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After creating the message, the sender encrypts it with their key and passes it to the recipient who then decrypts it by using a copy of the same key used to encrypt it.

Secret-key encryption does have some limitations, particularly with regard to key distribution. For privacy to be maintained, every transmitter of messages would need to provide a different key to everyone they intended to communicate with, otherwise every potential recipient would be able to read all messages whether it was intended for them or not.

Whilst this is manageable where a small number of parties are involved (for example, sending a private e-mail to a friend) it is not practical for Web commerce which can involve communicating with thousands of customers.

Another limitation with secret-key encryption is its inability to support non-repudiation. As both parties share the same key it is possible for one party to create a message with the shared secret key and falsely claim it had been sent by the other party.

Secret-key encryption on its own is therefore not suitable for Web commerce - instead a system known as public-key encryption is used.