Plaintext-based Attacks

Plaintext-based Attacks

Known Plaintext

As the name of this attack implies, the attacker has access to known samples of plaintext.

In fact, the attacker has access to both the ciphertext and the plaintext versions of the same message. Since the method or algorithm is always known, the goal of this type of attack is to find the relationship between the two that, of course, will be the cryptographic key that was used to encrypt the message. Once the key has been found, the attacker would then be able to decrypt all other messages that had been encrypted using that key.

Chosen Plaintext

In this type of cryptanalysis, the cryptanalyst is able to choose a quantity of plaintext and then obtain the corresponding encrypted text to try and recover the key.

To execute the chosen attacks, the attacker knows the algorithm used for the encryption, or even better, may have access to the cryptosystem used to do the encryption and is trying to determine the key. At this point, the attacker can run chosen pieces of plaintext through the algorithm and see what the result is. This may assist in a known plaintext attack. An adaptive chosen plaintext attack is where the attacker can modify the chosen input files repeatedly to see what effect that would have on the resulting ciphertext.

Linear cryptanalysis is a known plaintext attack and uses a linear approximation to try and describe the behavior of the block cipher.

Given sufficient pairs of plaintext and corresponding ciphertext, bits of information about the key can be obtained, and increased amounts of data will usually give a higher probability of success in possibly figuring out the correct key.

Differential Cryptanalysis

Differential cryptanalysis begins with a chosen plaintext approach.

The attacker makes minor changes in the chosen plaintext to see if there are corresponding minor changes in the resulting ciphertext. The idea is to obtain bits of clues regarding the key itself.

Combining the two attacks above, linear and differential cryptanalysis, have provided a variety of enhancements and improvements to each basic attack to improve the success of each.

Replay Attack

This attack is meant to disrupt and damage processing by the attacker through the resending of repeated files or input to the host.

If there are no checks such as timestamping, use of one-time tokens, or sequence verification codes in the receiving software or architecture, the system might process duplicate files or input, allowing access.