der_length_short_integer.c

来自「该压缩包中包括 tom的加密函数库及pdf说明 ,以及Rinick s ECC：」· C语言 代码 · 共 70 行

/* 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, [e-mail], http://libtomcrypt.org
 */
#include "tomcrypt.h"

/**
  @file der_length_short_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 integer 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_short_integer(unsigned long num, unsigned long *outlen)
{
   unsigned long z, y, len;

   LTC_ARGCHK(outlen  != NULL);

   /* force to 32 bits */
   num &= 0xFFFFFFFFUL;

   /* get the number of bytes */
   z = 0;
   y = num;
   while (y) {
     ++z;
     y >>= 8;
   }

   /* handle zero */
   if (z == 0) {
      z = 1;
   }

   /* we need a 0x02 to indicate it's INTEGER */
   len = 1;

   /* length byte */
   ++len;

   /* bytes in value */
   len += z;

   /* see if msb is set */
   len += (num&(1UL<<((z<<3) - 1))) ? 1 : 0;

   /* return length */
   *outlen = len; 
   return CRYPT_OK;
}

#endif

/* $Source: /cvs/libtom/libtomcrypt/src/pk/asn1/der/short_integer/der_length_short_integer.c,v $ */
/* $Revision: 1.3 $ */
/* $Date: 2005/05/23 01:35:38 $ */