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In cryptography, a **keystream** is a stream of random or pseudorandom characters that are combined with a plaintext message to produce an encrypted message (the ciphertext).

The "characters" in the keystream can be bits, bytes, numbers or actual characters like A-Z depending on the usage case.

Usually each character in the keystream is either added, subtracted or XORed with a character in the plaintext to produce the ciphertext, using modular arithmetic.

Keystreams are used in the one-time pad cipher and in most stream ciphers. Block ciphers can also be used to produce keystreams. For instance, CTR mode is a block mode that makes a block cipher produce a keystream and thus turns the block cipher into a stream cipher.

In this simple example we use the English alphabet of 26 characters from a-z. Thus we can not encrypt numbers, commas, spaces and other symbols. The random numbers in the keystream then have to be at least between 0 and 25.

To encrypt we add the keystream numbers to the plaintext. And to decrypt we subtract the same keystream numbers from the ciphertext to get the plaintext.

If a ciphertext number becomes larger than 25 we wrap it to a value between 0-25. Thus 26 becomes 0 and 27 becomes 1 and so on. (Such wrapping is called modular arithmetic.)

Here the plaintext message "attack at dawn" is combined by addition with the keystream "kjcngmlhylyu" and produces the ciphertext "kcvniwlabluh".

Plaintext | a | t | t | a | c | k | a | t | d | a | w | n |
---|---|---|---|---|---|---|---|---|---|---|---|---|

Plaintext as numbers | 0 | 19 | 19 | 0 | 2 | 10 | 0 | 19 | 3 | 0 | 22 | 13 |

Keystream | k | j | c | n | g | m | l | h | y | l | y | u |

Keystream as numbers | 10 | 9 | 2 | 13 | 6 | 12 | 11 | 7 | 24 | 11 | 24 | 20 |

Ciphertext as numbers | 10 | 28 | 21 | 13 | 8 | 22 | 11 | 26 | 27 | 11 | 46 | 33 |

Ciphertext as numbers wrapped to 0-25 |
10 | 2 | 21 | 13 | 8 | 22 | 11 | 0 | 1 | 11 | 20 | 7 |

Ciphertext as text | k | c | v | n | i | w | l | a | b | l | u | h |

*Handbook of Applied Cryptography*by Menezes, van Oorschot and Vanstone (2001), chapter 1, 6 and 7.