PGP in an acronym for Pretty Good Privacy.
Created by Phil Zimmermann, http://www.philzimmermann.com/ , it implements
a public/private key encryption environment where one is able to publish
a portion of one's key publically, to be used by someone wanting to send
you a private message.  Only those whose keys are used to encrypt the message
can, in theory, decrypt it using the software.  The software has had
some freely distributable portions as well as commercial portions, but it
is my understanding that some parts of the idea are patented.

Members of the [FSF] project, have created GNUPG, http://www.gnupg.org/ ,
as a freely available and open counter-part to PGP.

http://www.purl.org/NET/Tcl-FAQ/part4.html lists
 What: nn-tk
 Where:  http://www.general.uwa.edu.au/u/toivo/nn-tk
         ftp://ftp.uwa.edu.au/pub/nn/nn-tk/
         ftp://ftp.procplace.com/pub/tcl/sorted/packages-7.6/net/nn-tk.15.5/nn-tk.15.5.tar.gz
 Description: A Tk interface to nn (net newsreader).  It allows
        the use of exmh to post news, as well as to
        send and read MIME and PGP email messages as well as to turn URLs into
        buttons for passing to Netscape or other browsers.  Supports Tk 4.x
        You may need to update to the latest version of exmh to get the
        support for all of nn-tk's features.  Be sure to keep checking
        back at the ftp site for new releases.
 Updated: 08/1998
 Contact:  mailto:toivo@ucs.uwa.edu.au (Toivo Pedaste)

 What: NSBD
 Where:  http://www.bell-labs.com/nsbd/
 Description: GNU licensed software which provides an open WWW-based
        authenticating software distribution system.
        Uses digital signatures for maintainers of packages, and
        provides means to check for updates with auto download and install.
        Written in Tcl/Tk and C for Unix, and uses PGP.
        Watch web page for new release announcements.
        Latest version is 1.6 patchlevel 2002/01/09.
 Updated: 06/1997
 Contact:  mailto:dwd@bell-labs.com (Dave Dykstra)

 What: Postilion
 Where:  http://www.postilion.org/
         http://www.postilion.org/postilion-current.tar.gz
 Description: Mail User Agent based on TkRat, with a different user interface.
        Supports MIME, virtual folders, PGP, Unix file, MH, POP and IMAP mail
        support.  Current version is v0.9.3c.
 Updated: 04/1999
 Contact:  mailto:postilion-request@postilion.org

 What: tkpgp
 Where:  http://jfku.edu/%7Erunge/Programs/TkPgp/TkPgp.html
         http://jfku.edu/%7Erunge/Programs/TkPgp/tkpgp.tar.gz
 Description: Tk interface to the cryptology tool PGP.
        Can take input from Tk clipboard or files.  Places text into
        the clipboard for pasting.  It requires at least Tk 4.2 and PGP v2.6.2.
        Preliminary support for Windows.  Works better under Windows 95
        than Windows 3.1.  Currently at version 1.11.
 Updated: 11/1997
 Contact:  mailto:runge@crl.com (also http://www.geocities.com/SiliconValley/Chip/3708/tkpgp/tkpgp.htm ?)
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Also keep in mind the possibility of use of command-line utilities:
    exec gnupgp ...
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[Category Cryptography]