PkZIP

PkZip and PkUnzip are utilities which have the following purposes:

• Package groups of files together in a manner which permits easy transfer over a modem. Typically these groups make up a program, but single files, including text files, are often "Zipped"
• Compresses the files, typically to half their original size in the case of wordprocessor and text files.
• One favourable aspect of PKZip is that it tends to preserve the file's structure. This means that after a wordprocesor-text file is transferred over the Internet, it remains readable by the wordprocesor.
• PkZip is also convenient for archiving of files for backup, and can be used across a network.

PkZip/Unzip is shareware, and a $47. fee should be paid to PkWare.

Versions

There are two companies that make "zip" compression software for .zip file creation / extraction. There's actually 2 encrypted .ZIP formats: PKZIP will read WinZip's format, and vice-versa up to PKZip version 2.0.

WinZip's AES encryption is documented here [winzip.com]. PKWare's format is apparently proprietary.

PKUnZIP 2.1

pkzip 2 and below are most common. do not upgrade to 2.1 as it is mostly bloatware and incompatible with microsoft windows own unzip extraction built into file explorer.

pkzip / pkunzip 2.1 - bloatware / crapware