RC4

Question 1

Which kind of cypher is RC4?

Answer 1

Stream cypher

Question 2

What are the main characteristics of a stream cipher?

Answer 2

• Emulate one-time pad
• process message bit by bit (as stream)
• have a pseudo-random keystream
• combined (XOR) with plain text bit by bit

Question 3

Which is the main goal of the stream cyphers?

Answer 3

The main goal is the randomness of stream key completely destroys statistical properties in message

Question 4

What can cause security issues in a bad stream cipher implementation?

Answer 4

If the implementation reuses the stream key, it can compromise the implementation. Whenever the stream key is reused, we can recovery messages (using book cipher, for example).

Question 5

In which mode is RC4 operated?

Answer 5

RC4 is operated in the output feedback mode (OFB)

Question 6

How does RC4 work?

Answer 6

1. The encryption algorithm generates a pseudo-random sequence RC4(IV, K) that depends only on the key K and an initialization vector IV
2. The plain text is then XORed with the pseudo-random sequence to obtain the cipher text and vice versa.
   C1 = P1 XOR RC4(IV1, K)
   P1 = C1 XOR RC4(IV1, K)

Question 7

What is the keystream?

Answer 7

It is the pseudo-random sequence generated by RC4 based on IV (initialization vector) and K (the key).

Question 8

What is crucial to the security of the RC4 algorithm?

Answer 8

It is crucial to the security of the RC4 algorithm that the keystream is never reused, otherwise we would have two IV (initialization vectors) in which holds that IV1 = IV2 with the same key and then the XOR of two plain text can be obtained.
C1 XOR C2 = P1 XOR RC4(IV,K) XOR P2 XOR RC4(IV, K) = P1 XOR P2

Question 9

What is the key length of the RC4 algorithm?

Answer 9

Since the key is used only as a seed, the key length is variable up to 2048 bit.

Question 10

Explain in details how does RC4 work.

Answer 10

RC4 uses two byte arrays of 256 elements: S[0,255], K[0,255].

Step1: Initialize the arrays
     for all elements of S:
           S[i] = i;
     j := 0
     for i from 0 to 255
         j := (j + S[i] + key[i mod keylength]) mod 256
         swap values of S[i] and S[j]
     endfor

Step 2: generate the key stream
     i := 0
     j := 0
     while GeneratingOutput:
         i := (i + 1) mod 256
         j := (j + S[i]) mod 256
         swap values of S[i] and S[j]
         K := S[(S[i] + S[j]) mod 256]
         output K
     endwhile

Step 3: XOR the keystream with the plain text or the cipher text

Question 11

How is the security of RC4 regarding to brute force attacks?

Answer 11

Trying every possible key using bruteforce:

• the variable key length of up to 2048 bit allows to make this kind of attack impractical.
• By reducing the key length can also be made arbitrarily insecure.

Question 12

How is the security of RC4 regarding to differential and linear cryptanalysis attacks?

Answer 12

RSA claims that RC4 is immune to differential and linear cryptanalysis, and no small cycles are known.

Question 13

Is a 40bit key length secure enough?

Answer 13

NO! It is not secure against brute force attacks

Question 14

Which transfer protocol uses RC4 with a key length of 40bit?

Answer 14

SSL, which lacks security then.

Question 15

There are any known weaknesses of RC4?

Answer 15

Yes, depending on the details of the key scheduling method it leads to severe vulnerabilities.

Question 16

There is any recommendation to turn RC4 more secure?

Answer 16

Yes, to discard the first 3072 bits of the keystream.