This FAQ answer is excerpted from PGP(tm) User’s Guide;
Volume I: Essential Topics by Philip Zimmermann
PGP(tm) uses public-key encryption to protect E-mail and data files.
Communicate securely with people you’ve never met, with no secure
channels needed for prior exchange of keys. PGP is well featured and
fast, with sophisticated key management, digital signatures, data
compression, and good ergonomic design.
Pretty Good(tm) Privacy (PGP), from Phil’s Pretty Good Software, is a
high security cryptographic software application for MS-DOS, Unix,
VAX/VMS, and other computers. PGP allows people to exchange files or
messages with privacy, authentication, and convenience. Privacy means
that only those intended to receive a message can read it.
Authentication means that messages that appear to be from a particular
person can only have originated from that person. Convenience means
that privacy and authentication are provided without the hassles of
managing 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 a powerful
new technology called “public key” cryptography.
PGP combines the convenience of the Rivest-Shamir-Adleman (RSA)
public key cryptosystem with the speed of conventional cryptography,
message digests for digital signatures, data compression before
encryption, good ergonomic design, and sophisticated key management.
And PGP performs the public-key functions faster than most other
software implementations. PGP is public key cryptography for the
masses.
PGP Freeware 6.5.8 includes support for both RSA and Diffie-Hellman keys.
PGP may be downloaded from the MIT Distribution Center for PGP at http://web.mit.edu/network/pgp.html.
Many users have replaced PGP with GPG: GNU Privacy Guard.
