4. Conventional Cryptography

Question 1

What is symmetric encryption?

Answer 1

Symmetric encryption is also known as private-key encryption and is encryption where one key is used for both encryption and decryption of data. All classial encryption algorithms are private-key and private-key is by far the most widely used.

Question 2

What is the plaintext?

Answer 2

The original message before encryption / after decryption.

Question 3

What is the ciphertext?

Answer 3

The encrypted message after encryption / before decryption.

Question 4

What is a cipher?

Answer 4

A cipher is an algorithm used for transforming plaintext to ciphertext.

Question 5

What is a key?

Answer 5

A key is information used in a cipher known only to the sender/receiver.

Question 6

What is encipher?

Answer 6

Encipher is converting plaintext to ciphertext.

Question 7

What is decipher?

Answer 7

Recovering ciphertext from plaintext.

Question 8

What is cryptography?

Answer 8

Cryptography is the study of encryption principles / methods.

Question 9

What is cryptanalysis (codebreaking) ?

Answer 9

Cryptanalysis is the study of principlees / methods of deciphering ciphertext without knowing the key used to encipher it.

Question 10

What is cryptology?

Answer 10

Cryptology is the field of both crypography and cryptanalysis.

Question 11

What are the requirements for secure use of a symmetric encryption algorithm?

Answer 11

• A strong encryption algorithm.
• A secret key known only to the sender and receiver.

A secure channel to distribute the key is implied.

Question 12

How can the cryptographic system of an algorithm be characterized?

Answer 12

• Type of encryption operations used - substitution, transposition, product.
• Number of keys used - single key / two key.
• Way in which the plaintext is processed - block / stream.

Question 13

What is a brute force search?

Answer 13

A brute force search is an attempt to attempt every key possible until the correct one is found.

It is the most basic attack, is proportional to key size and assumes that the user either knows or can recognise the plaintext.

Question 14

What are the design elements of a cipher?

Answer 14

• Block Size
• Key Size
• Number of rounds
• Subkey generation algorithm
• Round Function
• Fast softwrae en/decryption
• Ease of analysis

Question 15

Name 3 symmetric encryption algorithms.

Answer 15

• DES (Data Encryption Standard)
• Triple DES (Triple Data Encryption Standard)
• AES (Advanced Encryption Standard)