Stream Cipher Structure

Dr. Benita Jaison , St. Francis College

 Random and Pseudorandom

1. Truly random - is defined as exhibiting ``true''

randomness.
2. Pseudorandom - is defined as having the
appearance of randomness, but nevertheless
exhibiting a specific, repeatable pattern.
3. Numbers calculated by a computer through a
deterministic process, cannot, by definition,
be random

Dr. Benita Jaison , St. Francis College

 Stream Cipher
• A typical stream cipher encrypts plaintext one byte at a
time.
• A stream cipher processes the input elements continuously
producing output one element at a time .
• A key is input to a pseudorandom bit generator that
produces a random stream bit numbers.
• The keystream output of the generator, is combined one
byte at a time with the plaintext stream using the bitwise
exclusive-OR (XOR) operation.
• The primary advantage of a stream cipher is that
stream ciphers are almost always faster and use far
less code than do block ciphers.

Dr. Benita Jaison , St. Francis College

 Stream Cipher Properties
The following important design considerations for a stream cipher:
• The encryption key sequence should have a large period, the
longer the period of repeat the more difficult it will be to do
cryptanalysis.
• The keystream should approximate the properties of a true
random number stream as close as possible, the more random-
appearing the keystream is, the more randomized the
ciphertext is, making cryptanalysis more difficult.
• To guard against brute-force attacks, the key needs to be
sufficiently long.Thus, with current technology, a key length of
at least 128 bits is desirable.
• With a properly designed pseudorandom number generator, a
stream cipher can be as secure as block cipher of comparable
key length. The primary advantage of a stream cipher is that
stream ciphers are almost always faster and use far less code
than do block ciphers. A stream cipher can be constructed with
any cryptographically strong PRNG.
Dr. Benita Jaison , St. Francis College
Encryption Speed

Dr. Benita Jaison , St. Francis College

 RC4
• RC4 is a stream cipher designed in 1987 by Ron Rivest
for RSA Security.
• It is a variable key-size stream cipher with byte-
oriented operations.
• The algorithm is based on the use of a random
permutation.
• The period of the cipher is greater than 10^100.
• Eight to sixteen machine operations are required per
output byte, and the cipher can be expected to run
very quickly in software.
• RC4 is the most widely used stream cipher.
• It is used in the SSL/TLS secure web protocol, & in the
WEP & WPA wireless LAN security protocols.
Dr. Benita Jaison , St. Francis College
 RC 4 Algorithm
• Designed by Ron Rivest for RSA Security
• Variable key-size from 1 to 256 bytes.
• S[255] is a state vector S, which contains a 8
bit number or a byte.
• Key is used to initialise the state vector S.
• T is a temporary vector of length 256 bytes.
• K is repeatedly copied in to vector T, to fill out
T.
Dr. Benita Jaison , St. Francis College
 RC 4 Algorithm
• Key Scheduling
• Pseudo - Random Generation Algorithm
(Stream Generation)

• CT = PT xor KeyStream
• PT = CT xor KeyStream

Dr. Benita Jaison , St. Francis College

 KEY SCHEDULING
ALGORITHM
j=0
for i = 0 to 255 do
j = j + S[i] + K[i] mod 256
swap S[i] and S[j]
end for

Dr. Benita Jaison , St. Francis College

 KEY SCHEDULING
• Uses an S Array of length 256
• where S[0] = 0 to S[255] = 255
• Suppose the key is K={"I AM THE KEY“};
• Key encoding using ASCII gives K={ 73, 32, 65, 77,
32, 84, 72, 69, 32, 75, 69, 89}
• Key Array, K is of length 256, K[0] to K[255]
• Element of the key array is repeated copied to
vector T.
• T[0] = 73,T[1] = 32, T[2] = 65, T[3] = 77, T[4] = 32,
T[5] =84,……. ,T[11] = 89, T[12] = 73, T[13] = 32,….
T[255] = ?
Dr. Benita Jaison , St. Francis College
 PSEUDO- RANDOM GENERATION
ALGORITHM
set i=0 and j= 0
for i = i + 1
j = j + S[i] mod 256
swap S[i] and S[j]
t = S[i] + S[j] mod 256
KeyStream = S[t]
end for

