cryptography

The algorithm is correct and the key never leaves the chip. The attacker reads it anyway from how long, how loud or how hot the computation is, or breaks it by making the chip skip one instruction.

WireGuard's configuration is a list of public keys and the IP ranges each may use. That small idea replaces most of what IPsec negotiates, and it is the part interviewers ask about.

The most common screen question in security. The answer that scores sorts all four by one property: whether there is a key, and who holds it.

The trap is answering 'which is better'. Every protocol you use daily runs both, each doing the job the other cannot.

Message names are table stakes. The interviewer is listening for why CertificateVerify exists and what Finished actually checks.

Checking the signature is one step of seven. The bugs that matter live in the other six, and in code that switches validation off.

SHA-256 with a salt sounds right and fails. The answer that scores explains the attacker's economics and names the parameters.

Both detect tampering. The difference is who else could have made the tag, and that decides webhooks, tokens and audit evidence.

The question behind the question: if your server's private key leaks next year, what happens to the traffic someone recorded today?

'Yes, always' is the right default and the wrong answer. The interviewer wants the list of attackers it does nothing against.

Encryption without integrity has been exploited for decades. Nonce reuse under GCM is worse than most candidates think: it breaks integrity too.

Data keys, key-encryption keys, tenant isolation, rotation and crypto-shredding. The design is standard; the trade-offs at each level are the interview.

Most cryptographic failures are misuse of good primitives, not broken algorithms. The scored answer has a checklist that catches the common ones and a strong default to replace them.

Preimage, second preimage and collision resistance sound alike and protect different things. The birthday bound explains why the last one costs half the bits.

Being able to switch algorithms is essential. Letting an attacker choose which algorithm you switch to is a vulnerability with a long history.

Most candidates can recite TLS 1.3. Interviewers ask for 1.2 because it is still everywhere, and because every change in 1.3 is the fix for an attack on 1.2.

An HSM's promise is narrow and strong: the key cannot leave in plaintext. Everything else, including who may ask the HSM to use the key, is your design.