A single key is used for both functions; hence, this
method is called the symmetric encryption process.
Figure 14-3. Symmetric Key Encryption
[View full size image]
Symmetric key cryptography ciphers are generally categorized in two modes:
Stream cipher: A symmetric cipher that encrypts the plaintext digits (bits or bytes) one by one. The
transformation of encrypted output varies during the encryption cycle. There are several varying types of
stream ciphers, such as synchronous stream cipher and asynchronous stream cipher. RC4 is one of the
most common stream cipher designs.
Block cipher: A symmetric key cipher that encrypts the plaintext on a fixed-length group of bits, with an
unvarying transformation during the encryption cycle. Block ciphers encrypt blocks of data by using the
same key on each block. For example, a block cipher can take a 128-bit block of plaintext as input and
generate a corresponding 128-bit block of ciphertext output. DES and AES are examples of common block
cipher designs.
In general, a block cipher mode yields the same ciphertext from a block of plaintext when using the same key,
whereas a stream cipher mode yields different ciphertext from the same plaintext.
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