OpenSSL

http://www.madboa.com/geek/openssl/is a blog that contains really good explanations and examples of how to use OpenSSL.

For example, here's how to encrypt a text string-
It’s also possible to do a quick command-line encoding of a string value:

$ echo "encode me" | openssl enc -base64

ZW5jb2RlIG1lCg==

Note that echo will silently attach a newline character to your string. Consider using its -n option if you want to avoid that situation, which could be important if you’re trying to encode a password or authentication string.
$ echo -n "encode me" | openssl enc -base64

ZW5jb2RlIG1l

Use the -d (decode) option to reverse the process.
$ echo "ZW5jb2RlIG1lCg==" | openssl enc -base64 -d

encode me