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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 $ */ |
