An Improved Known Plaintext Attack on PKZIP Encryption Algorithm

Abstract

The PKZIP encryption algorithm has been widely used to protect the contents of compressed archives despite the known security weakness. Biham and Kocher proposed a known plaintext attack with the complexity 2⁴⁰ when 12 plaintext bytes are given. Stay suggested a different way to attack and addressed an idea which makes the complexity be reduced if the information of additional files encrypted under the same password is provided. However, the complexity of Stay’s attack is quite large when only one file is used.

In this paper, we propose a new attack based on Biham and Kocher’s attack. We introduce a method to reduce the complexity using the information of multi-files, so our attack can have the both advantages of previous two attacks. As a result, our attack becomes about (3.4)^l times faster than the attack of Biham and Kocher when l additional files are used. Our experiment shows that ours is at least 10 times faster than Stay’s. In addition, our attack can be improved in the chosen ciphertext model. It is about (21.3)^l times faster than Biham and Kocher’s attack with chosen plaintext of l additional files.