This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
NSA (National Security Agency)
The following information is from the sci.crypt FAQ:
The NSA is the official communications security body of the U.S.
government. It was given its charter by President Truman in the early
50's, and has continued research in cryptology till the present. The
NSA is known to be the largest employer of mathematicians in the
world, and is also the largest purchaser of computer hardware in the
world. Governments in general have always been prime employers of
cryptologists. The NSA probably possesses cryptographic expertise many
years ahead of the public state of the art, and can undoubtedly break
many of the systems used in practice; but for reasons of national
security almost all information about the NSA is classified.
PGP (Pretty Good Privacy)
The program we're discussing. See question 1.1.
PKP (Public Key Partners)
A patent holding company that holds many public-key patents, including
(supposedly) the patent on public-key cryptography itself. Several of
its patents are not believed by some to be valid, including their
patent on RSA (which affects PGP).
RSA is the public key encryption method used in PGP. RSA are the
initials of the developers of the algorithm which was done at taxpayer
expense. The basic security in RSA comes from the fact that, while it
is relatively easy to multiply two huge prime numbers together to
obtain their product, it is computationally difficult to go the
reverse direction: to find the two prime factors of a given composite
number. It is this one-way nature of RSA that allows an encryption key
to be generated and disclosed to the world, and yet not allow a
message to be decrypted.
This is the free library RSA Data Security, Inc., made available for
the purpose of implementing freeware PEM applications. It implements
several encryption algorithms, including (among others) RSA. MIT PGP
uses RSAREF's RSA routines to avoid the alleged patent problems
associated with other versions of PGP.