To encrypt data, enter the data ("plaintext") and an encryption key to the
encryption portion of the algorithm. To decrypt the "ciphertext," a proper
decryption key is used at the decryption portion of the algorithm. Those
keys, which contains simply a string of numbers, are called public key and
private key, respectively. For example, suppose Alice intends to send
e-mail to Bob. Through a public-key directory, she finds his public key.
Then, she encrypts her message using the key and send it to Bob. This
public key, however, will not decrypt the ciphertext. Knowledge of Bob's
public key will not help an eavesdropper. In order for Bob to decrypt his
ciphertext, he must use his private key. If Bob wants to respond to Alice,
he encrypts his message using **her** public key.

The challenge of public-key cryptography is developing a system in which it is impossible to determine the private key. This is accomplished through the use of a one-way function. With a one-way function, it is relatively easy to compute a result given some input values. However, it is extremely difficult, nearly impossible, to determine the original values if you start with the result. In mathematical terms, given x, computing f(x) is easy, but given f(x), computing x is nearly impossible. The one-way function used in RSA is multiplication of prime numbers. It is easy to multiply two big prime numbers, but for most very large primes, it is exremely time-consuming to factor them. Public-key cryptography uses this function by building a cryptosystem which uses two large primes to build the private key and the product of those primes to build the public key.

Instructions:

- This model is best carried out by two persons. Name them Alice and Bob.
- Alice will pick two available primes. Public and private keys will be generated by computer.
- Record the public key containing the exponent value, E and the product of the primes, N. And, record the private key, D.
- Give E and N to Bob (your partner).
- Bob will go to another page to pick a letter to encrypt. Enter E and N. The encrypted message / number will be generated.
- Bob will send or give the encrypted message to Alice.
- Alice will go to decryption page. Enter the message, D and N. The message will be decrypted to the original letter. Later, Alice can check with Bob to see if it is the right letter.

Remember, the main purpose of this model is understanding the RSA algorithm, not necessarily for encryption purpose. A lot of simplification has been made, while the mathematics and algorithm stay the same. So, ENJOY !

Now, proceed to: key generation page, encryption page, or decryption page.

For more information about RSA algorithm, check out RSA homepage.

sullivca@ucs.orst.edu

makmur@flop.engr.orst.edu

This is page is created on June 12, 1996.

Last updated on Wed Dec 31 19:00:00 1969.