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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).
With this form of cryptography, it is obvious that the key must be known to both the sender and the receiver; that, in fact, is the secret.
A block cipher is so-called because the scheme encrypts one block of data at a time using the same key on each block.
In general, the same plaintext block will always encrypt to the same ciphertext when using the same key in a block cipher whereas the same plaintext will encrypt to different ciphertext in a stream cipher.
The most common construct for block encryption algorithms is the Feistel cipher, named for cryptographer Horst Feistel (IBM).
Stream ciphers come in several flavors but two are worth mentioning here (Figure 2). The Advanced Encryption Standard (AES) and Rijndael 5.10. Because a single key is used for both functions, secret key cryptography is also called symmetric encryption. The receiver applies the same key to decrypt the message and recover the plaintext. Stream ciphers operate on a single bit (byte or computer word) at a time and implement some form of feedback mechanism so that the key is constantly changing. Secret key cryptography schemes are generally categorized as being either stream ciphers or block ciphers.