Session 3: Stream Ciphers-Cryptanalysis

Fast Software Encryption

Volume 1008 of the series Lecture Notes in Computer Science pp 144-153


A known plaintext attack on the PKZIP stream cipher

  • Eli BihamAffiliated withComputer Science Department, Technion - Israel Institute of Technology
  • , Paul C. KocherAffiliated withIndependent cryptographic consultant


The PKZIP program is one of the more widely used archive/ compression programs on personal computers. It also has many compatible variants on other computers, and is used by most BBS's and ftp sites to compress their archives. PKZIP provides a stream cipher which allows users to scramble files with variable length keys (passwords).

In this paper we describe a known plaintext attack on this cipher, which can find the internal representation of the key within a few hours on a personal computer using a few hundred bytes of known plaintext. In many cases, the actual user keys can also be found from the internal representation. We conclude that the PKZIP cipher is weak, and should not be used to protect valuable data.