3rd Cryptography Midterm quiz

Cryptography is the practice and study of hiding and securing information so that only the intended recipient can read or process it. It transforms readable data (plaintext) into an unreadable scrambled format (ciphertext) using mathematical algorithms and keys.

Modern cryptography relies on four main pillars:• Confidentiality: Ensures only authorized people can view the data.• Integrity: Guarantees data has not been changed or tampered with during storage or transfer.• Authentication: Verifies the identity of the sender and receiver.• Non-repudiation: Prevents a sender from denying they sent a

Created by: ssock

A message authentication code (MAC) is attached to a message before it is sent. Which property does a MAC by itself provide?
- Integrity and authenticity: a receiver who holds the shared secret key can be confident the message came from someone who knows that key and was not altered in transit.
- Confidentiality: an eavesdropper who intercepts the message and its tag cannot recover any information about the message.
- Non-repudiation: the sender cannot later deny having sent the message, even to a third party who does not hold the secret key.
- Availability: it guarantees the message will eventually be delivered even if the network drops packets.
In the basic MAC picture, Alice sends Bob a message m along with a tag. What does Bob need in order to check that the tag is valid?
- Only Alice's public key.
- The same secret key k that Alice used to generate the tag.
- A certificate issued by a trusted third party vouching for Alice's identity.
- Nothing beyond the message and tag themselves; the tag is self-verifying.
An early idea for message integrity was to send a message with a CRC (cyclic redundancy check) checksum attached, instead of a proper MAC. Why is this insecure against an active adversary?
- CRC computations are far too slow to use on modern networks.
- CRC produces a longer tag than a proper MAC, wasting bandwidth.
- CRC uses no secret key, so an attacker who modifies the message can simply recompute a matching CRC; CRC is designed to catch accidental errors, not intentional tampering.
- CRC only works on messages shorter than one block, so it cannot protect longer files.
The standard way of defining a secure MAC imagines an attacker who interacts with the MAC before attempting an attack. What is the attacker given during that interaction?
- The secret key k itself, so the definition can test what happens once the key leaks.
- A single ciphertext, with no corresponding tag.
- A promise that the same key will never be reused for any other message.
- Valid tags for messages of the attacker's own choosing: for messages m1, m2, ..., the attacker receives the genuine tags t1, t2, ... produced under k.
A source machine sends AES-CBC packets under a fresh random IV to a server, which decrypts each one and forwards the payload to whichever port the packet header names. There is no MAC. What can an attacker who knows the header layout do to a packet in transit?
- Recover the key K after observing enough packets.
- Read the payload without holding the key.
- Change the destination port so the server forwards the decrypted payload to him.
- Nothing, because CBC with a random IV is provably secure.
Why does altering the IV let the attacker control what the first plaintext block decrypts to?
- CBC decryption XORs the IV directly into the first plaintext block, so flipping a bit of the IV flips the same bit of that block.
- The IV is encrypted under a weaker key than the payload.
- The IV is used as the encryption key for the first block.
- The server reuses the IV as an integrity check value.
Which property does authenticated encryption NOT provide on its own?
- Confidentiality of the payload.
- Detection of a ciphertext modified in transit.
- Protection against chosen ciphertext attacks.
- Protection against replay of a ciphertext the sender genuinely produced.
In the chosen ciphertext attack model, what is the adversary given that he does not have in the weaker chosen plaintext model?
- The ability to obtain encryptions of messages he chooses.
- The ability to obtain decryptions of ciphertexts he chooses, excluding only the challenge ciphertext.
- The encryption key itself.
- The ability to choose the nonce used for each message.
Which protocol combined the two operations in the order the lecture identifies as the correct one?
- SSL/TLS
- SSH
- All three used the same order.
- IPsec
Why is encrypt-then-MAC the preferred order?
- The tag covers exactly the bytes that arrived, so a forged ciphertext can be rejected before the decryption function is ever invoked.
- It produces a shorter ciphertext than the alternatives.
- It allows a single key to be used for both operations safely.
- It authenticates the plaintext, which is what the receiver ultimately cares about.
What is the weakness of computing the authentication tag over the plaintext and sending it alongside the ciphertext, as SSH did?
- The tag cannot be verified at all without first decrypting.
- It makes the ciphertext malleable in the way the tampering attack exploited.
- The tag is a function of the plaintext and travels in the clear, so it can leak information -- identical plaintexts produce identical tags.
- It requires three separate keys instead of two.
In an AEAD scheme, how is the associated data treated?
