This question would be more appropriate on sci.crypt.

See http://www.hedgie.com/passwords/index.html
Here are a few programs to break pkzip encryption:
ftp://idea.sec.dsi.unimi.it/pub/security/crypt/code/zipcrack.c.gz
ftp://idea.sec.dsi.unimi.it/pub/security/crypt/rpub.cl.msu.edu/crypt/msdos/zipcrack*
ftp://idea.sec.dsi.unimi.it/pub/security/crypt/rpub.cl.msu.edu/crypt/other/zipcrack.c
ftp://ftp.ox.ac.uk/pub/crypto/cryptanalysis/fzc104.zip
ftp://ftp.ox.ac.uk/pub/crypto/cryptanalysis/pkcrack.zip
ftp://ftp.ox.ac.uk/pub/crypto/cryptanalysis/zipcrk20.zip

These are brute force crackers. A known plaintext attack is also possible,
see http://www.unix-ag.uni-kl.de/~conrad/krypto/pkcrack.html
or ftp://ftp.sunet.se/pub/security/docs/crypt/ripem.msu.edu/kocher-pkzip-attack.ps.gz

          End of part 1 of the comp.compression faq.

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