding of num
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@param num The int to get the size of
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@param outlen [out] The length of the DER encoding for the given integer
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@return CRYPT_OK if successful
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*/
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int der_length_integer(void *num, unsigned long *outlen)
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{
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unsigned long z, len;
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int leading_zero;
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LTC_ARGCHK(num != NULL);
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LTC_ARGCHK(outlen != NULL);
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if (mp_cmp_d(num, 0) != LTC_MP_LT) {
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/* positive */
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/* we only need a leading zero if the msb of the first byte is one */
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if ((mp_count_bits(num) & 7) == 0 || mp_iszero(num) == LTC_MP_YES) {
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leading_zero = 1;
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} else {
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leading_zero = 0;
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}
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/* size for bignum */
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z = len = leading_zero + mp_unsigned_bin_size(num);
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} else {
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/* it's negative */
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/* find power of 2 that is a multiple of eight and greater than count bits */
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z = mp_count_bits(num);
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z = z + (8 - (z & 7));
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if (((mp_cnt_lsb(num)+1)==mp_count_bits(num)) && ((mp_count_bits(num)&7)==0)) --z;
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len = z = z >> 3;
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}
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/* now we need a length */
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if (z < 128) {
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/* short form */
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++len;
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} else {
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/* long form (relies on z != 0), assumes length bytes < 128 */
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++len;
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while (z) {
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++len;
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z >>= 8;
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}
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}
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/* we need a 0x02 to indicate it's INTEGER */
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++len;
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/* return length */
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*outlen = len;
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return CRYPT_OK;
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}
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#endif
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/* ref: HEAD -> master, tag: v1.18.2 */
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/* git commit: 7e7eb695d581782f04b24dc444cbfde86af59853 */
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/* commit time: 2018-07-01 22:49:01 +0200 */
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