Let's talk in more detail about how keys and ciphers
work together. Many digital cipher algorithms are called
*symmetric-key*
ciphers, because they use the same key
value for encoding as they do for decoding (e = d).
Let's just call the key k.

In a symmetric key cipher, both a sender and a receiver need to have the same shared secret key, k, to communicate. The sender uses the shared secret key to encrypt the message and sends the resulting ciphertext to the receiver. The receiver takes the ciphertext and applies the decrypting function, along with the same shared secret key, to recover the original plaintext (Figure 14-7).

Figure 14-7. Symmetric-key cryptography algorithms use the same key for encoding and decoding

Some popular symmetric-key cipher algorithms are DES, Triple-DES, RC2, and RC4.

It's very important that secret keys stay secret. In most cases, the encoding and decoding algorithms are public knowledge, so the key is the only thing that's secret!

A good cipher algorithm forces the enemy to try every single possible
key value in the universe to crack the code. Trying all key values by
brute force is called an *enumeration
attack*
. If there are only a few possible key values, a bad guy can go through all of them by brute force and eventually crack the code. But if there are a lot of possible key values, it might ...

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