Cryptography Applications Flashcards

Question 1

What is the difference between symmetric and asymmetric encryption?

Accepted Answer

Symmetric encryption uses the same secret key to encrypt and decrypt data, so both parties must already share that key securely. Asymmetric encryption uses a mathematically linked key pair, where a public key encrypts or verifies and a private key decrypts or signs. Symmetric methods are fast, while asymmetric methods simplify key sharing.

Question 2

What is the purpose of a cryptographic hash function?

Accepted Answer

A cryptographic hash function takes any input and produces a fixed-length digest that acts like a fingerprint of the data. Good hash functions are one-way, so the input is hard to recover from the output, and collision resistant, so two different inputs rarely produce the same digest. Hashes help detect whether data has changed.

Question 3

What is the difference between encryption and encoding?

Accepted Answer

Encryption transforms data so that only someone with the correct key can read it, which provides confidentiality. Encoding, such as Base64, converts data into a different format for transport or storage and needs no secret at all, so anyone can reverse it. Encoding offers no security, and it should never be mistaken for encryption.

Question 4

What do confidentiality, integrity, and authentication mean in security?

Accepted Answer

Confidentiality means only authorized parties can read protected data. Integrity means data cannot be altered without detection. Authentication means the parties can confirm who they are communicating with or who created a message. Many real systems need all three, and a tool that provides one does not automatically provide the others.

Question 5

What does Kerckhoffs's principle say about the security of a cipher?

Accepted Answer

Kerckhoffs's principle states that a cryptosystem should remain secure even if everything about it is public except the secret key. Security should depend on keeping the key secret, not on hiding the algorithm. Publicly reviewed, widely tested algorithms are therefore trusted more than secret designs, because many experts can search for weaknesses.

Question 6

What is a message authentication code and what problem does it solve?

Accepted Answer

A message authentication code, or MAC, is a tag computed from a message and a shared secret key. The receiver recomputes the tag with the same key, and a match shows the message was not altered. A MAC provides integrity and authenticity, but it does not hide the message.

Question 7

What is a nonce, and why should it never repeat for the same key?

Accepted Answer

A nonce is a number used only once, often random or a counter, included in a cryptographic operation to make each use unique. Reusing a nonce with the same key can break security, for example by letting an attacker compare ciphertexts or replay messages. Nonces help protocols resist replayed messages.

Question 8

What is the difference between a block cipher and a stream cipher?

Accepted Answer

A block cipher encrypts fixed-size blocks of data, typically 128 bits, and uses a mode of operation to handle longer messages. A stream cipher generates a keystream and combines it with plaintext one bit or byte at a time. Both are symmetric, so key management matters.

Question 9

What is the main purpose of Transport Layer Security?

Accepted Answer

Transport Layer Security, or TLS, protects data as it travels between a client and a server over a network. It provides confidentiality and integrity for the connection and lets the client verify the server's identity using certificates. TLS is the protocol behind HTTPS, which is used for most secure web browsing.

Question 10

Why do secure protocols usually combine asymmetric and symmetric encryption?

Accepted Answer

Asymmetric encryption solves the problem of establishing a shared secret between strangers, but it is slow for large amounts of data. Protocols therefore use it during a handshake to agree on or transport a session key, then switch to fast symmetric encryption for the actual traffic. This combination is called hybrid encryption.

Question 11

What is forward secrecy and why does it matter?

Accepted Answer

Forward secrecy means that each session uses temporary keys, so compromising a long-term private key does not expose past recorded sessions. Protocols use ephemeral key exchange, generating fresh key pairs for each connection and discarding them. It matters because attackers who record encrypted traffic cannot decrypt it later by stealing a key.

Question 12

What does end-to-end encryption protect against that transport encryption does not?

Accepted Answer

End-to-end encryption keeps messages encrypted from the sender's device to the recipient's device, so servers in between cannot read content. Transport encryption protects data only on each hop, so the service handling the message can see it. Metadata, such as who talks to whom, can still be visible.

Question 13

How does a VPN use cryptography to protect network traffic?

Accepted Answer

A virtual private network creates an encrypted tunnel between a device and a VPN server, so traffic crossing an untrusted network is unreadable to observers. Both endpoints authenticate each other and encrypt every packet in the tunnel. A VPN protects the path, but it does not automatically secure the applications using it.

Question 14

What is a session key and why is it temporary?

