CASP+ Cryptography and PKI 1 — Questions and Answers
Question 1: An organization wants to ensure that digital signatures on contracts remain legally valid even if the signing algorithm is later deprecated. Which mechanism BEST addresses this?
- Re-signing all documents with a new algorithm immediately
- Trusted timestamping combined with long-term validation (LTV) signatures (Correct answer)
- Storing documents in encrypted archives
- Using symmetric encryption for document storage
Correct answer: Trusted timestamping combined with long-term validation (LTV) signatures
LTV signatures embed the certificate chain and revocation data at signing time, and a trusted timestamp proves the signature existed before algorithm deprecation.
Question 2: Which cryptographic algorithm provides BOTH confidentiality and data authentication in a single operation?
- AES-CBC with HMAC-SHA256 applied separately
- AES-GCM (Galois/Counter Mode) (Correct answer)
- RSA-2048 encryption
- SHA-256 hashing
Correct answer: AES-GCM (Galois/Counter Mode)
AES-GCM is an authenticated encryption mode that simultaneously provides confidentiality via CTR mode and integrity/authentication via the GHASH function.
Question 3: A PKI administrator needs to revoke a certificate immediately due to a key compromise. Which mechanism provides the FASTEST revocation propagation?
- CRL published on a daily schedule
- OCSP stapling with short-validity responses (Correct answer)
- Manual notification to all relying parties
- Waiting for the certificate to expire naturally
Correct answer: OCSP stapling with short-validity responses
OCSP stapling embeds a recent, CA-signed revocation response directly in the TLS handshake, providing near-real-time revocation status without client round-trips.
Question 4: Which key exchange protocol provides perfect forward secrecy (PFS)?
- RSA key exchange
- Static Diffie-Hellman (DH)
- Ephemeral Diffie-Hellman (DHE) or ECDHE (Correct answer)
- Pre-shared key (PSK) exchange
Correct answer: Ephemeral Diffie-Hellman (DHE) or ECDHE
Ephemeral DH/ECDHE generates a new key pair for each session, so past session keys cannot be derived even if the server's long-term private key is later compromised.
Question 5: An enterprise is designing a PKI hierarchy. Which structure BEST protects the root CA while maintaining operational flexibility?
- A single root CA used directly for all certificate issuance
- An offline root CA with online intermediate/issuing CAs (Correct answer)
- A flat PKI with no intermediate CAs
- A root CA hosted on an internet-connected server for availability
Correct answer: An offline root CA with online intermediate/issuing CAs
An offline root CA is never exposed to network attack; intermediate CAs handle day-to-day issuance, and if an intermediate is compromised only that subtree needs replacement.
Question 6: Which elliptic curve is RECOMMENDED by NIST for use in US federal systems at the 128-bit security level?
- Curve25519
- P-256 (secp256r1) (Correct answer)
- P-521
- secp256k1
Correct answer: P-256 (secp256r1)
NIST P-256 is the FIPS-approved curve at the 128-bit security level, recommended in NIST SP 800-186 for federal cryptographic applications.
