Symmetric Key Algorithms Questions and Answers

Q: Advanced Encryption Standard (AES) is based on :

AES is based on a Symmetric key algorithm.The Advanced Encryption Standard (the Rijndael cipher, adopted by NIST in 2001) uses the same secret key to encrypt and to decrypt. It is a block cipher with a 128-bit block and keys of 128, 192 or 256 bits.Asymmetric (public key) algorithms such as RSA use a pair of different keys, and key exchange protocols such as Diffie-Hellman establish a shared key rather than encrypting data, so neither describes AES.

Q: In symmetric key cryptography, if every person in a group of 10 people needs to communicate with every other person in the group, how many secret keys are required ?

The group needs 45 secret keys.In symmetric cryptography each pair of people shares one secret key. The number of pairs among n people is\[ \frac{n(n - 1)}{2} = \frac{10 \times 9}{2} = 45 \]90 counts each pair twice (once for each person), 9 is the number of keys one person needs, and 10 is just the number of people.

Q: In symmetric-key cryptography, if every person in a group of 10 people needs to communicate with every other person in another group of 10 people, how many minimum secret keys are needed ?

The number of keys is 100.In symmetric-key cryptography each communicating pair needs a secret key known only to the two of them; if one key were shared by more people, any holder could read the others' messages. Every one of the 10 people in the first group talks to every one of the 10 in the second:\[ 10 \times 10 = 100 \]People in the same group are not said to talk to each other, so the formula n(n − 1)/2 for one group of n does not apply. A figure of 10 would give each person a single key, and 1 a single shared key, neither of which keeps pairs private. 50 matches no counting.

Q: Which type of encryption is preferred to encrypt the data stored on a conventional hard-disk ?

Symmetric-key cryptography is preferred.Disk encryption has to encrypt and decrypt large volumes of data quickly, on every read and write. Symmetric ciphers such as AES do this efficiently and are what full-disk encryption systems use, with the same key used to encrypt and decrypt.Asymmetric cryptography is far slower and is used for key exchange and digital signatures, not bulk data (at most it protects the symmetric key). Hashing is one-way, so data hashed could never be read back. Quantum cryptography concerns key distribution over communication channels, not data at rest.

Q: The key length of a 3-key Triple Data Encryption Standard (3TDES) is :

The key length of 3-key Triple DES is 168 bits.Triple DES applies DES three times (encrypt-decrypt-encrypt) with keys K1, K2 and K3. Each DES key has 56 effective bits (64 bits including 8 parity bits), so with three independent keys:\[ 3 \times 56 = 168 \text{ bits} \]112 bits is the 2-key variant, where K3 = K1. 256 and 512 bits are not Triple DES key sizes (256 bits is the largest AES key).

Q: In symmetric-key cryptography, how many keys are needed for more secure communication if Alice and Bob want to communicate with each other ?

The keyed answer is 2.In symmetric-key cryptography, the same secret key is used for encryption and decryption, and in principle Alice and Bob can share a single key for both directions. For more secure communication, each direction uses its own key: one key for messages from Alice to Bob and another for messages from Bob to Alice. Then a compromise of one key exposes only one direction of the conversation. That makes two keys.3, 4 and 8 have no basis: even in public-key cryptography the two of them would hold four keys in all (a public and a private key each), and symmetric communication between a pair never needs more than one key per direction.

Q: The Advanced Encryption Standard (AES) was designed by

The key's answer is the National Institute of Standards and Technology.AES was produced by NIST, the US standards body, which ran an open international competition from 1997, chose the winning cipher in 2000 and published it as the Advanced Encryption Standard (FIPS 197) in 2001. The cipher itself, Rijndael, was devised by the Belgian cryptographers Joan Daemen and Vincent Rijmen, who are not among the options; the key credits the standard to the body that defined it.IBM designed the earlier DES cipher, not AES, and HP and Intel had no part in it.

Advanced Encryption Standard (AES) is based on :

A.Asymmetric key algorithm.
B.Symmetric key algorithm
C.Public key algorithm
D.Key exchange

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: B

AES is based on a Symmetric key algorithm.

The Advanced Encryption Standard (the Rijndael cipher, adopted by NIST in 2001) uses the same secret key to encrypt and to decrypt. It is a block cipher with a 128-bit block and keys of 128, 192 or 256 bits.

Asymmetric (public key) algorithms such as RSA use a pair of different keys, and key exchange protocols such as Diffie-Hellman establish a shared key rather than encrypting data, so neither describes AES.

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In symmetric key cryptography, if every person in a group of 10 people needs to communicate with every other person in the group, how many secret keys are required ?