Accepted Answer

A session key is a symmetric key generated for a single communication session, such as one browsing connection or one call. Because it exists only for that session and is discarded afterward, a leaked session key exposes only that session's data. Fresh keys each time also reduce how much data any one key protects.

Question 15

How do secure protocols prevent a replayed message from being accepted?

Accepted Answer

Protocols add unique values such as nonces, sequence numbers, or timestamps to messages, so the receiver can detect when an old message is sent again. The receiver tracks which values it has already accepted and rejects duplicates or messages that fall outside an expected window. Replay defenses rely on integrity protection covering those values.

Question 16

What is a downgrade attack on a secure connection?

Accepted Answer

A downgrade attack tricks two parties into negotiating an older, weaker protocol version or cipher suite than they both support. A network attacker may alter handshake messages so that a weak option is chosen, then exploit its flaws. Defenders reduce this risk by disabling obsolete versions and protecting the negotiation itself with integrity checks.

Question 17

What does a digital signature provide that a message authentication code cannot?

Accepted Answer

A digital signature provides non-repudiation, meaning anyone holding the signer's public key can verify it, and only the private key holder could have created it. A MAC uses a shared secret, so either key holder could have produced it. Signatures therefore suit public proof of origin.

Question 18

Why are digital signatures usually computed over a hash of the message?

Accepted Answer

Public key signing operations are relatively slow and work on limited input sizes, so signing a hash gives a short, fixed-size value that represents the whole message. Any change to the message changes its hash, which causes signature verification to fail. Signing the digest is faster and still binds the signature to the content.

Question 19

What is a public key certificate and what does it contain?

Accepted Answer

A public key certificate is a signed document that binds a public key to an identity, such as a domain name or organization. It includes the subject name, the public key, the issuer name, a validity period, and the issuer's digital signature. Clients check that signature and the validity period before trusting the key.

Question 20

How does a certificate authority help a browser trust a website's public key?

Accepted Answer

A certificate authority is a trusted organization that verifies a site's identity and signs its certificate. Browsers ship with trusted root certificates, and a certificate is accepted when a chain of signatures leads back to one of them. If the chain is broken, expired, or names the wrong host, the browser warns the user.

Question 21

Why is comparing key fingerprints useful when verifying a contact in a messaging app?

Accepted Answer

A key fingerprint is a short hash of a public key that two people compare through a separate channel, such as in person. If they match, no key was swapped in transit, which would enable a man in the middle attack. Checking them adds trust that does not depend on the messaging service alone.

Question 22

How does challenge-response authentication differ from sending a reusable password?

Accepted Answer

Challenge-response schemes never send the secret, while a reusable password sent over a network can be captured and replayed. In it, the server sends a fresh random challenge, and the client proves knowledge of a secret by computing a response over that challenge. Since each challenge is new, a captured response is useless later.

Question 23

How does a digital signature differ from encrypting a message with a public key?

Accepted Answer

Signing with a private key proves the signer created the data, because anyone holding the public key can verify the signature. Encrypting with a recipient's public key protects confidentiality, because only the matching private key can decrypt. Mixing up these two operations is a common source of design errors.

Question 24

How does multi-factor authentication improve on a single password?

Accepted Answer

Multi-factor authentication requires proof from at least two independent categories, such as a password and a hardware token. An attacker must compromise more than one separate factor, so stealing a password alone is no longer enough. Device-bound keys, for example, can make phishing much harder.

Question 25

Why should systems avoid storing passwords in plaintext?

Accepted Answer

Storing plaintext passwords means a database breach immediately exposes every account. Systems instead store a one-way password hash, so the original password is not directly revealed. Hashes also let the system verify a login by hashing the attempt and comparing the results, without keeping the password itself.

Question 26

What is the purpose of a salt in password hashing?

Accepted Answer

A salt is a random value unique to each user, combined with the password before hashing. It ensures that two users with the same password get different hashes, and it prevents attackers from using precomputed tables built for common passwords. The salt is stored beside the hash because it need not be secret.

Question 27

Why are password hashing functions intentionally slow?

Accepted Answer

Password hashing functions are deliberately designed to take measurable time, so each guess an attacker makes is expensive. Fast general-purpose hashes let attackers test enormous numbers of guesses quickly on specialized hardware, while slow password hashes sharply reduce that rate. Examples include bcrypt, scrypt, and Argon2, and some of these also require substantial memory.

Question 28

What does full disk encryption protect against, and what does it not protect?

