CipherText
The four most common modes are ECB, CBC, CFB, and OFB. In what follows, Ek denotes the encryption function of the block cipher E parameterized by key K, while E-1k denotes decryption. A plaintext message x = x1 …xt is assumed to consist of n-bit blocks for ECB and CBC modes, and r-bit blocks for CFB and OFB modes for appropriate fixed r<=n. Although public-key encryption algorithms (such as RSA) also use block ciphers, their analysis requires a different approach than symmetric-key algorithms.
First of all, you must realize that unlike encryption with symmetric key, which can be performed on plaintext of an arbitrary size, the maximum number of plaintext bytes that can be encrypted using a public key depends on the size of the key (and the type of padding).
Block ciphers take a block of plaintext, whose size depends on the cryptosystem, and use a fixed key of some block length also depending on the cryptosystem, to produce a block of ciphertext, usually the same length as the block of plaintext. Such encryption is ``stand-alone'' and does not depend on what happened before. (Such a block cipher does not have state or memory.) Having ciphertext block size the same as plaintext block size is important, because then there is no data expansion with encryption.
Encryption itself provides a function from each possible plaintext block to a ciphertext block, with no duplicate values occurring. Similarly decryption gives the inverse mapping, which would not be uniquely defined if there were duplicates. Encryption in this type of cryptosystem is essentially a parameterized collection of encryption functions, one for each key value.