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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 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[change | change source]

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 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

References[change | change source]