Encryption, sometimes referred to as privacy, means that your data is protected from
disclosure to a would-be attacker “sniffing” or eavesdropping on the wire (see the
Threats section for more details). Ciphers are the mechanism by which Secure Shell
encrypts and decrypts data being sent over the wire. A block cipher is the most common
form of symmetric key algorithms (e.g. DES, 3DES, Blowfish, AES, and Twofish).
These operate on a fixed size block of data, use a single, secret, shared key, and generally
involve multiple rounds of simple, non-linear functions. The data at this point is
“encrypted” and cannot be reversed without the shared key.