Keystream

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In cryptography, a keystream is a stream of random or pseudorandom characters that are combined with a cleartext 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 cleartext 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.

[edit] Example

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 cleartext. And to decrypt we subtract the same keystream numbers from the ciphertext to get the cleartext.

Note that 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.

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

Cleartext	a	t	t	a	c	k	a	t	d	a	w	n
Cleartext 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	j	c	v	n	i	w	l	a	b	l	u	h

Stream ciphers v • d • e
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Theory: Shift register | LFSR | NLFSR | Shrinking generator | T-function | IV
Standardization: eSTREAM
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