# Definition:Cipher (Cryptography)

## Definition

A **cipher**, in the context of cryptography, is an algorithm used to transform a message into another string of symbols in order to hide its meaning from a third party.

A **cipher** operates by replacing each symbol in the original string with a word in some alphabet.

The specific nature of the replacement depends on the specifics of the **cipher**.

### Plaintext

In the context of cryptography, the **plaintext** is the message being transformed into an encrypted string.

### Ciphertext

In the context of cryptography, the **ciphertext** is the ciphered string after the **plaintext** has been processed by the **cipher**.

### Key

In the context of cryptography, a **key** is a piece of information which:

- is used to convert plaintext to ciphertext
- allows the ciphertext to be transformed back into the original plaintext.

These two processes may use different **keys**.

## Also see

- Results about
**ciphers**in the context of**cryptography**can be found**here**.

## Linguistic Note

The word **cipher** can also be found in its less common spelling: **cypher**.

The word ultimately derives from the Arabic **صِفْر** (**ṣifr**), meaning **zero** or **empty**.

## Sources

- 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**cipher (cypher)**:**3.** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**cipher (cypher)**:**3.**