Cipher
In cryptography, cipher is an algorithm that used to encrypt and decrypt data.
When using a cipher the original information (plaintext) is transformed to the encrypted form (ciphertext) with a series of well-defined steps. The ciphertext message contains all the information of the plaintext message, but it is not readable by a human or computer and looks like random data. In order to read the plaintext message, one should decrypt the cipher text first with the same algorithm used for encryption.
The operation of encryption algorithm depends on auxiliary data known as encryption key.
Encryption key must be selected before using a cipher to encrypt a plaintext message. Without knowledge of the key, it is practically impossible to decrypt the resulting ciphertext into readable plaintext.
Ciphers can be categorized by the way they process input data. Ciphers that work with continuous stream of symbols are called stream ciphers. If the cipher processes its input by grouping symbols into fixed-size blocks, it is called a block cipher.
Symmetric ciphers use the same key is used for encryption of plaintext and decryption of resulting ciphertext. Another class of ciphers called assymetric ciphers use different keys for encryption and decryption of data.
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