| Date of Issue: 06-01-2023 | Rate this Study Guide |
Question 1.
A caesar cipher is:
(Choose 2)
| a) | A unidirectional message digest which can validate message authorship |
| b) | A symmetric letter substitution cipher |
| c) | A modern cipher process that uses only 23 English letters instead of all 26 |
| d) | A cipher that originally operated as C = P + K mod 23, where C is the Ciphertext, P is the Plaintext, and K is the key |
Question 2.
When traffic matches a "permit" statement in a crypto access list,
| a) | it will receive the protection set by the crypto map on this interface. |
| b) | it will be allowed to bypass the protection set by the crypto map on this interface. |
| c) | it will not be blocked by a regular access list. |
| d) | it will receive both AH and ESP protection. |
Question 3.
Source nonrepudiation is useful in securing communications because
| a) | Senders sometimes later deny having sent a message |
| b) | It can validate when a document was created as well as by whom |
| c) | It makes digital forgeries more difficult |
| d) | All of these are correct |
Answers
Question 1.
A caesar cipher is:
(Choose 2)
| a) | A unidirectional message digest which can validate message authorship |
| b) | A symmetric letter substitution cipher |
| c) | A modern cipher process that uses only 23 English letters instead of all 26 |
| d) | A cipher that originally operated as C = P + K mod 23, where C is the Ciphertext, P is the Plaintext, and K is the key |
| b) | A symmetric letter substitution cipher |
| d) | A cipher that originally operated as C = P + K mod 23, where C is the Ciphertext, P is the Plaintext, and K is the key |
Explanation
A caesar cipher is a symmetric cipher using letter substitution. The original caesar cipher, as used by Roman emperors, was a letter substitution that offset the alphabet by a certain number of letters (Julius Caesar used 3, Augustus Caesar used 2). Since the Romans only had a 23-letter alphabet, their use would be expressed mathematically as C = P + K mod 23. If used with modern English, it would be C = P + K mod 26. It had no capability to provide any kind of message digest.
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Question 2.
When traffic matches a "permit" statement in a crypto access list,
| a) | it will receive the protection set by the crypto map on this interface. |
| b) | it will be allowed to bypass the protection set by the crypto map on this interface. |
| c) | it will not be blocked by a regular access list. |
| d) | it will receive both AH and ESP protection. |
| a) | it will receive the protection set by the crypto map on this interface. |
Explanation
Crypto access lists are independent of regular access lists; they have nothing to do with traffic being permitted to pass through an interface or being blocked from it. Traffic that matches a "permit" statement in a crypto access list will be afforded the protection described by the crypto map associated with that interface, which may be AH, ESP, or both.
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Question 3.
Source nonrepudiation is useful in securing communications because
| a) | Senders sometimes later deny having sent a message |
| b) | It can validate when a document was created as well as by whom |
| c) | It makes digital forgeries more difficult |
| d) | All of these are correct |
| d) | All of these are correct |
Explanation
Nonrepudiation establishes the source of a message and/or that the destination has received it. The postal equivalents are proof of sending and proof of delivery/return receipt.
This is typically done by taking a hash of a plaintext, in conjunction with a private key, and creating a digital signature. As long as the private key has been kept secure, only the holder's device or account could have created this message; no other device or account could have (reasonably) created this signature.
When used in conjunction with a timestamp, source nonrepudiation can validate when a document was created. If the hash created under a given algorithm (and key, if used) is the same for a current document as for a document validated as existing at a given time, they must be the same document. The document's existence at the prior time is established; this is important in intellectual property matters.
Finally, as long as the private key used to create the digital signature is not known to have been compromised, the resolution of the hash with the public key validates the signature; a forgery is not practical.
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