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@gmail.com, http://libtomcrypt.com
10 */
11#include "tomcrypt.h"
12
13/**
14  @file der_length_integer.c
15  ASN.1 DER, get length of encoding, Tom St Denis
16*/
17
18
19#ifdef LTC_DER
20/**
21  Gets length of DER encoding of num
22  @param num    The int to get the size of
23  @param outlen [out] The length of the DER encoding for the given integer
24  @return CRYPT_OK if successful
25*/
26int der_length_integer(void *num, unsigned long *outlen)
27{
28   unsigned long z, len;
29   int           leading_zero;
30
31   LTC_ARGCHK(num     != NULL);
32   LTC_ARGCHK(outlen  != NULL);
33
34   if (mp_cmp_d(num, 0) != LTC_MP_LT) {
35      /* positive */
36
37      /* we only need a leading zero if the msb of the first byte is one */
38      if ((mp_count_bits(num) & 7) == 0 || mp_iszero(num) == LTC_MP_YES) {
39         leading_zero = 1;
40      } else {
41         leading_zero = 0;
42      }
43
44      /* size for bignum */
45      z = len = leading_zero + mp_unsigned_bin_size(num);
46   } else {
47      /* it's negative */
48      /* find power of 2 that is a multiple of eight and greater than count bits */
49      leading_zero = 0;
50      z = mp_count_bits(num);
51      z = z + (8 - (z & 7));
52      if (((mp_cnt_lsb(num)+1)==mp_count_bits(num)) && ((mp_count_bits(num)&7)==0)) --z;
53      len = z = z >> 3;
54   }
55
56   /* now we need a length */
57   if (z < 128) {
58      /* short form */
59      ++len;
60   } else {
61      /* long form (relies on z != 0), assumes length bytes < 128 */
62      ++len;
63
64      while (z) {
65         ++len;
66         z >>= 8;
67      }
68   }
69
70   /* we need a 0x02 to indicate it's INTEGER */
71   ++len;
72
73   /* return length */
74   *outlen = len;
75   return CRYPT_OK;
76}
77
78#endif
79
80/* $Source: /cvs/libtom/libtomcrypt/src/pk/asn1/der/integer/der_length_integer.c,v $ */
81/* $Revision: 1.4 $ */
82/* $Date: 2006/04/22 01:22:55 $ */
83