Lecture 11 – Cryptography and Blockchain

These flashcards cover essential concepts from Lecture 11, including cryptographic goals, classical ciphers, attack models, hashing and password security, public-key infrastructure, digital signatures, and blockchain fundamentals.

What does the word “cryptography” literally mean based on its Greek roots?

Hidden (crypto) writing (graphy).

Which core security property is provided by limiting access to information?

Confidentiality.

What ancient Greek device implemented one of the first transposition ciphers?

The Scytale (Skytali).

In a Caesar cipher with a shift of 3, what does the letter “A” become?

“D.”

Give the general encryption formula for the Caesar cipher.

E(x,k) = (x + k) mod 26.

Which letter is the most frequent in English plaintext, making it useful for frequency analysis?

The letter “E.”

Name the electromechanical encryption machine famously broken in WWII.

The Enigma Machine.

List the four main goals of cryptography highlighted in the lecture.

Confidentiality, Integrity, Authentication, and Non-Repudiation.

Define “plaintext” in cryptographic terminology.

The original, readable message before encryption.

Define “ciphertext.”

The encrypted, unreadable version of a message.

What is a “cipher”?

An algorithm that transforms plaintext into ciphertext and vice-versa.

What is a “key” in cryptography?

Secret information required to encrypt or decrypt a message.

What must hold for correctness of a cryptosystem?

For every plaintext p and key k, D(E(p,k),k) = p.

Describe a Known-Ciphertext attack model.

The attacker only has access to ciphertexts and tries to recover information or the key.

Describe a Chosen-Plaintext attack model.

The attacker can submit chosen messages to be encrypted and observe the resulting ciphertexts.

How many possible shifts must be tested in a brute-force attack on the Caesar cipher?

25 (or 26 including the identity), giving O(26) time complexity.

What cryptographic property means altering one bit of input yields a completely different hash?

Avalanche effect (shown with SHA-256 examples).

In public-key cryptography, what are the two distinct keys called?

Public key (KU) and private key (KR).

What major drawback does symmetric (private-key) cryptography suffer from?

Securely distributing/ sharing the secret key.

To send an encrypted message to Alice, which key does Bob use in a public-key system?

Alice’s public key.

How can Bob use his private key to provide authentication in a public-key system?

Encrypt (sign) the message with his private key; recipients verify with Bob’s public key.

Name four desired properties of a cryptographic hash function.

Deterministic, Irreversible (one-way), Collision-resistant/Unique, and Fast to compute.

Why should passwords be stored as hashes rather than plaintext?

If the database is breached, attackers cannot directly read the original passwords.

What is a ‘rainbow table’?

A precomputed dictionary of hashes used to reverse weak or unsalted password hashes by lookup.

How does adding a unique salt to each password hash improve security?

It makes identical passwords produce different hashes, thwarting rainbow-table and hash-matching attacks.

What three assurances does a digital signature provide?

Authentication, Integrity, and Non-Repudiation.

Outline the two high-level steps of digital signature verification.

1) Hash the received document. 2) Decrypt the sender’s signature with their public key and compare the two hashes; if they match, the signature is valid.

State the main motivation for blockchain technology expressed in the lecture.

Removing centralized intermediaries in digital transactions to avoid single points of failure, cost, and delay.

Give a concise definition of blockchain.

A distributed, immutable ledger that records transactions across a network of nodes.

List four key properties of blockchain.

Decentralization, Immutability, Transparency, and Consensus.

What three main elements does each block normally contain?

Hash of previous block, transaction data, and a timestamp.

Briefly describe the five-step blockchain transaction flow (Bitcoin example).

1) Broadcast transaction, 2) Validation by nodes, 3) Grouping into a block, 4) Consensus (e.g., PoW) to add the block, 5) Linking the block to the chain via hash.

What is a consensus mechanism in a blockchain?

An algorithm enabling distributed nodes to agree on the ledger’s state without a central authority.

Explain Proof of Work (PoW) in one sentence.

Miners expend computational effort solving a puzzle; the first solver earns the right to add the next block.

Name one major advantage of Proof of Stake (PoS) over Proof of Work.

Energy efficiency (less power consumption).

Give one cryptocurrency that uses Proof of Work and one that uses Proof of Stake.

Bitcoin (PoW) and Ethereum (PoS after “Merge”).

The “Nothing-at-stake” problem is associated with which consensus mechanism?

Proof of Stake (PoS).

In what year was the Bitcoin white paper published?

2008.

What fundamental problem of symmetric encryption does public-key encryption solve?

The key distribution problem.

Define a “cryptosystem.”

A structured suite of cryptographic algorithms with associated key, plaintext, and ciphertext spaces for secure communication.