Compile: gcc -o PKZIP PKZIP.c

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Usage: PKZIP inputfile outputfile direction

where inputfile == file to read input
      outputfile == file write output
      direction == 0 to encrypt, 1 to decrypt

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The file ciphertext.txt was generated from

    PKZIP plaintext.txt ciphertext.txt 0

The 96-bit key used was
0x01234567,0xdecea5ed,0xfedcba98
which is hardcoded into the program. This is the
so-called internal representation of the key. In PKZIP,
this internal representation is derived from a password.

Note: Since the PKZIP stream cipher differs slightly 
in encryption and decryption modes, this program 
generates the keystream bytes and encrypts/decrypts
(as opposed to just generating keystream bytes).
