In cryptography

Cryptography

Cryptography is the practice and study of techniques for secure communication in the presence of third parties...

, the term ciphertext expansion refers to the length increase of a message when it is encrypted

Encryption

In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...

. Many modern cryptosystems cause some degree of expansion during the encryption process, for instance when the resulting ciphertext

Ciphertext

In cryptography, ciphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext that is unreadable by a human or computer without the proper cipher...

 must include a message-unique Initialization Vector

Initialization vector

In cryptography, an initialization vector is a fixed-size input to a cryptographic primitive that is typically required to be random or pseudorandom...

 (IV). Probabilistic encryption

Probabilistic encryption

Probabilistic encryption is the use of randomness in an encryption algorithm, so that when encrypting the same message several times it will, in general, yield different ciphertexts...

 schemes cause ciphertext expansion, as the set of possible ciphertexts is necessarily greater than the set of input plaintexts. Certain schemes, such as Cocks Identity Based Encryption, or the Goldwasser-Micali cryptosystem

Goldwasser-Micali cryptosystem

The Goldwasser–Micali cryptosystem is an asymmetric key encryption algorithm developed by Shafi Goldwasser and Silvio Micali in 1982. GM has the distinction of being the first probabilistic public-key encryption scheme which is provably secure under standard cryptographic assumptions...

 result in ciphertexts hundreds or thousands of times longer than the plaintext.

Ciphertext expansion may be offset or increased by other processes which compress or expand the message, e.g., data compression

Data compression

In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....

 or error correction coding.