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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 |
