der_length_bit_string.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 55 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_bit_string.c
  ASN.1 DER, get length of BIT STRING, Tom St Denis
*/

#ifdef LTC_DER
/**
  Gets length of DER encoding of BIT STRING 
  @param nbits  The number of bits in the string to encode
  @param outlen [out] The length of the DER encoding for the given string
  @return CRYPT_OK if successful
*/
int der_length_bit_string(unsigned long nbits, unsigned long *outlen)
{
   unsigned long nbytes;
   LTC_ARGCHK(outlen != NULL);

   /* get the number of the bytes */
   nbytes = (nbits >> 3) + ((nbits & 7) ? 1 : 0) + 1;
 
   if (nbytes < 128) {
      /* 03 LL PP DD DD DD ... */
      *outlen = 2 + nbytes;
   } else if (nbytes < 256) {
      /* 03 81 LL PP DD DD DD ... */
      *outlen = 3 + nbytes;
   } else if (nbytes < 65536) {
      /* 03 82 LL LL PP DD DD DD ... */
      *outlen = 4 + nbytes;
   } else {
      return CRYPT_INVALID_ARG;
   }

   return CRYPT_OK;
}

#endif


/* $Source: /cvs/libtom/libtomcrypt/src/pk/asn1/der/bit/der_length_bit_string.c,v $ */
/* $Revision: 1.1 $ */
/* $Date: 2005/05/16 15:08:11 $ */