Chapter 13 Crptography

1. With a rotation of 4, what does erwaiv decrypt to?

A. waive

B. wave

C. answer

D. decrypt

. C. This is a rotation cipher with a key of 4. When you rotate the alphabet by 4, you end up with e = a, r = n, w = s, and so on. In addition to not being the right decryption, none of the others has the correct number of letters. In a substitution cipher like a rotation cipher, you will always have the same number of letters in the output as you do in the input.

2. What do you call a message before it is encrypted?

A. Text

B. Plain text

C. Bare words

D. Bare text

B. In cryptography, any data or message that is in an unencrypted state is called plaintext. The output from a cryptographic process is ciphertext. While you may have text as input to an encryption process, the word text would be ambiguous in this context. The other two are unrelated to cryptography.

3. What does PGP use to verify identity?

A. Central authority

B. Web of users

C. Web of trust

D. Central trust authority

C. Where certificate authorities use a centralized mechanism for verification of users or certificate subjects, PGP uses a decentralized model. PGP calls this a web of trust, where individual users sign keys that belong to other people to validate that they are who they say they are. All of the other answers are made‐up terms.

4. What principle is used to demonstrate that a signed message came from the owner of the key that signed it? A. Nonrepudiation

B. Nonverifiability

C. Integrity

D. Authority

A. Integrity is part of the CIA triad but isn't the principle that ties a signed message back to the subject of the signing certificate. Nonverifiability is nonsense, and authority isn't relevant here. Instead, nonrepudiation means someone can't say they didn't send a message if it was signed with their key. This assumes the key was in their possession and password protected, meaning no one else could use it.

5. What is Diffie‐Hellman used for?

A. Key management

B. Key isolation

C. Key exchange

D. Key revocation

. C. Certificates can be revoked, but that's not what Diffie–Hellman is used for. Key management is a much broader topic than key exchange, which is what Diffie–Hellman is used for. It is a process that allows two parties to an encrypted conversation to mutually derive the same key starting with the same base value.

6. How did 3DES improve on DES?

A. Made the key longer

B. Used two keys

C. Changed algorithms

D. Used three keys

D. 3DES, or Triple DES, uses three keys. The first key is used to encrypt the plaintext. The second key is used to decrypt the ciphertext resulting from the first round of encryption. Finally, the third key is used to encrypt the ciphertext that resulted from the decryption with the second key. The key wasn't made longer because the 168 bits used in 3DES aren't used in a single key. The underlying DES algorithm is still used.

7. What improvement does elliptic curve cryptography make?

A. Smaller keys improve speed

B. Algorithm is more complex

C. Doesn't use factoring, which is better

D. Longer keys

A. Algorithms used for elliptic curve cryptography are not more complex necessarily. While they don't use factoring, that fact alone doesn't necessarily make the algorithms better. Instead, elliptic curve cryptography relies on the assumption that a discrete logarithm of a point on an elliptic curve can't be computed in a consistent way. The keys that result from elliptic key cryptography are actually smaller than those that result from factoring with large prime numbers.

8. What is it called when two different data sets yield the same cryptographic hash?

A. Paradox

B. Collision

C. Crash

D. Unrealistic

B. When two different data sets yield the same cryptographic hash, it is called a collision. It relates to a mathematical problem called the birthday paradox, but two values being the same is not a paradox. It's also not unrealistic, nor is it a crash.

9. Which of the following terms can be used in a description of asymmetric key encryption?

A. Defined key

B. Multifactor

C. Public key

D. Single factor

. C. Asymmetric key cryptography uses two related keys. One key is used for encryption, and one is used for decryption. These keys are referred to as the public and private keys. Because it's the public key that is used to encrypt messages to the owner of the paired private key, this type of encryption is commonly referred to as public key cryptography. It is neither single‐factor nor multifactor since it's not authentication.

10. If Alice were to send an email to Bob, what key would she use to encrypt the message?

A. PGP key

B. Private key

C. Public key

D. Symmetric key

C. Public key cryptography works because the public key can be provided to anyone. The only thing you can do with the public key is encrypt a message that could be decrypted by the matched private key. This process uses asymmetric encryption, so it's not a symmetric key. The private key has to be with the owner of the key and protected. If that key gets out, any messages encrypted to the owner by the public key could be decrypted. PGP uses public/private keys and does not have its own type of key.