Dr. Benita Jaison , St. Francis College

 KEY SCHEDULING
• Suppose the S-Array is of length 8
• [S0, S1, S2, S3, S4, S5, S6, S7] = [0, 1, 2, 3, 4, 5, 6, 7]
• Let the key be K = [3 1 4 1 5]
• The T array thus is
• [T0, T1, T2, T3, T4, T5, T6, T7] = [3, 1, 4, 1, 5, 3, 1,4]

Dr. Benita Jaison , St. Francis College

 Initialisation
• S is initialised from 0 to 255 in ascending order
s[0]=0,s[1]=1,s[2]=2,s[3]=3,…..s[255]=255.
• Key value K is transferred to T. If keylen is lesser than 256
bytes then K is repeatedly copied to T ,to fill out T.
/* Initialisation */
• for i=0 to 255
• S[i]=i;
(S[0]=0,S[1]=1,S[2]=2,…..S[255]=255)
• T[i]=k[i mod keylen];

• → T[0]=k[0 mod 256] =k[0]

Dr. Benita Jaison , St. Francis College
 Initial permutation of S
• S[0],S[1],….S[255] is having values in ascending
order from 0 to 255. Using permutation, S values
are swapped with one another.
/* Initial Permutation */
j=0;
for i=0 to 255 do
j=(j + S[i]+T[i]) mod 256;
Swap(S[i],S[j]);

Dr. Benita Jaison , St. Francis College

 Stream Generation
• The key K is only used for the initialisation of S vector. The key stream
is generated using the S values. Stream generation involves cycling
through all the elements of S[i] and for each S[i],swapping S[i] with
another byte in S, once it reaches till S[255],it again starts from S[0].

/* Stream generation */

i=0,j=0;
While (true)
for i=0 to 255)
j=(j+S[i])mod 256;
Swap(S[i],S[j]);
t=(S[i] + S[j]) mod 256;
k=S[t];

Dr. Benita Jaison , St. Francis College

• k=S[t].

• To encrypt, XOR the value k with the next byte

of plaintext.
• To decrypt ,XOR the value k with the next byte
of cipher text.

Dr. Benita Jaison , St. Francis College

 Example
• Let K be of keylen 8 byte.(K can vary from 1 to
256 bytes).
• Let S be the State vector and T be the
temporary vector.

Dr. Benita Jaison , St. Francis College

 Initialisation
S[0]=0,S[1]=1,S[2]=2,…..S[255]=255

T[0]=k[0 mod 256] =k[0]

T[1]=k[1 mod 256] =k[1]
T[2]=k[2 mod 256] =k[2]
T[3]=k[3 mod 256] =k[3]
T[4]=k[4 mod 256] =k[4]
T[5]=k[5 mod 256] =k[5]
T[6]=k[6 mod 256] =k[6]
T[7]=k[7 mod 256] =k[7]
T[8]=k[0 mod 256] =k[0]
T[9]=k[1 mod 256] =k[1]
T[10]=k[2 mod 256] =k[2]
T[11]=k[3 mod 256] =k[3]
T[12]=k[4 mod 256] =k[4]
T[13]=k[5 mod 256] =k[5]
T[14]=k[6 mod 256] =k[6]
T[15]=k[7 mod 256] =k[7]
T[16]=k[0 mod 256] =k[0]
Dr. Benita Jaison , St. Francis College
Initial permutation of S

Dr. Benita Jaison , St. Francis College

Initial permutation of S

Dr. Benita Jaison , St. Francis College

Stream Generation

Dr. Benita Jaison , St. Francis College