Applied Cryptography – Lecture 0x02

Question-and-answer flashcards covering key terms, algorithms, attacks and concepts from the Applied Cryptography lecture.

What are the four primary goals of cryptography?

Confidentiality, Integrity, Authenticity, and Non-Repudiation.

What is the difference between cryptography, cryptology, and cryptanalysis?

Cryptography is the practical engineering of encryption systems, cryptology is the mathematical study of them, and cryptanalysis is the art of breaking them.

In symmetric encryption, what is shared between sender and receiver?

A single secret key used for both encryption and decryption.

Why is availability NOT considered a goal of cryptography?

Cryptography focuses on protecting data (confidentiality, integrity, authenticity, non-repudiation) rather than ensuring systems are reachable.

Explain Kerckhoffs’s principle in one sentence.

An encryption system should remain secure even if everything about it except the key is public knowledge.

What Boolean operation underlies many stream ciphers and demonstrates perfect reversibility?

The XOR (exclusive OR) operation.

State two critical requirements for a one-time pad to provide perfect secrecy.

The key must be truly random and at least as long as the plaintext, and each key is used only once.

Why is an N-time pad (re-using a one-time-pad key) insecure?

Because XORing two ciphertexts encrypted with the same key reveals the XOR of the two plaintexts, enabling statistical recovery of both messages and the key.

Differentiate between block ciphers and stream ciphers.

Block ciphers encrypt fixed-size blocks of data; stream ciphers encrypt data bit-by-bit (or byte-by-byte) and are typically faster but generally considered less robust.

Name three common block cipher modes of operation.

ECB (Electronic Code Book), CBC (Cipher Block Chaining), and CTR (Counter) mode.

Why is ECB mode usually discouraged in practice?

Identical plaintext blocks produce identical ciphertext blocks, revealing patterns in the data.

What advantage does CBC mode provide over ECB?

It chains each ciphertext block with the next plaintext block using an IV, so repeated plaintext does not create repeated ciphertext.

What is a nonce in CTR mode and why is it important?

A unique value combined with a counter to ensure each keystream block is unique; re-using a nonce compromises security.

Give the key sizes for DES, 3DES, and AES (standard options).

DES – 56 bits, 3DES – 168/112/56 bits, AES – 128, 192, or 256 bits (block size 128 bits).

What mathematical problem underpins Diffie-Hellman key exchange?

The difficulty of computing discrete logarithms in modular arithmetic (finding a from A = g^a mod p).

Describe the main purpose of Diffie-Hellman.

To allow two parties to agree on a shared secret key over an insecure channel without prior shared secrets.

Why does symmetric key management scale poorly with many users?

N users need N(N-1)/2 unique shared keys, which grows quadratically.

How does asymmetric encryption solve key-distribution problems?

Each user publishes a public key for encryption while keeping a private key for decryption, eliminating the need for pairwise secret exchange.

What hard mathematical problem secures RSA?

Factoring a large composite number n into its prime factors p and q.

In RSA, what is signed to create a digital signature?

A message (or its hash) is encrypted with the sender’s private key.

Define a cryptographic hash function.

A one-way, fixed-length output function that is pre-image resistant, second-pre-image resistant, and collision resistant.

Give two popular but now-broken hash functions mentioned in the lecture.

MD5 and SHA-1 (both have practical collision attacks).

What real-world incident demonstrated MD5 collision attacks?

The Flame malware (2012) forged a Microsoft code-signing certificate using an MD5 collision.

Why are Certificate Authorities (CAs) essential in public key infrastructures?

They bind public keys to entity identities by issuing digitally signed certificates that others can verify.

Outline the three-step TLS handshake simplified in the lecture.

1) Client requests TLS and receives server certificate, 2) Client verifies certificate with CA, 3) Client sends session key encrypted with server’s public key; secure channel begins.

What four components make up a TLS cipher suite string?

(1) Key Exchange, (2) Authentication, (3) Symmetric Cipher/Mode, (4) Message Digest.

Give recommended key sizes from the lecture for symmetric, asymmetric, and hash algorithms.

Symmetric: AES-256, Asymmetric: RSA-2048, Hash: SHA-256 or SHA-512.

Why is simply encrypting stored passwords a bad practice?

If the encryption key is compromised, all passwords are exposed; plus identical passwords encrypt to identical ciphertext.

What is salting in password hashing?

Adding a random value to each password before hashing to prevent pre-computed and rainbow-table attacks.

Does salting make brute-forcing a single password harder?

No; it mainly thwarts large-scale pre-computation attacks but not per-password brute force.

Name two slow, adaptive password-hashing algorithms recommended in the lecture.

BCrypt and Argon2 (also SCrypt, PBKDF2).

How do slow hashes defend against GPU-accelerated cracking?

By iterating the hash thousands of times and/or requiring large memory, they drastically reduce guesses per second and hinder parallelisation.

What is a rainbow table?

A pre-computed table mapping plaintext passwords to their hashes, used to reverse unsalted hashes quickly.

Which properties of XOR make it suitable for encryption?

It is reversible (A⊕K⊕K = A), commutative, and associative, allowing decryption by applying the same key again.

Explain Shannon’s maxim in cryptography.

‘The enemy knows the system’; security must rely only on the secrecy of keys, not algorithms.

What is frequency analysis and which classical ciphers are vulnerable to it?

A technique using character frequency statistics to break substitution and Vigenère ciphers.

How does a transposition cipher differ from a substitution cipher?

Transposition ciphers rearrange the positions of characters without altering them; substitution ciphers replace characters with others.

Why is ECB mode still occasionally used despite its weakness?

It is simple, parallelisable, and sometimes acceptable for random or non-patterned data, though generally discouraged.

What does the term ‘nonce’ stand for and what is its critical property?

‘Number used once’; it must be unique for each encryption operation to prevent keystream reuse.

List two common password-cracking tools mentioned in the lecture.

John the Ripper and Hashcat (offline); THC-Hydra and Brutus (online).