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I am using zip on Arch and the version is given as "Zip 3.0 (July 5th 2008) by Info-Zip. Compiled with gcc 11.2.0 on Mar 22 2022. The encryption compilation information reads "[encryption, version 2.91 of 05 JAN 2007] (modified for Zip 3)"

What is the actual encryption algorithm used?

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Zip 3.0 uses the same encryption as the original PKZIP, since it was meant to be able to produce compatible zip files. Files contained in the zip file are encrypted using a proprietary cipher informally known as ZipCrypto. Briefly, it's a symmetric cipher that uses three 32-bit keys derived from the password provided to zip. Somewhat ironically, probably the best academic description of it can be found in the research paper "A Known-Plaintext Attack on the PKZIP Stream Cipher".

ZipCrypto has been considered insecure for quite some time, and tools are available to break it with little effort. Beta releases of Zip 3.1 supporting AES-256 have been floating around since at least 2015.

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