The Free On-line Dictionary of Computing (30 December 2018):

Pretty Good Privacy
PGP

    (PGP) A high security RSA public-key
   encryption application for MS-DOS, Unix, VAX/VMS, and
   other computers.  It was written by Philip R. Zimmermann
    of Phil's Pretty Good(tm) Software and later
   augmented by a cast of thousands, especially including Hal
   Finney, Branko Lankester, and Peter Gutmann.

   PGP was distributed as "guerrilla freeware".  The authors
   don't mind if it is distributed widely, just don't ask Philip
   Zimmermann to send you a copy.  PGP uses a public-key
   encryption algorithm claimed by US patent #4,405,829.  The
   exclusive rights to this patent are held by a California
   company called Public Key Partners, and you may be
   infringing this patent if you use PGP in the USA.  This is
   explained in the PGP User's Guide, Volume II.

   PGP allows people to exchange files or messages with privacy
   and authentication.  Privacy and authentication are provided
   without managing the keys associated with conventional
   cryptographic software.  No secure channels are needed to
   exchange keys between users, which makes PGP much easier to
   use.  This is because PGP is based on public-key
   cryptography.

   PGP encrypts data using the International Data Encryption
   Algorithm with a random session key, and uses the RSA
   algorithm to encrypt the session key.

   In December 1994 Philip Zimmermann faced prosecution for
   "exporting" PGP out of the United States but in January 1996
   the US Goverment dropped the case.  A US law prohibits the
   export of encryption software out of the country.
   Zimmermann did not do this, but the US government hoped to
   establish the proposition that posting an encryption program
   on a BBS or on the Internet constitutes exporting it - in
   effect, stretching export control into domestic censorship.
   If the government had won it would have had a chilling effect
   on the free flow of information on the global network, as well
   as on everyone's privacy from government snooping.

   FAQ (ftp://ftp.netcom.com/pub/mp/mpj/getpgp.asc).  UK FTP
   (ftp://src.doc.ic.ac.uk/packages/pgp/).  USA FTP
   (http://web.mit.edu/network/pgp-form.html).
   (http://pegasus.esprit.ec.org/people/arne/pgp.html).

   Justice Dept. announcement
   (http://eff.org/pub/Alerts/usatty_pgp_011196.announce).

   ["Protect Your Privacy: A Guide for PGP Users", William
   Stallings, Prentice-Hall, ISBN 0-13-185596-4].

   (1996-04-07)