First 8 byes of my encrypted data corrupting using 3DES and CBC

Question

I'm using PyCrypto in an application to encrypt data, but for some reason the first 8 bytes (corresponding to the first block) are coming through corrupt no matter what I do. ``` >>> from Crypto.Cipher import DES3 >>> from Crypto import Random >>> iv = Random.new().read(DES3.block_size) >>> key = Random.new().read(DES3.key_size[-1]) >>> des3 = DES3.new(key, DES3.MODE_CBC, iv) >>> des3.decrypt(des3.encrypt('12345678abcdefgh12345678')) 't\x1b\x0f\xcbD\x15M\xababcdefgh12345678' ``` I've read that that's a sign that the IV is corrupt, but those sources also say that using a mode other than CBC would result in the entire message corrupting. That isn't the case: ``` >>> des3 = DES3.new(key, DES3.MODE_CFB, iv) >>> des3.decrypt(des3.encrypt('12345678abcdefgh12345678')) '\xe1\x85\xae,\xf1m\x83\x9cabcdefgh12345678' ``` I can also rule out the cipher as the cause: ``` >>> from Crypto.Cipher import AES >>> from Crypto import Random >>> iv = Random.new().read(AES.block_size) >>> key = Random.new()…

Accepted Answer

You have to reset IV vector before decryption. Try this code:

```
>>> from Crypto.Cipher import DES3
>>> from Crypto import Random
>>> iv = Random.new().read(DES3.block_size)
>>> key = Random.new().read(DES3.key_size[-1])
>>> des3enc = DES3.new(key, DES3.MODE_CBC, iv)
>>> des3dec = DES3.new(key, DES3.MODE_CBC, iv)
>>> des3dec.decrypt(des3enc.encrypt('12345678abcdefgh12345678'))
```

IV vector is changing after encryption / decryption each block. You used the same instance of DES3 class for encrypting and decrypting the message, therefore you had incorrect IV for decryption.

Hope above code works - I didn't test it.

More about CBC mode: http://en.wikipedia.org/wiki/Block_cipher_mode_of_operation

First 8 byes of my encrypted data corrupting using 3DES and CBC

Solution

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