This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
The following was taken from the sci.crypt FAQ:
How do I send encrypted mail under UNIX? [PGP, RIPEM, PEM, ...]?
Here's one popular method, using the des command:
cat file | compress | des private_key | uuencode | mail
Meanwhile, there is a de jure Internet standard in the works called
PEM (Privacy Enhanced Mail). It is described in RFCs 1421 through
1424. To join the PEM mailing list, contact email@example.com.
There is a beta version of PEM being tested at the time of this
There are also two programs available in the public domain for
encrypting mail: PGP and RIPEM. Both are available by FTP. Each has
its own news group: alt.security.pgp and alt.security.ripem. Each has
its own FAQ as well. PGP is most commonly used outside the USA since
it uses the RSA algorithm without a license and RSA's patent is valid
only (or at least primarily) in the USA.
[ Maintainer's note: The above paragraph is not fully correct, as MIT
PGP uses RSAREF as well now. ]
RIPEM is most commonly used inside the USA since it uses the RSAREF
which is freely available within the USA but not available for
shipment outside the USA.
Since both programs use a secret key algorithm for encrypting the body
of the message (PGP used IDEA; RIPEM uses DES) and RSA for encrypting
the message key, they should be able to interoperate freely. Although
there have been repeated calls for each to understand the other's
formats and algorithm choices, no interoperation is available at this
time (as far as we know).