Mercurial > dropbear

annotate libtomcrypt/src/pk/asn1/der/integer/der_length_integer.c @ 1612:470aa609c7e4

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author Matt Johnston <matt@ucc.asn.au>
date Tue, 24 Jul 2018 20:19:05 +0800
parents 6dba84798cd5
children
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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 #include "tomcrypt.h"
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10
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11 /**
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12 @file der_length_integer.c
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13 ASN.1 DER, get length of encoding, Tom St Denis
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14 */
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17 #ifdef LTC_DER
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18 /**
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19 Gets length of DER encoding of num
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20 @param num The int to get the size of
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21 @param outlen [out] The length of the DER encoding for the given integer
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22 @return CRYPT_OK if successful
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23 */
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24 int der_length_integer(void *num, unsigned long *outlen)
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25 {
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26 unsigned long z, len;
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27 int leading_zero;
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28
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29 LTC_ARGCHK(num != NULL);
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30 LTC_ARGCHK(outlen != NULL);
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31
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32 if (mp_cmp_d(num, 0) != LTC_MP_LT) {
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33 /* positive */
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34
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35 /* we only need a leading zero if the msb of the first byte is one */
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36 if ((mp_count_bits(num) & 7) == 0 || mp_iszero(num) == LTC_MP_YES) {
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37 leading_zero = 1;
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38 } else {
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39 leading_zero = 0;
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40 }
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42 /* size for bignum */
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43 z = len = leading_zero + mp_unsigned_bin_size(num);
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44 } else {
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45 /* it's negative */
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46 /* find power of 2 that is a multiple of eight and greater than count bits */
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47 z = mp_count_bits(num);
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48 z = z + (8 - (z & 7));
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49 if (((mp_cnt_lsb(num)+1)==mp_count_bits(num)) && ((mp_count_bits(num)&7)==0)) --z;
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50 len = z = z >> 3;
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51 }
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52
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53 /* now we need a length */
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54 if (z < 128) {
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55 /* short form */
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56 ++len;
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57 } else {
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58 /* long form (relies on z != 0), assumes length bytes < 128 */
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59 ++len;
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60
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61 while (z) {
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62 ++len;
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63 z >>= 8;
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64 }
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65 }
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66
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67 /* we need a 0x02 to indicate it's INTEGER */
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68 ++len;
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69
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70 /* return length */
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71 *outlen = len;
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72 return CRYPT_OK;
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73 }
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75 #endif
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76
1471
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77 /* ref: $Format:%D$ */
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78 /* git commit: $Format:%H$ */
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79 /* commit time: $Format:%ai$ */