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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 /* Exports a positive bignum as DER format (upto 2^32 bytes in size) */ |
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15 int der_encode_integer(mp_int *num, unsigned char *out, unsigned long *outlen) |
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16 { |
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17 unsigned long tmplen, x, y, z; |
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18 int err, leading_zero; |
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19 |
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20 _ARGCHK(num != NULL); |
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21 _ARGCHK(out != NULL); |
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22 _ARGCHK(outlen != NULL); |
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23 |
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24 /* find out how big this will be */ |
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25 if ((err = der_length_integer(num, &tmplen)) != CRYPT_OK) { |
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26 return err; |
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27 } |
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28 |
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29 if (*outlen < tmplen) { |
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30 return CRYPT_BUFFER_OVERFLOW; |
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31 } |
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32 |
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33 /* we only need a leading zero if the msb of the first byte is one */ |
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34 if ((mp_count_bits(num) & 7) == 7 || mp_iszero(num) == MP_YES) { |
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35 leading_zero = 1; |
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36 } else { |
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37 leading_zero = 0; |
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38 } |
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39 |
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40 /* get length of num in bytes (plus 1 since we force the msbyte to zero) */ |
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41 y = mp_unsigned_bin_size(num) + leading_zero; |
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42 |
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43 /* now store initial data */ |
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44 *out++ = 0x02; |
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45 if (y < 128) { |
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46 /* short form */ |
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47 *out++ = (unsigned char)y; |
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48 } else { |
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49 /* long form (relies on y != 0) */ |
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50 |
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51 /* get length of length... ;-) */ |
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52 x = y; |
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53 z = 0; |
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54 while (x) { |
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55 ++z; |
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56 x >>= 8; |
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57 } |
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58 |
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59 /* store length of length */ |
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60 *out++ = 0x80 | ((unsigned char)z); |
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61 |
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62 /* now store length */ |
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63 |
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64 /* first shift length up so msbyte != 0 */ |
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65 x = y; |
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66 while ((x & 0xFF000000) == 0) { |
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67 x <<= 8; |
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68 } |
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69 |
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70 /* now store length */ |
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71 while (z--) { |
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72 *out++ = (unsigned char)((x >> 24) & 0xFF); |
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73 x <<= 8; |
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74 } |
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75 } |
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76 |
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77 /* now store msbyte of zero if num is non-zero */ |
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78 if (leading_zero) { |
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79 *out++ = 0x00; |
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80 } |
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81 |
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82 /* if it's not zero store it as big endian */ |
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83 if (mp_iszero(num) == MP_NO) { |
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84 /* now store the mpint */ |
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85 if ((err = mp_to_unsigned_bin(num, out)) != MP_OKAY) { |
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86 return mpi_to_ltc_error(err); |
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87 } |
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88 } |
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89 |
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90 /* we good */ |
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91 *outlen = tmplen; |
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92 return CRYPT_OK; |
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93 } |
