Cryptography Multiple Choice Questions & Answers on “RC4 and RC5”.
1. RC5 encryption uses Right shift and decryption uses Left shift.
a) True
b) False
Answer: b
Clarification: RC5 encryption rounds use left shift operations and the decryption uses right shift operations.
2. Till when was the RC4 algorithm kept a secret?
a) 1990
b) 1992
c) 1996
d) 1994
Answer: d
Clarification: In September 1994, the RC4 algorithm was anonymously posted on the Internet on the Cypherpunks anonymous remailers list.
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)
Answer: b
Clarification:
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];
4. “RC5 uses the Feistel Structure.”
a) True
b) False
Answer: b
Clarification: 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];
Answer: a
Clarification: 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
c) RC5-Cipher Padding
d) RC5-CipherText Stealing
Answer: c
Clarification: RFC 2040 [BALD96] defines four different modes of operation:
1. RC5 block cipher
2. RC5-CBC
3. RC5-CBC-Pad
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
c) RC5-Cipher Block Chaining Pad
d) RC5-CipherText Stealing
Answer: c
Clarification: The Cipher Block Chaining Pad mode produces a ciphertext output as such.
8. RC4 algorithm uses the concept of Block Cipher System.
a) True
b) False
Answer: b
Clarification: RC4 algorithm uses the concept of Stream Cipher and uses a “stream key”.
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
Answer: c
Clarification:
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
Answer: c
Clarification: RC4 is used in Secure Socket Layer, Transport Layer Security and Wired Equivalency Privacy. Not used in File Transfer Protocol.