 # Keystream facts for kids

Kids Encyclopedia Facts

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 example, the CTR mode (also known as Integer Counter Mode (ICM)) is a block mode that makes a block cipher produce a keystream then turns the block cipher into a stream cipher.

## Example

In this simple example we will use only the English alphabet of 26 characters from a-z, therefore we will not encrypt numbers, commas, spaces and other symbols. As we have only 26 characters the random numbers in the keystream 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. I.e. 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 Plaintext as numbers Keystream Keystream as numbers Ciphertext as numbers Ciphertext as numbers Ciphertext as text wrapped to 0-25 a t t a c k a t d a w n 0 19 19 0 2 10 0 19 3 0 22 13 k j c n g m l h y l y u 10 9 2 13 6 12 11 7 24 11 24 20 10 28 21 13 8 22 11 26 27 11 46 33 10 2 21 13 8 22 11 0 1 11 20 7 k c v n i w l a b l u h Keystream Facts for Kids. Kiddle Encyclopedia.