Symmetric Encryption I

Which two main branches constitute the field of cryptology?

Cryptography and cryptanalysis.

In a symmetric cryptosystem, what is the relationship between the encryption key and the decryption key?

They are the same shared secret key.

Which security goal ensures that a message remains unreadable to unauthorized eavesdroppers?

Confidentiality.

Which security goal ensures that a message has not been altered during transmission?

Integrity.

What security principle ensures that a party in a transaction cannot deny having authorized an action?

Non-repudiation.

Non-repudiation is established if both non-forgeability and _____ are fulfilled.

Non-mutability.

In cryptographic notation, what does the letter M typically represent?

The plaintext message.

In cryptographic notation, what does the letter C typically represent?

The ciphertext.

What is the mathematical consistency requirement for decryption functions (D_K) and encryption functions (E_K)?

D_K(E_K(M)) = M.

How many positions are shifted in a standard Caesar cipher?

Three.

The Caesar cipher encryption formula is C_i = E(M_i) = symbol(M_i) + 3 _____.

mod 26.

How many total unique keys are required for n parties to communicate securely using symmetric encryption?

n(n-1)/2.

In a public-key cryptosystem, which key is used by the sender to encrypt a message for a specific recipient?

The recipient's public key.

In a public-key cryptosystem, which key does the recipient use to decrypt a message encrypted with their public key?

Their private key.

How many total keys are required in a public-key system involving n users?

2n keys.

What is the ideal mathematical complexity for encryption and decryption algorithms?

Linear complexity.

How does the length of the ciphertext typically compare to the length of the plaintext in symmetric schemes?

They are typically the same length.

Mathematically, encryption and decryption are _____ on the set of all n-bit arrays.

Bijections.

If the probability p(i) of a message is 1, what is the resulting information measure I(i)?

0 bits.

What is the estimated entropy (information content) of natural English per character?

1.25 bits.

In the language redundancy formula D = 1 - a, what does a represent?

Entropy.

What is the term for the ciphertext length at which a brute-force attack is expected to yield a unique plaintext?

Unicity distance.

How many total decryption keys must be attempted in a brute-force attack on a k-bit key?

2^k

What type of cipher uniquely replaces each letter in a plaintext with another letter from the alphabet?

A simple substitution cipher.

How many possible keys exist for a simple substitution cipher using a 26-letter alphabet?

26!.

What specific shift value is used in the ROT13 cipher?

13.

Which classical cryptographic tool uses a coordinate system of numbers to represent letters?

The Polybius square.

Which type of cipher combines the Polybius square with substitution techniques?

A polygraphic substitution cipher.

What is the term for modern cryptographic components that perform substitution on binary data?

S-boxes.

How does a Trithemius Progressive Key cipher differ from a standard Caesar cipher?

The shift value increases for every letter in the word.

Why are simple substitution ciphers highly vulnerable to frequency analysis?

Letters in natural language are not uniformly distributed.

What is the Vigenère cipher's primary advantage over the Trithemius progressive key?

It uses a secret key to determine the shifts.

The Vigenère cipher decryption formula is C = (k + m) _____.

mod 26.

What is the only cryptosystem that provides perfect secrecy if its key is truly random and never reused?

The One-Time Pad.

What are the two primary requirements for a Pseudo Random Number Generator (PRNG)?

Uniform distribution and independence.

What does the acronym LCG stand for in the context of PRNGs?

Linear Congruential Generator.

By what logical operation is a plaintext bit combined with a keystream bit in a stream cipher?

XOR.

In which type of stream cipher is the keystream obtained solely from the secret key K?

Synchronous stream cipher.

Which keystream generation method was developed by RSA but is now considered to fall below high security standards?

RC4.

Name one modern stream cipher mentioned as an alternative to RC4.

ChaCha20 (or SEAL, FISH, ISAAC).

What was the result of the Snowden revelations in 2013 regarding global security trends?

A significant increase in interest in security and privacy.

In identity security, what does non-forgeability imply about the communicating partners?

The sender or receiver has a unique signature.

In identity security, what does non-mutability prevent?

The use of a unique signature on a letter that does not belong to the sender.

What is the range of possible shift values for a Caesar cipher using a 26-letter alphabet?

1 to 25.

What is the primary practical use of public-key distribution in modern symmetric sessions?

To distribute the symmetric session key safely.

Which modern symmetric cipher category processes plaintext bits or bytes one at a time?

Stream ciphers.

What are the two classical methods of manipulating plaintext letters?

Substitution and transposition.

What is the term for a cipher that uses a key instead of a progressive sequence of shifts?

Vigenère cipher.

The probability of a valid English message within the total space of n-bit strings is 2 _____.

^(-0.8n).

What logical state is achieved when both forgeability and mutability are prevented?

Non-repudiation.

A synchronous stream cipher generates its keystream using only the _____.

Secret key (K).

Encryption and decryption functions should ideally be _____ for maximum efficiency.

Efficient algorithms.

What is the main classification of ciphers that 'beats' classical frequency analysis?

Modern cryptography.