A.9
B.90
C.45
D.10

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

The group needs 45 secret keys.

In symmetric cryptography each pair of people shares one secret key. The number of pairs among n people is

\[ \frac{n(n - 1)}{2} = \frac{10 \times 9}{2} = 45 \]

90 counts each pair twice (once for each person), 9 is the number of keys one person needs, and 10 is just the number of people.

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In symmetric-key cryptography, if every person in a group of 10 people needs to communicate with every other person in another group of 10 people, how many minimum secret keys are needed ?

A.1
B.10
C.100
D.50

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

The number of keys is 100.

In symmetric-key cryptography each communicating pair needs a secret key known only to the two of them; if one key were shared by more people, any holder could read the others' messages. Every one of the 10 people in the first group talks to every one of the 10 in the second:

\[ 10 \times 10 = 100 \]

People in the same group are not said to talk to each other, so the formula n(n − 1)/2 for one group of n does not apply. A figure of 10 would give each person a single key, and 1 a single shared key, neither of which keeps pairs private. 50 matches no counting.

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Which type of encryption is preferred to encrypt the data stored on a conventional hard-disk ?

A.Symmetric-key cryptography
B.Asymmetric-key cryptography
C.Hashing
D.Quantum cryptography

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: A

Symmetric-key cryptography is preferred.

Disk encryption has to encrypt and decrypt large volumes of data quickly, on every read and write. Symmetric ciphers such as AES do this efficiently and are what full-disk encryption systems use, with the same key used to encrypt and decrypt.

Asymmetric cryptography is far slower and is used for key exchange and digital signatures, not bulk data (at most it protects the symmetric key). Hashing is one-way, so data hashed could never be read back. Quantum cryptography concerns key distribution over communication channels, not data at rest.

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The key length of a 3-key Triple Data Encryption Standard (3TDES) is :

A.112 bits
B.256 bits
C.512 bits
D.168 bits

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: D

The key length of 3-key Triple DES is 168 bits.

Triple DES applies DES three times (encrypt-decrypt-encrypt) with keys K1, K2 and K3. Each DES key has 56 effective bits (64 bits including 8 parity bits), so with three independent keys:

\[ 3 \times 56 = 168 \text{ bits} \]

112 bits is the 2-key variant, where K3 = K1. 256 and 512 bits are not Triple DES key sizes (256 bits is the largest AES key).

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In symmetric-key cryptography, how many keys are needed for more secure communication if Alice and Bob want to communicate with each other ?

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: A

The keyed answer is 2.

In symmetric-key cryptography, the same secret key is used for encryption and decryption, and in principle Alice and Bob can share a single key for both directions. For more secure communication, each direction uses its own key: one key for messages from Alice to Bob and another for messages from Bob to Alice. Then a compromise of one key exposes only one direction of the conversation. That makes two keys.

3, 4 and 8 have no basis: even in public-key cryptography the two of them would hold four keys in all (a public and a private key each), and symmetric communication between a pair never needs more than one key per direction.

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The Advanced Encryption Standard (AES) was designed by

A.National Institute of Standards and Technology
B.IBM
C.HP
D.Intel

Exam Year(s)

Tripura Public Service Commission - 2019,

Answer: A

The key's answer is the National Institute of Standards and Technology.

AES was produced by NIST, the US standards body, which ran an open international competition from 1997, chose the winning cipher in 2000 and published it as the Advanced Encryption Standard (FIPS 197) in 2001. The cipher itself, Rijndael, was devised by the Belgian cryptographers Joan Daemen and Vincent Rijmen, who are not among the options; the key credits the standard to the body that defined it.

IBM designed the earlier DES cipher, not AES, and HP and Intel had no part in it.

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AES is a round cipher based on the Rijndal Algorithm that uses a 128-bit block of data. AES has three different configurations. ______ rounds with a key size of 128 bits, ______ rounds with a key size of 192 bits and ______ rounds with a key size of 256 bits.

A.5, 7, 15
B.10, 12, 14
C.5, 6, 7
D.20, 12, 14

Exam Year(s)

UGC NET - 2012,

Answer: B

AES, based on the Rijndael algorithm, has 10 rounds with a 128-bit key, 12 rounds with a 192-bit key and 14 rounds with a 256-bit key, for a 128-bit block of data. The larger key needs more rounds to give the extra strength.

The other sets, such as 5, 7, 15 or 5, 6, 7 or 20, 12, 14, do not give the true number of rounds, and the 128 bit key does not need 20 rounds. Only 10, 12, 14 is correct.

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