---
title: "Cipher - Glossary | MDN"
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source: "https://developer.mozilla.org/en-US/docs/Glossary/Cipher"
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  1. [Glossary](https://developer.mozilla.org/en-US/docs/Glossary)
  2. [Cipher](https://developer.mozilla.org/en-US/docs/Glossary/Cipher)

# Cipher
In [cryptography](https://developer.mozilla.org/en-US/docs/Glossary/Cryptography), a **cipher** is an algorithm that can [encrypt](https://developer.mozilla.org/en-US/docs/Glossary/Encryption) [plaintext](https://developer.mozilla.org/en-US/docs/Glossary/Plaintext) to make it unreadable, and to [decrypt](https://developer.mozilla.org/en-US/docs/Glossary/Decryption) the encoded data back to plaintext again.
Ciphers were common long before the information age (e.g., [substitution ciphers](https://en.wikipedia.org/wiki/Substitution_cipher), [transposition ciphers](https://en.wikipedia.org/wiki/Transposition_cipher), and [permutation ciphers](https://en.wikipedia.org/wiki/Permutation_cipher)), but none of them were cryptographically secure except for the [one-time pad](https://en.wikipedia.org/wiki/One-time_pad).
In the modern age, ciphers have evolved dramatically. [AES](https://en.wikipedia.org/wiki/Advanced_Encryption_Standard), [RSA](https://en.wikipedia.org/wiki/RSA_\(cryptosystem\)) and [Blowfish](https://en.wikipedia.org/wiki/Blowfish_\(cipher\)) are examples of ciphers that are integral part of contemporary encryption standards and systems.
Modern ciphers are designed to withstand attacks discovered through [cryptanalysis](https://developer.mozilla.org/en-US/docs/Glossary/Cryptanalysis). There is no guarantee that all attack methods have been discovered, so each algorithm is [recommended for different purposes](https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto#supported_algorithms) based on known classes of attacks.
Ciphers operate either as [block ciphers](https://en.wikipedia.org/wiki/Block_cipher) on successive blocks (or buffers) of data, or as [stream ciphers](https://en.wikipedia.org/wiki/Stream_cipher) on a continuous data flow (often of sound or video).
Ciphers are also classified according to how their [keys](https://developer.mozilla.org/en-US/docs/Glossary/Key) are handled:
  * [symmetric key](https://developer.mozilla.org/en-US/docs/Glossary/Symmetric-key_cryptography) algorithms use the same key to encode and decode a message. The key also must be sent securely if the message is to stay confidential.
  * [asymmetric key](https://developer.mozilla.org/en-US/docs/Glossary/Public-key_cryptography) algorithms use one key for encryption and the other for decryption.

## In this article
  * [See also](https://developer.mozilla.org/en-US/docs/Glossary/Cipher#see_also)

##  [See also](https://developer.mozilla.org/en-US/docs/Glossary/Cipher#see_also)
  * [Web Crypto API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Crypto_API)
  * [SubtleCrypto supported algorithms](https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto#supported_algorithms)
  * Related glossary terms: 
    * [Block cipher mode of operation](https://developer.mozilla.org/en-US/docs/Glossary/Block_cipher_mode_of_operation)
    * [Ciphertext](https://developer.mozilla.org/en-US/docs/Glossary/Ciphertext)
    * [Cipher suite](https://developer.mozilla.org/en-US/docs/Glossary/Cipher_suite)
    * [Cryptanalysis](https://developer.mozilla.org/en-US/docs/Glossary/Cryptanalysis)
    * [Cryptography](https://developer.mozilla.org/en-US/docs/Glossary/Cryptography)
    * [Decryption](https://developer.mozilla.org/en-US/docs/Glossary/Decryption)
    * [Encryption](https://developer.mozilla.org/en-US/docs/Glossary/Encryption)
    * [Key](https://developer.mozilla.org/en-US/docs/Glossary/Key)
    * [Plaintext](https://developer.mozilla.org/en-US/docs/Glossary/Plaintext)
    * [Public-key cryptography](https://developer.mozilla.org/en-US/docs/Glossary/Public-key_cryptography)
    * [Symmetric-key cryptography](https://developer.mozilla.org/en-US/docs/Glossary/Symmetric-key_cryptography)
  * [Cipher](https://en.wikipedia.org/wiki/Cipher) on Wikipedia