- It is encrypted but not authenticated.
- It is authenticated but not encrypted.
- It is neither encrypted nor authenticated; it is simply carried along.
- It is encrypted twice, once under each key.
Authenticated encryption was formalized as a notion only around 2000; before that, cryptographic libraries typically offered encryption and MAC as separate tools and left developers to combine them.
- True
- False
In the tampering attack on the unauthenticated CBC system, the attacker must first recover the encryption key K before he can redirect the packet.
- True
- False
When building an authenticated-encryption scheme by hand, the encryption and MAC operations should share one key so that the two stay synchronized.
- True
- False
Fill in the blank. GCM combines CTR-mode encryption with a ____________ MAC, computed over the ciphertext.
- Carter Wegman
- CBC
- HMAC
- Davies-Meyer
Fill in the blank. The mode that combines a CBC-MAC with CTR-mode encryption, and that secures modern Wi-Fi under the 802.11i standard, is ____________.
- GCM
- EAX
- CCM
- OCB
Fill in the blank. The adversary's goal in the chosen ciphertext attack model is to break ____________.
- the key schedule of the underlying cipher
- collision resistance
- the birthday bound
- semantic security
Every password-storage scheme in this lecture is judged against a single starting assumption. What is that assumption?
- An attacker can watch network traffic but never reaches the server itself.
- The password file will be stolen.
- Users will choose strong passwords if the policy requires it.
- The encryption key can be kept secret indefinitely.
A team proposes storing E(k, password) for every account, encrypted under one system key. Several objections are raised. Which one is the decisive argument for hashing instead?
- Encryption is slower than hashing.
- Encryption algorithms have historically been subject to export controls.
- Encrypted values cannot be compared to check a login.
- Verifying a password never requires recovering it, so storing something reversible takes on risk for no benefit.
A password file stores each password encrypted with a stream-style mode under a single key. Before an attacker does anything at all to the key, what has the file already given away?
- The length of every password, and which users share the same password.
- Nothing. The file is encrypted.
- The plaintext of the most commonly used passwords.
- The salt belonging to each user.
In the early UNIX scheme, how was the value stored for each user computed?
- The password was encrypted twenty-five times under a system-wide key.
- The password was hashed with MD5 and the result truncated to 56 bits.
- The password was used as the key to encrypt an all-zero block, twenty-five times over.
- The password was XORed with a twelve-bit salt and the result stored directly.
Why did the early UNIX scheme effectively ignore everything a user typed after the eighth character?
- The password file format reserved exactly eight bytes for each entry.
- DES takes a 56-bit key, and 56 bits is eight characters of seven-bit ASCII.
- Longer passwords were truncated to prevent buffer overflows.
- The twelve-bit salt left room for only eight characters.
An attacker has stolen a salted password file and wants Alice's password specifically. How much does the salt slow him down?
- Completely. Without knowing the salt he cannot test any guess.
- About 2 to the 12th power times, because the salt is twelve random bits.
- Twice as much, since he must now hash the salt as well as the password.
- Not much. Her salt is in the same file he stole, so guessing with it costs what guessing without it cost.
State precisely what adding a distinct per-user salt accomplishes.
- It makes each individual password harder for an attacker to guess.
- It keeps the stored password file unreadable to anyone lacking the salt.
- It destroys reusable precomputation, and it hides which accounts share a password.
- It slows every hash computation down by a fixed factor.
A server stores x = H^100(s) for Alice. She logs in by sending p = H^99(s); the server checks that H(p) equals x, accepts her, and replaces x with p. What must she send at her next login?
- H^98(s)
- H^100(s)
- H^99(s) again
- The secret s itself
A website that is able to email you your existing password must be storing that password in a form it can reverse.
- True
- False
With a twelve-bit salt, an offline dictionary attack against an entire password file becomes roughly 2 to the 12th power times more expensive.
- True
- False
In the hotel-door scheme, a guest who enters the room many times will eventually advance the lock past their own key and be locked out.
- True
- False
Fill in the blank. In the hotel-door scheme, the lock unlocks AND advances its stored value x only when ____________ equals x.
- k, the key on the guest's card
- H(k)
- H(x)
- H(H(k))
Fill in the blank. The 2009 RockYou breach exposed about 32 million passwords; the single most common one, chosen by more than 290,000 accounts, was ____________.
- letmein
- 123456
- password
- qwerty
Fill in the blank. Modern systems store passwords using deliberately slow, memory-hard functions such as bcrypt, scrypt, and ____________.
- SHA-256
- CRC32
- Argon2
- HMAC

3rd Cryptography Midterm quiz

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