Cryptography Test on “Pseudorandom Number Generators and Stream Ciphers”.
1. Which mode is less prone to decryption : PRNG using CTR / PRNG using OFB?
a) OFB
b) CTR
c) Both are equally prone
d) Both can’t be decrypted
Answer: b
Clarification: CTR mode is more prone than the OBF mode.
2. ANSI X9.17 uses a seed of size
a) 56 bits
b) 64 bits
c) 32 bits
d) 128 bits
Answer: b
Clarification: The seed in ANSI X9.17 is of 64 bits and the randomly generated keys are of size 56 bits.
3. How many 3DES blocks are present in the ANSI X9.17 PRNG?
a) 2
b) 3
c) 4
d) 5
Answer: b
Clarification: There are 3 3DES blocks in the ANSI X9.17 PRNG.
4. How many DES encryptions take place in the ANSI X9.17 PRNG?
a) 3
b) 6
c) 9
d) 12
Answer: d
Clarification: Since there are 3 block, 12 DES encryptions take place.
5. How many keys are used in the ANSI X9.17 PRNG?
a) 3
b) 2
c) 4
d) 6
Answer: a
Clarification: 2 keys are used for the 3DES Encrypt Decrypt Encrypt method.
6. Which of the following PRNGs is used in most recent Intel Chips?
a) ANSI X9.17 PRNG
b) NIST CTR_DRBG
c) ANSI standard X9.82
d) None of the mentioned
Answer: b
Clarification: NIST CTR_DRBG is the PRNG used in most Intel Chips.
7. For the NIST CTR_DRBG , Seed length =
a) Output Block Length
b) Reseed Interval
c) Reseed Interval + Output Block Length
d) Output Block Length + Key Length
Answer: d
Clarification: Seed length = Output Block Length + Key Length
8. The CTR mode uses a Key K and an Initial Vector V. The Intel Digital Random Number generator has these values initially as
a) K = 0 ; V = 1
b) K = 0 ; V = 0
c) K = 1 ; V = 1
d) K = 1 ; V = 0
Answer: b
Clarification: K = 0 ; V = 0 for Intel Digital Random Number generator. (Initial Values)
9. What is the Seed length (seedlen) length for AES-192 ?
a) 428
b) 384
c) 320
d) 512
Answer: c
Clarification: Seed length = Output Block Length + Key Length = 128 + 192.
10. What is the Seed length (seedlen) length for 3DES?
a) 284
b) 260
c) 232
d) 216
Answer: c
Clarification: Seed length = Output Block Length + Key Length = 64 + 232.
11. The Generate function precedes the Update phase in CTR_DRBG.
a) True
b) False
Answer: a
Clarification: The Generate function comes after the Update Phase function.
12. With reference to stream ciphers, the output of the generator is called
a) Byte Stream
b) Re-Seed Interval
c) Key Length
d) Keystream
Answer: d
Clarification: The generator produces the Keystream.
13. Which of the following is not a valid design parameter to be considered in designing stream ciphers?
a) Keystream should be truly as random as possible
b) Encryption sequence should have a large value
c) Output of the PRNG (the key) should be sufficiently large
d) All of the mentioned are valid points that should be considered while designing stream cipher blocks
Answer: d
Clarification: a, b and c are all valid design parameters.