11. What property allows you to trust someone trusted by a certificate authority you trust?

A. Commutative property

B. Associative property

C. Communicative property

D. Transitive property

D. What this says is that if A trusts B and B trusts C, then A can trust C. This is an application of the transitive property. The commutative and associative properties are both also mathematical principles. There is no such thing as a communicative property.

12. Why is symmetric key encryption typically used over asymmetric key encryption?

A. It's faster.

B. It's more secure.

C. It's easier to implement.

D. It isn't encumbered with patents.

A. Symmetric key encryption is generally used instead of asymmetric key encryption because symmetric key encryption uses shorter keys and fewer resources, resulting in shorter times for encryption and decryption. This does not make it more secure, even if that word were to be defined in this context. Symmetric key is not easier to implement, and asymmetric key is not encumbered with patents, which is why C and D are wrong.

13. What is it called when both symmetric and asymmetric keys are used?

A. Fast cryptosystem

B. Hybrid cryptosystem

C. Super‐symmetric cryptosystem

D. Dual key cryptosystem

. B. When both symmetric and asymmetric keys are used, typically where the asymmetric key is used to protect the symmetric key, it is called a hybrid cryptosystem. The other options don't exist.

14. What is MD5 or SHA‐1 commonly used for in cryptography?

A. Media access control (MAC)

B. Machine authentication code (MAC)

C. Message access code (MAC)

D. Message authentication code (MAC)

D. Media access control (MAC) is an address attached to physical network interfaces. The correct answer is message authentication code because SHA‐1 and MD5 are used as message authentication codes to ensure that a message has not been tampered with. This means it is being authenticated.

15. What type of encryption does PGP use?

A. Null key

B. Asymmetric key

C. Trusted key

D. Web key

B. PGP uses public and private keys. The public key is stored in a public place like a key repository. Since there are two keys, PGP uses asymmetric key encryption, sometimes known as public key encryption.

16. What tool would you use to identify ciphersuites in use on a web server?

A. Hydra

B. sslscan

C. tlsscan

D. cipherscan

B. Hydra is used to brute‐force passwords against network services. The tools tlsscan and cipherscan don't exist. While the SSL protocol has been deprecated for many years, it is still commonly used, so the tool is sslscan.

17. How does AES protect against related‐key attacks?

A. Longer key lengths

B. Better initialization vectors

C. Implementation doesn't allow related keys

D. Upgrading to AES‐2

C. Protecting against related keys is done in the implementation rather than the specification. The implementation of AES should not allow related keys to be issued. None of the other answers are true.

18. What is one advantage of using a certificate authority?

A. Trusted third party doing validation

B. A certificate authority is faster

C. Stronger keys are offered

D. They support more cipher suites

A. A certificate authority is a trusted third party that can validate users and their identity, relieving everyone from having to verify every user's identity themselves. A certificate authority is not faster, and they don't offer stronger keys. The certificate authority is not responsible for ciphersuites being supported, just the certificate.

19. How does a certificate authority keep a list of valid certificates up‐to‐ date?

A. Periodic CA update

B. Hashing the list

C. Re‐validating identities

D. Certificate revocation lists

D. A certificate revocation list is used to indicate when a certificate is no longer valid. Hashing the list won't help and re‐validating identities may only be necessary when a certificate has fully expired. When a certificate expires it may be put on the certificate revocation list. A periodic CA update is ambiguous, but may be updating software, which won't keep a certificate list up‐to‐date.

20. What security property suggests that an email signed by an individual's key must have come from that person? A. Confidentiality

B. Integrity

C. Availability

D. Nonrepudiation

D. The property of nonrepudiation says that a key belonging to an individual, where the private key is protected by a password and not accessible to everyone, will not be used by anyone but that individual, so any message signed by their key must have come from them. While encrypted messages are related to confidentiality, signing a message is not encrypting it. The other properties are not related to this scenario.