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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 "mycrypt.h" |
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12 |
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13 #ifdef CBC |
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14 |
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15 int cbc_decrypt(const unsigned char *ct, unsigned char *pt, symmetric_CBC *cbc) |
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16 { |
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17 int x, err; |
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18 unsigned char tmp[MAXBLOCKSIZE], tmp2[MAXBLOCKSIZE]; |
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19 |
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20 _ARGCHK(pt != NULL); |
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21 _ARGCHK(ct != NULL); |
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22 _ARGCHK(cbc != NULL); |
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23 |
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24 /* decrypt the block from ct into tmp */ |
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25 if ((err = cipher_is_valid(cbc->cipher)) != CRYPT_OK) { |
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26 return err; |
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27 } |
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28 cipher_descriptor[cbc->cipher].ecb_decrypt(ct, tmp, &cbc->key); |
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29 |
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30 /* is blocklen valid? */ |
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31 if (cbc->blocklen < 0 || cbc->blocklen > (int)sizeof(cbc->IV)) { |
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32 return CRYPT_INVALID_ARG; |
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33 } |
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34 |
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35 /* xor IV against the plaintext of the previous step */ |
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36 for (x = 0; x < cbc->blocklen; x++) { |
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37 /* copy CT in case ct == pt */ |
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38 tmp2[x] = ct[x]; |
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39 |
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40 /* actually decrypt the byte */ |
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41 pt[x] = tmp[x] ^ cbc->IV[x]; |
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42 } |
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43 |
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44 /* replace IV with this current ciphertext */ |
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45 for (x = 0; x < cbc->blocklen; x++) { |
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46 cbc->IV[x] = tmp2[x]; |
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47 } |
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48 #ifdef CLEAN_STACK |
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49 zeromem(tmp, sizeof(tmp)); |
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50 zeromem(tmp2, sizeof(tmp2)); |
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51 #endif |
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52 return CRYPT_OK; |
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53 } |
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54 |
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55 #endif |
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56 |
