OpenSSL contains both DES and RSA, and even more encryption methods. The name "OpenSSL" is misleading: it's an all-purpose encryption library, not just for SSL.
For example, to encrypt using a symmetric encryption algorithm like DES:
openssl enc -e -des -in plaintext.txt -out encrypted.dat -pass file:/well/protected/cryptokey.txt
And to decrypt:
openssl enc -d -des -in encrypted.dat -out decrypted.txt -pass file:/well/protected/cryptokey.txt
Note that the simple DES is too weak for today's technology. Even triple-DES (des3) is barely adequate for low-level protection.
Each sub-command of OpenSSL has a separate man page: to get more information about "openssl enc", use "man enc" after installing OpenSSL.
To use an asymmetric encryption algorithm like RSA, you must first generate a pair of keys. A proper RSA key pair has an interesting feature: when data is encrypted using one half of the key pair, it cannot be decrypted using that same key... you must use the _other_ half of the key pair to decrypt the data. Usually, one half of the key pair is called "private key" and the other "public key".
In addition, encrypting large amounts of data using plain RSA is not very efficient: usually, some symmetric encryption algorithm is used to encrypt the data with a random key ("session key") which is used only once. Then the key is protected using RSA and the encrypted key is sent along with the data. The recipient then uses RSA to decrypt the session key, then uses the session key to decrypt the data.
"RSA signature" may not be an encryption method at all: it's more like a way to prove that the data is from you and it has not been tampered with. If you need this, OpenSSL has tools for this too.
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