der_encode_bit_string.c

这是由Rinick编写的加解密函数库。最近找了不少关于加解密的C源码

/* 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_encode_bit_string.c
  ASN.1 DER, encode a BIT STRING, Tom St Denis
*/


#ifdef LTC_DER

/**
  Store a BIT STRING
  @param in       The array of bits to store (one per char)
  @param inlen    The number of bits tostore
  @param out      [out] The destination for the DER encoded BIT STRING
  @param outlen   [in/out] The max size and resulting size of the DER BIT STRING
  @return CRYPT_OK if successful
*/
int der_encode_bit_string(const unsigned char *in, unsigned long inlen,
                                unsigned char *out, unsigned long *outlen)
{
   unsigned long len, x, y, buf;
   int           err;

   LTC_ARGCHK(in     != NULL);
   LTC_ARGCHK(out    != NULL);
   LTC_ARGCHK(outlen != NULL);

   /* avoid overflows */
   if ((err = der_length_bit_string(inlen, &len)) != CRYPT_OK) {
      return err;
   }

   if (len > *outlen) {
      return CRYPT_BUFFER_OVERFLOW;
   }

   /* store header (include bit padding count in length) */
   x = 0;
   y = (inlen >> 3) + ((inlen&7) ? 1 : 0) + 1;

   out[x++] = 0x03;
   if (y < 128) {
      out[x++] = y;
   } else if (y < 256) {
      out[x++] = 0x81;
      out[x++] = y;
   } else if (y < 65536) {
      out[x++] = 0x82;
      out[x++] = (y>>8)&255;
      out[x++] = y&255;
   }

   /* store number of zero padding bits */
   out[x++] = (8 - inlen) & 7;

   /* store the bits in big endian format */
   for (y = buf = 0; y < inlen; y++) {
       buf |= (in[y] ? 1 : 0) << (7 - (y & 7));
       if ((y & 7) == 7) {
          out[x++] = buf;
          buf      = 0;
       }
   }
   /* store last byte */
   if (inlen & 7) {
      out[x++] = buf;
   }
   *outlen = x;
   return CRYPT_OK;
}

#endif

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