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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 |
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15 /* decodes a DER INTEGER in [in]. You have to tell this function |
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16 * how many bytes are available [inlen]. It will then attempt to |
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17 * read the INTEGER. If all goes well it stores the number of bytes |
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18 * read in [inlen] and the number in [num]. |
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19 */ |
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20 int der_decode_integer(const unsigned char *in, unsigned long *inlen, mp_int *num) |
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21 { |
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22 unsigned long tmplen, y, z; |
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23 |
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24 _ARGCHK(num != NULL); |
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25 _ARGCHK(in != NULL); |
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26 _ARGCHK(inlen != NULL); |
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27 |
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28 /* save copy of max output size */ |
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29 tmplen = *inlen; |
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30 *inlen = 0; |
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31 |
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32 /* min DER INTEGER is 0x02 01 00 == 0 */ |
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33 if (tmplen < (1 + 1 + 1)) { |
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34 return CRYPT_INVALID_PACKET; |
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35 } |
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36 |
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37 /* ok expect 0x02 when we AND with 0011 1111 [3F] */ |
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38 if ((*in++ & 0x3F) != 0x02) { |
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39 return CRYPT_INVALID_PACKET; |
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40 } |
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41 ++(*inlen); |
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42 |
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43 /* now decode the len stuff */ |
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44 z = *in++; |
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45 ++(*inlen); |
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46 |
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47 if ((z & 0x80) == 0x00) { |
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48 /* short form */ |
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49 |
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50 /* will it overflow? */ |
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51 if (*inlen + z > tmplen) { |
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52 return CRYPT_INVALID_PACKET; |
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53 } |
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54 |
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55 /* no so read it */ |
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56 (*inlen) += z; |
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57 return mpi_to_ltc_error(mp_read_unsigned_bin(num, (unsigned char *)in, z)); |
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58 } else { |
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59 /* long form */ |
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60 z &= 0x7F; |
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61 |
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62 /* will number of length bytes overflow? (or > 4) */ |
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63 if (((*inlen + z) > tmplen) || (z > 4)) { |
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64 return CRYPT_INVALID_PACKET; |
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65 } |
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66 |
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67 /* now read it in */ |
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68 y = 0; |
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69 while (z--) { |
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70 y = ((unsigned long)(*in++)) | (y << 8); |
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71 ++(*inlen); |
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72 } |
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73 |
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74 /* now will reading y bytes overrun? */ |
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75 if ((*inlen + y) > tmplen) { |
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76 return CRYPT_INVALID_PACKET; |
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77 } |
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78 |
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79 /* no so read it */ |
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80 (*inlen) += y; |
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81 return mpi_to_ltc_error(mp_read_unsigned_bin(num, (unsigned char *)in, y)); |
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82 } |
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83 } |
