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      1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis
      2  *
      3  * LibTomCrypt is a library that provides various cryptographic
      4  * algorithms in a highly modular and flexible manner.
      5  *
      6  * The library is free for all purposes without any express
      7  * guarantee it works.
      8  *
      9  * Tom St Denis, tomstdenis (at) gmail.com, http://libtomcrypt.com
     10  */
     11 #include "tomcrypt.h"
     12 
     13 /**
     14   @file der_encode_short_integer.c
     15   ASN.1 DER, encode an integer, Tom St Denis
     16 */
     17 
     18 
     19 #ifdef LTC_DER
     20 
     21 /**
     22   Store a short integer in the range (0,2^32-1)
     23   @param num      The integer to encode
     24   @param out      [out] The destination for the DER encoded integers
     25   @param outlen   [in/out] The max size and resulting size of the DER encoded integers
     26   @return CRYPT_OK if successful
     27 */
     28 int der_encode_short_integer(unsigned long num, unsigned char *out, unsigned long *outlen)
     29 {
     30    unsigned long len, x, y, z;
     31    int           err;
     32 
     33    LTC_ARGCHK(out    != NULL);
     34    LTC_ARGCHK(outlen != NULL);
     35 
     36    /* force to 32 bits */
     37    num &= 0xFFFFFFFFUL;
     38 
     39    /* find out how big this will be */
     40    if ((err = der_length_short_integer(num, &len)) != CRYPT_OK) {
     41       return err;
     42    }
     43 
     44    if (*outlen < len) {
     45       *outlen = len;
     46       return CRYPT_BUFFER_OVERFLOW;
     47    }
     48 
     49    /* get len of output */
     50    z = 0;
     51    y = num;
     52    while (y) {
     53      ++z;
     54      y >>= 8;
     55    }
     56 
     57    /* handle zero */
     58    if (z == 0) {
     59       z = 1;
     60    }
     61 
     62    /* see if msb is set */
     63    z += (num&(1UL<<((z<<3) - 1))) ? 1 : 0;
     64 
     65    /* adjust the number so the msB is non-zero */
     66    for (x = 0; (z <= 4) && (x < (4 - z)); x++) {
     67       num <<= 8;
     68    }
     69 
     70    /* store header */
     71    x = 0;
     72    out[x++] = 0x02;
     73    out[x++] = (unsigned char)z;
     74 
     75    /* if 31st bit is set output a leading zero and decrement count */
     76    if (z == 5) {
     77       out[x++] = 0;
     78       --z;
     79    }
     80 
     81    /* store values */
     82    for (y = 0; y < z; y++) {
     83       out[x++] = (unsigned char)((num >> 24) & 0xFF);
     84       num    <<= 8;
     85    }
     86 
     87    /* we good */
     88    *outlen = x;
     89 
     90    return CRYPT_OK;
     91 }
     92 
     93 #endif
     94 
     95 /* $Source: /cvs/libtom/libtomcrypt/src/pk/asn1/der/short_integer/der_encode_short_integer.c,v $ */
     96 /* $Revision: 1.7 $ */
     97 /* $Date: 2006/12/04 21:34:03 $ */
     98