1. A non-algebraic method of encrypting and decrypting data.

2. At the terminals a sequence of keys is generated for use in decrypting the ADUs.

3. A co-processor (44) executes an arithmetical algorithm that computes modular exponentiation equations for encrypting or decrypting data.

4. The invention concerns a method for asymmetric communications, in particular for encrypting and decrypting messages whose length is known or not known a priori.

5. Symmetric Cryptography involves encrypting and decrypting data using the same key, which is only known to the sender and receiver of the message