core/lib/libtomcrypt/src/pk/asn1/der/integer/der_length_integer.c

// SPDX-License-Identifier: BSD-2-Clause
/* 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.
 */
#include "tomcrypt_private.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, err;

   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 */
      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 */
      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 >> 3;
   }

   if ((err = der_length_asn1_length(len, &z)) != CRYPT_OK) {
      return err;
   }
   *outlen = 1 + z + len;

   return CRYPT_OK;
}

#endif

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