In this lab we'll try to implement the CBC-padding attack of Serge Vaudenay: http://link.springer.com/content/pdf/10.1007%2F3-540-46035-7_35.pdf

See sections 3.1 to 3.3 of chapter 3 in the paper above.

Given a message of length L bytes, we need to pad with b more bytes such that L+b is a multiple of the cipher's block size (e.g. 16 bytes in the case of AES). Each of the last b bytes will have the value b. For example, for a message that has 30 bytes, we shall add two bytes with the value 0x02 at the end, obtaining m = [b1 | b2 | .. | b30 | 0x02 | 0x02], where bi are the 30 original bytes.

The idea in short is the following: given a 1-block ciphertext c = [c1 | c2 | … | c16], you can find its last byte by crafting a 2-block ciphetext c' = [r | c], where r = [r1 | r2 | … | r16] contains random bytes. Then, by using an oracle that tells you whether the padding of the decryption of c' is correct or not, you can determine the value of c16. For this you just try all possible values of r16, until for one of them you will find that the oracle returns a successful result (it will do that for one value). In that case, due to the workings of CBC, you found out that r16 ⊕ LSB(Dec(k, c)) = 1. Hence you find that the last byte of the decryption of c is r16 ⊕ 1.

Once you found the last byte c16 of the decryption of c, you can work your way towards the following byte, c15. This time you need to target the two-byte padding [0x02 | 0x02]. For this you simply use for r16 the value LSB(Dec(k, c)) ⊕ 0x02, and try all values for r15 just as we did above for r16. In the same manner you can decrypt all the ciphertext blocks c1 … c16.

You can then repeat the above process for any ciphertext of arbitrary length. Simply apply the attack on all blocks.

You are given the following ciphertext: 553b43d4b821332868fece8149eea14a2b0a98c7bed43cc1cf75f4e778cb315dc1d92 8d0340e0aab4900ca8af9adaee761e2affa3e9996d81483e950b913492b

and a padding oracle (see below in the Python script), which knows the key that was used to encrypt the ciphertext and which will answer 1 if the padding of the ciphertext is correct and 0 otherwise.

Your task is to decrypt the message by using the CBC-padding attack.

<note tip> You are not allowed to use the key from the file, but you can (and you may have to) use the IV in order to decrypt the entire message.