Question

8. Suppose you saw a ciphertext stream that you know was encrypted with DES-CBC, and you observed that C3 = C6 (i.e., the third and sixth output ciphertext blocks were the same). What information does this tell you about the plaintext input?

Answer 1

Explanation:

The DES-CBC is a symmetric secret key algorithm where the key size is 64 bits. However it is commonly known as 56 bit key as the key has 56 significant bits as the least significanr bit in each byte is the parity bit.

In cryptography, the block cipher mode of operation is an algorithm that uses a block cipher to provide information security in terms of confidentiality or authenticity. A block cipher by itself is only suitable for the secure cryptographic transformation in the form of encryption or decryption of 1 fixed length group of bits called the block. The mode of operation describes how to repeatedly apply the single block operation of the cipher to securely transform amounts of data larger than a block.

The modes of operation of block ciphers are configuration methods that allow those ciphers to work with large data streams without the risk of compromising the security provided.

Although not recommended, yet while working with block ciphers it is possible to use the same secret key bits for encrypting the same plaintext parts. The use of one deterministic algorithm for a number of identical input data results insome number of identical ciphertext blocks. This situation might be very dangerous for the users of the cipher because an intruder would be able to get much information by knowing the distribution of identical message parts of the input data, even though the intruder would not be able to break the cipher and discover the original input messages.

Answer:

In the light of the above discussions, in the scenario under consideration if it is observed that C3 = C6 i.e., the third and sixth output ciphertext blocks are the same, it can be easily understood that the 3rd and the 6th input blocks of the original message are the same although the actual message or the content of those two input blocks might not be decrypted or deciphered.