This applet lets you encrypt and decrypt messages using an RSA-like asymmetric encryption scheme. With this implementation I chose to be able to represent any ASCII character at the cost of small message sizes. This means that for every three digits of n, you can encode one character. If you have more to say than that, split it up into two messages.
There are two parts to this applet, key generation and encryption/decryption. To generate a key, pick two prime numbers, p and q, and a public encryption key e - a number coprime to (p-1)(q-1) = phi(n), and it will give you n = pq and d, such that e*d = 1 mod phi(n). n and e are what you want to distribute - your public key.
To work with messages, enter n, and e for encryption or d for decryption. ASCII messages go in the message box for encryption, and encrypted codes go in the code box for decryption.
This applet works in base 10. The source is here. This applet was designed for Netscape. Suggestions for improvement are welcome!