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.
Exampleedit
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
Referencesedit
Handbook of Applied Cryptography by Menezes, van Oorschot and Vanstone (2001), chapter 1, 6 and 7.
May 08, 2024
keystream, cryptography, keystream, stream, random, pseudorandom, characters, that, combined, with, plaintext, message, produce, encrypted, message, ciphertext, characters, keystream, bits, bytes, numbers, actual, characters, like, depending, usage, case, usua. 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 Example editIn 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 hReferences editHandbook of Applied Cryptography by Menezes van Oorschot and Vanstone 2001 chapter 1 6 and 7 Retrieved from https en wikipedia org w index php title Keystream amp oldid 1136438276, wikipedia, wiki, book, books, library,
