# Symmetric Ciphers Questions and Answers – RC4 and RC5 – II

This set of Cryptography Multiple Choice Questions & Answers focuses on “RC4 and RC5”.

1. RC5 encryption uses Right shift and decryption uses Left shift.
a) True
b) False

Explanation: RC5 encryption rounds use left shift operations and the decryption uses right shift operations.

2. The below round is the last step in encryption /decryption in RC5.

a) True
b) False

Explanation: The last round is the last step in encryption but the first step in decryption process.

3. The code bellow is the RC5 encryption pseudo code in C language.
What is the error?
LE0 = A + S[0];
RE0 = B + S [1];
for i = 1 to r do
LEi = ((LEi-1 ⊕ REi-1) >>> REi-1) + S [2xi];
REi = ((REi-1 ⊕ LEi) <<< LEi) + S [2xi+1];

a) The left shift operation (should be right shift)
b) The right shift operation (should be left shift)
c) LEi-1 and REi-1 should be LEi and REi
d) The for loop runs from 1 to r-1 ( not 1 to r)

Explanation:
Correct code –
LE0 = A + S[0];
RE0 = B + S [1];
for i = 1 to r do
LEi = ((LEi-1 ⊕ REi-1) <<<REi-1) + S [2xi];
REi = ((REi-1 ⊕ LEi) <<< LEi) + S [2xi+1];
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4. “RC5 uses the Feistel Structure.”
a) True
b) False

Explanation: RC5 does not use the classic Feistel structure.

5. Find the error in the Decryption pseudo code for RC5 –
for i = 1 to r do
RDi-1 = ((RDi – S [2xi+1] >>> LDi ) ⊕ LDi);
LDi-1 = ((LDi – S [2xi] >>> RDi-1) ⊕ RDi-1);
B= RD0 + S [1];
A = LD0 – S [0];

a) B= RD0 + S [1];
b) for i = 1 to r do
c) LDi-1 = ((LDi – S [2xi] >>> RDi-1) ⊕ RDi-1);
d) A = LD0 – S [0];

Explanation: B= RD0 + S [1]; is incorrect as there should be a ‘-’ instead.

6. Which one of the following is not a RC5 mode of operation?
a) RC5 block cipher
b) RC5-Cipher Block Chaining
d) RC5-CipherText Stealing

Explanation: RFC 2040 [BALD96] defines four different modes of operation:
1. RC5 block cipher
2. RC5-CBC
4. RC5-CTS.

7. Which RC5 mode will have the ciphertext longer than the plaintext by at most the size of a single RC5 block?
a) RC5 block cipher
b) RC5-Cipher Block Chaining
d) RC5-CipherText Stealing

Explanation: The Cipher Block Chaining Pad mode produces a ciphertext output as such.

8. Which RC5 mode of operation is this?

a) RC5 block cipher
b) RC5-Cipher Block Chaining
d) RC5-CipherText Stealing

Explanation: This mode of operation is the RC5 CTS mode.

9. Which of these is not a characteristic of block ciphers?
a) Variable key length / block size / number of rounds
b) Mixed operators, data/key dependent rotation
c) Key independent S-boxes
d) More complex key scheduling

Explanation:
Features seen in modern block ciphers are:
– Variable key length / block size / number of rounds
– Mixed operators, data/key dependent rotation
– Key dependent S-boxes
– More complex key scheduling
– Operation of full data in each round
– Varying non-linear functions.

10. Which one of the following RC4 algorithm not used in?
a) SSL
b) TLS
c) FTP
d) WEP

Explanation: RC4 is used in Secure Socket Layer, Transport Layer Security and Wired Equivalency Privacy. Not used in File Transfer Protocol.

11. Till when was the RC4 algorithm kept a secret?
a) 1990
b) 1992
c) 1996
d) 1994

Explanation: In September 1994, the RC4 algorithm was anonymously posted on the Internet on the Cypherpunks anonymous remailers list.

12. RC4 algorithm uses the concept of Block Cipher System.
a) True
b) False

Explanation: RC4 algorithm uses the concept of Stream Cipher and uses a “stream key”.

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