A known plaintext attack on the PKZIP stream cipher

Session 3: Stream Ciphers-Cryptanalysis
Part of the Lecture Notes in Computer Science book series (LNCS, volume 1008)


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.


Internal Representation Stream Cipher Internal Memory File Header Plaintext Attack 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


  1. 1.
    PKWARE, Inc., General Format of a ZIP File, technical note, included in PKZIP 1.10 distribution (pkz110.exe: file appnote.txt).Google Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 1995

Authors and Affiliations

  1. 1.Computer Science DepartmentTechnion - Israel Institute of TechnologyHaifaIsrael
  2. 2.Independent cryptographic consultantCorvallisUSA

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