Mercurial > dropbear

annotate libtomcrypt/src/pk/asn1/der/integer/der_length_integer.c @ 322:84aa4e60bd3c

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author Matt Johnston <matt@ucc.asn.au>
date Wed, 07 Jun 2006 13:41:19 +0000
parents 1b9e69c058d2
children 0cbe8f6dbf9e
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rev   line source
285
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1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis
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2 *
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3 * LibTomCrypt is a library that provides various cryptographic
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4 * algorithms in a highly modular and flexible manner.
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5 *
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6 * The library is free for all purposes without any express
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7 * guarantee it works.
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8 *
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9 * Tom St Denis, [email protected], http://libtomcrypt.org
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10 */
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11 #include "tomcrypt.h"
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12
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13 /**
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14 @file der_length_integer.c
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15 ASN.1 DER, get length of encoding, Tom St Denis
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16 */
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17
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18
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19 #ifdef LTC_DER
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20 /**
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21 Gets length of DER encoding of num
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22 @param num The mp_int to get the size of
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23 @param outlen [out] The length of the DER encoding for the given integer
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24 @return CRYPT_OK if successful
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25 */
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26 int der_length_integer(mp_int *num, unsigned long *outlen)
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27 {
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28 unsigned long z, len;
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29 int leading_zero;
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30
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31 LTC_ARGCHK(num != NULL);
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32 LTC_ARGCHK(outlen != NULL);
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33
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34 if (mp_cmp_d(num, 0) != MP_LT) {
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35 /* positive */
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36
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37 /* we only need a leading zero if the msb of the first byte is one */
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38 if ((mp_count_bits(num) & 7) == 0 || mp_iszero(num) == MP_YES) {
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39 leading_zero = 1;
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40 } else {
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41 leading_zero = 0;
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42 }
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43
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44 /* size for bignum */
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45 z = len = leading_zero + mp_unsigned_bin_size(num);
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46 } else {
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47 /* it's negative */
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48 /* find power of 2 that is a multiple of eight and greater than count bits */
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49 leading_zero = 0;
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50 z = mp_count_bits(num);
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51 z = z + (8 - (z & 7));
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52 len = z = z >> 3;
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53 }
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54
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55 /* now we need a length */
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56 if (z < 128) {
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57 /* short form */
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58 ++len;
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59 } else {
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60 /* long form (relies on z != 0), assumes length bytes < 128 */
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61 ++len;
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62
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63 while (z) {
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64 ++len;
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65 z >>= 8;
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66 }
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67 }
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68
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69 /* we need a 0x02 to indicate it's INTEGER */
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70 ++len;
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71
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72 /* return length */
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73 *outlen = len;
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74 return CRYPT_OK;
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75 }
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76
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77 #endif
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78
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79 /* $Source: /cvs/libtom/libtomcrypt/src/pk/asn1/der/integer/der_length_integer.c,v $ */
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80 /* $Revision: 1.1 $ */
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81 /* $Date: 2005/05/16 15:08:11 $ */