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The important point here is that it does not matter which key is applied first, but that both keys are required for the process to work (Figure 1B).
As shown in Figure 1A, the sender uses the key to encrypt the plaintext and sends the ciphertext to the receiver.
Secret key cryptography schemes are generally categorized as being either stream ciphers or block ciphers.
The biggest difficulty with this approach, of course, is the distribution of the key (more on that later in the discussion of public key cryptography).
While stream ciphers do not propagate transmission errors, they are, by their nature, periodic so that the keystream will eventually repeat.
A block cipher is so-called because the scheme encrypts one block of data at a time using the same key on each block.
Stream ciphers come in several flavors but two are worth mentioning here (Figure 2).