annotate der_length_integer.c @ 154:b4c6e7f276e9 libtomcrypt

Makefile.in: don't compile things we don't need
author Matt Johnston <matt@ucc.asn.au>
date Mon, 20 Dec 2004 14:46:40 +0000
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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
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12 #include "mycrypt.h"
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13
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14 /* Gets length of DER encoding of num */
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15
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16 int der_length_integer(mp_int *num, unsigned long *outlen)
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17 {
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18 unsigned long z, len;
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19 int leading_zero;
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20
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21 _ARGCHK(num != NULL);
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22 _ARGCHK(outlen != NULL);
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23
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24 /* we only need a leading zero if the msb of the first byte is one */
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25 if ((mp_count_bits(num) & 7) == 7 || mp_iszero(num) == MP_YES) {
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26 leading_zero = 1;
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27 } else {
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28 leading_zero = 0;
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29 }
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30
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31 /* size for bignum */
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32 z = len = leading_zero + mp_unsigned_bin_size(num);
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33
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34 /* we need a 0x02 */
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35 ++len;
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36
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37 /* now we need a length */
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38 if (z < 128) {
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39 /* short form */
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40 ++len;
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41 } else {
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42 /* long form (relies on z != 0) */
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43 ++len;
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44
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45 while (z) {
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46 ++len;
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47 z >>= 8;
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48 }
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49 }
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50
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51 *outlen = len;
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52 return CRYPT_OK;
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53 }
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54