RSA is a method of public-key encryption suitable for both document or file sender-authentication and file or information encrytion. This is an example of asymmetrical encrytion, where the receiver of an encypted message necesarily posseses more information regarding the encryption key than does the sender. It is necessary to understand modular arithmetic in order to fully understand RSA encryption.

Joe says: Asymetrical. Right. That's what I was trying to think of in class. The analogy is this: I want you to send me a secure letter.

1. I send you an unlocked safe.
2. You put the letter in the safe and lock it. You don't have the combination.
3. You send the safe to me.
4. I get the safe and open it, since I have the combination.
5. I read the letter inside.

Note that no one along the way could intercept the letter and read it, because no one else but me has the combination to open the safe and get to the letter.

With Public Key encryption, everyone has access to one of my unlocked safes (the public key). I make the public key available: you can download it from my website, I can send it to you in an unsecure email, I can paint it on a billboard, whatever. Anyone can use the public key to encrypt the message. Only I have the private key that can be used to decrypt the message.

It's asymetrical. Others have the encryption key (the public key), but only I have the decryption key as well (the private key). What's magical about the keys is that they share mathematical properties that make them work together but extremely hard to figure out the private key from the public key.