Accepted Answer

Full disk encryption protects stored data when a device is lost, stolen, or accessed without authorization while powered off. It makes the data unreadable without the key, which is released after login. It does not protect data while the system is running and unlocked, and it cannot stop malware that runs with the user's privileges.

Question 29

What does authenticated encryption add to plain encryption?

Accepted Answer

Authenticated encryption combines confidentiality with integrity and authenticity. Plain encryption can hide data but may let an attacker modify ciphertext in ways that produce predictable changes to the plaintext. An authenticated mode attaches an integrity tag, so tampered ciphertext is rejected before any decryption output is used. AES-GCM is one example.

Question 30

What is a key derivation function used for?

Accepted Answer

A key derivation function turns a password or shared secret into one or more cryptographic keys of the required length. Password-based key derivation functions add a salt and many iterations, which slows guessing attacks against the password. They are used when a system must generate encryption keys from something a person remembers.

Question 31

What is the difference between data at rest and data in transit?

Accepted Answer

Data at rest is stored on a device, database, or backup, while data in transit is moving across a network. Each state needs protection, because an attacker may reach either one through theft or interception. Encryption at rest often uses managed keys, while transit encryption usually relies on TLS.

Question 32

How does a password manager use encryption to protect stored credentials?

Accepted Answer

A password manager encrypts its vault with a key derived from the user's master password, so stored credentials are unreadable without it. The master password is not stored, and decryption occurs on the device. Because the provider holds only encrypted data, a server breach should not reveal passwords, although weak master passwords remain a risk.

Question 33

Why is key management often harder than choosing an algorithm?

Accepted Answer

Strong algorithms are useless if keys are generated poorly, shared insecurely, stored carelessly, or never retired. Key management covers creation, distribution, storage, rotation, and destruction, and each stage creates opportunities for error. Many real breaches come from exposed or misused keys rather than from broken ciphers.

Question 34

What is key rotation and why is it useful?

Accepted Answer

Key rotation means replacing an existing key with a new one on a planned schedule or after a suspected exposure. It limits how much data and time any single key protects, so a compromise has a bounded impact. Rotation also forces systems to handle key changes correctly, which tests whether key management processes work.

Question 35

Why should developers avoid designing their own cryptographic algorithms?

Accepted Answer

Designing a secure algorithm is extremely difficult, and subtle flaws often remain hidden until skilled reviewers analyze the design over time. Well-studied, standard primitives have been examined by many experts, which gives more confidence than an untested invention. Developers should instead use established libraries and well-known constructions, and focus on correct configuration.

Question 36

Why must a random number generator used for keys be cryptographically secure?

Accepted Answer

Keys must be unpredictable, so an attacker cannot guess them by reconstructing the generator's output. An ordinary random number generator, designed only for statistics or simulations, may produce values whose sequence is predictable once some state is known. A cryptographically secure generator is designed to resist such prediction.

Question 37

What is a hardware security module used for?

Accepted Answer

A hardware security module is a tamper-resistant device that generates, stores, and uses cryptographic keys without exposing them to the host computer. Operations such as signing or decryption happen inside the module, so private keys never leave it. Certificate authorities and payment systems commonly use these devices to protect high-value keys.

Question 38

What is a side-channel attack against cryptographic software?

Accepted Answer

A side-channel attack recovers secret information by observing indirect effects of computation, such as timing, power consumption, electromagnetic emissions, or cache behavior, rather than breaking the math. Code that branches or indexes memory based on secret values can leak key bits this way. Defenses include constant-time implementations and careful hardware design.

Question 39

What is a zero-knowledge proof and what does it reveal?

Accepted Answer

A zero-knowledge proof lets one party convince another that a statement is true without revealing any information beyond the truth of the statement. For example, a prover can show knowledge of a password-derived secret without transmitting it. Zero-knowledge techniques support privacy-preserving authentication and credential systems.

Question 40

Why should organizations plan to retire outdated cryptographic algorithms?

Accepted Answer

Cryptographic algorithms can weaken over time as researchers find flaws and attack capabilities improve, so an algorithm once considered safe may become unacceptable. Keeping deprecated options available invites downgrade attacks and can leave stored data exposed. Planning for migration, with cryptographic agility that allows algorithms to be swapped, reduces disruption when change becomes necessary.

Cryptography Applications Flashcards

1Core Cryptographic Building Blocks

2Secure Communication Protocols

3Signatures, Certificates, and Identity

4Protecting Stored Data and Passwords

5Keys, Systems, and Practical Risks

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