Theopensslapplication that ships with the OpenSSL libraries can perform a wide range of crypto operations. This HOWTO provides some cookbook-style recipes for using it.
OpenSSL is a very powerful cryptography
utility, perhaps a little too powerful for the average user. With all the
different command line options, it can be a daunting task figuring out
how to do exactly what you want to do. Here are several common tasks you
may find useful. Note that some commands may require version 0.9.8a and
above.