Week8：Security : Encrypting and Signing

Week8：Security : Encrypting and Signing

1. Which of the following is true of security?
Perfect security is achievable and requires a trade-off with cost

2. What is the difference between active and passive wiretapping?
In passive wiretapping the network is snooped whereas in active wiretapping the network data is altered

3. Integrity is preserved if
The information you receive is from who you think it is and has not been modified since it was sent

4. Which of the following factors has the smallest effect on the strength of a cryptosystem?
The data being transmitted

5. What is one possible advantage of public-key cryptosystems over secret-key ones?
Public-key cryptosystems do not have the problem of secure key distribution

6.What does it mean if a cryptosystem is symmetric-key in nature?
The key used for encryption is not the same as the key used for decryption

7. The following question is encrypted using a Caesar Cipher with a shift of 13. You can use www.rot13.com to decrypt the question.
Jub vf perqvgrq nf orvat bar bs gur vairagbef bs Rgurearg?
Bob Metcalfe

8. The following question is encrypted using a Caesar Cipher with a shift of 13. You can use www.rot13.com to decrypt the question and answers.

• Jung qbrf gur Gjvggre unfugnt #VUGF fgnaq sbe?
• Vagrearg Uvfgbel, Grpuabybtl, naq Frphevgl

9. What is the SHA-1 hash of the string below as computed by http://www.dr-chuck.com/sha1.php

• The Transport Layer does retransmission
• 1399edc7e55f7be8dbc7024bcb8830527722e179

10. What does a cryptographic hash function do?
It takes a block of data and returns a fixed-size bit string called the hash value

11. What critical element does simple digest-based Message Signing as described in lecture depend on?
The sharing of a secret transported securely ‘out of band’

12. What is the problem with secret key distribution via the internet?
The communication of the secret key is insecure

13. You are going to send the message below using shared secret of IHTS. Use http://www.dr-chuck.com/sha1.php to compute your message digest using the technique from lecture. What will the first six characters of the digest/signature that you send along with the message?

• Be sure to drink more Ovaltine
• 8b4258

14. Select the valid signed message(s) from Annie if your shared secret is IHTS? Use http://www.dr-chuck.com/sha1.php to compute your message digests using the technique from lecture. Only the first 6 characters of the SHA1 message digest are shown below.

• Bring me cookies51be4e
• Everything is all right7dd244