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CompTIA Security+ study material: Cryptography

62 questions of the 611 in the CompTIA Security+ quiz. Each opens with its answer, the reasoning and where that is written down.

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The questions

1. 158. An administrator raises the password hashing work factor until each login takes three seconds. What new risk has been introduced?
2. 185. A self-encrypting drive must be sanitised in minutes rather than hours. Which technique achieves this?
3. 191. Which protection applies to data in use, as distinct from data at rest or in transit?
4. 202. Which property ensures a sender cannot credibly deny having sent a message?
5. 219. What distinguishes a hardware security module from software key storage?
6. 220. Which component anchors full-disk encryption and boot integrity to one specific machine?
7. 225. What does code signing let an organisation verify, and what does it enable operationally?
8. 247. A team encrypts every secret in its vault at rest. What problem does this leave unsolved?
9. 251. A developer replaces sensitive identifiers with a reversible transformation and calls the data protected. What is the correct assessment?
10. 263. How must a verifier store passwords so they resist offline attack?
11. 264. What is the minimum salt length NIST specifies for stored passwords, and what does the salt achieve?
12. 265. NIST suggests an additional keyed operation over the stored password hash. Where should that secret key live?
13. 307. Why does certificate-based authentication resist network capture better than a password?
14. 312. Why must AAL3 authenticators use public-key cryptography to protect the authentication secret?
15. 391. An organisation terminates TLS at its load balancer. What is the security trade-off?
16. 393. Why does every TLS 1.3 handshake provide forward secrecy?
17. 394. Which two features were removed in TLS 1.3? Choose two.
18. 398. A phishing site presents a valid domain-validated certificate. What does this certificate actually prove?
19. 399. What risk does a wildcard certificate concentrate?
20. 400. What does a certificate signing request contain?
21. 401. A browser warns about a self-signed certificate on an internal site. What is actually missing?
22. 458. An attacker steals a password hash file. Why does the login endpoint's rate limiting no longer help?
23. 459. Which defence makes rainbow tables useless against a stolen password database?
24. 460. Why is SHA-256 unsuitable for storing passwords even when salted?
25. 461. A team pre-hashes passwords with plain SHA-512 before passing them to bcrypt. Which attack does this enable?
26. 486. Why is disabling old TLS versions on the server necessary rather than merely deprioritising them?
27. 487. Which property of a hash function does a birthday attack exploit?
28. 488. A collision is found in the hash function underlying a signature scheme. What has been broken, even though no key was compromised?
29. 489. An attacker recovers a key by measuring how long cryptographic operations take. What class of attack is this?
30. 507. What does recomputing a hash of an evidence image months after acquisition demonstrate?
31. 531. Why is hashing rather than encryption the right tool for storing passwords?
32. 532. How does a pepper differ from a salt?
33. 533. A pepper is compromised. Why can it not simply be rotated?
34. 534. Which password hashing algorithm does OWASP recommend first, and why?
35. 535. An organisation must use FIPS-140 validated implementations. Which password hashing algorithm should it choose?
36. 536. An application uses bcrypt and accepts passwords of any length. What must it also enforce?
37. 537. When is the usual moment to re-hash a user's password with an increased work factor?
38. 538. A team refuses to document which password hashing algorithm it uses, calling it sensitive. What is the correct position?
39. 539. What determines the appropriate cryptoperiod for a key?
40. 540. Why does a signing key's usage period end well before the period during which its signatures must still verify?
41. 541. Why should the same key not be used both to sign and to encrypt?
42. 542. An implementation seeds its key generation from the system clock. What is the consequence?
43. 543. An organisation wants to rotate its encryption key without re-encrypting terabytes of stored data. Which technique achieves this?
44. 544. What is the trade-off of escrowing an encryption key?
45. 545. Why do protocols such as TLS use asymmetric cryptography only to establish a symmetric session key?
46. 546. An adversary records encrypted sessions today and obtains the server's long-term private key next year. Which property prevents them decrypting the recordings?
47. 547. Which two properties does a digital signature provide that an HMAC does not? Choose two.
48. 548. What problem does a public key infrastructure solve?
49. 549. In a public key infrastructure, which component verifies the requester's identity?
50. 550. A mobile application pins the certificate of its backend. What does this prevent, and what does it complicate?
51. 551. What security property does a blockchain provide?
52. 552. A laptop with full-disk encryption is stolen while running and unlocked. What protection remains?
53. 553. How does steganography differ from encryption?
54. 554. What does key stretching achieve against an offline attacker?
55. 555. What is the difference between using an approved algorithm and using a FIPS 140 validated module?
56. 556. Why does NIST apply stricter FIPS 140 requirements to verifiers than to authenticators?
57. 557. Why can a protocol using only static long-term keys not provide forward secrecy?
58. 558. Why does SHA-1 still appear in some systems despite being unsuitable for new designs?
59. 559. An attacker obtains valid query access to a database whose files are encrypted at rest. What still protects individual sensitive fields?
60. 560. An attacker steals a table of salted password hashes. What can they do with it directly?
61. 561. Beyond choosing an algorithm, which two concerns does key management cover? Choose two.
62. 608. Besides forward secrecy, what did TLS 1.3 improve about the handshake?