CPSC 42500 Kings College London Applications of Encryption Questions

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CPSC 42500

Kings College London

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Name _________________________ Score: ____ / 24 CPSC 42500 Homework 9: Certificates and PKI, Applications of Encryption 1. (3 pts) Translate the following expression for a simple digitally-signed certificate to a formula, using the same notation and variable names used in the lecture. “Bob’s public key pkB is concatenated with the string ‘Bob, Inc.’ and the date string ‘202004-27’. The resulting string is hashed, and the result is encrypted with the certificate authority’s secret key skCA to produce signature s.” 2. (2 pts) Explain why the use of Certificate Authorities does not eliminate the need for trust on the web. Since it does not do this, what is it good for? 3. (3 pts) Use a web browser to find the following parameters of the security certificate used by the website https://www.yahoo.com : expiration date (not valid after), certificate signature algorithm, size of the certificate signature. 4. (3 pts) Suppose a CA is malicious. What bad things could the CA accomplish? 5. (2 pts) Suppose the malicious CA in the previous question is not a root CA, but an intermediate CA in a hierarchical system. How could the damage they can cause be limited or stopped? Page 1 of 2 6. (8 pts) a. Say an eavesdropper on a wired network sniffs packets that are part of a TLS-encrypted HTTP session. No other encryption is added. State whether the eavesdropper will see each of the following elements as encrypted or in plaintext: 1) application data, 2) HTTP headers, 3) Layer 4 headers, 4) Layer 3 headers, 5) Layer 2 headers. Justify your answer. b. Answer the same question as part (a), but for traffic encrypted with IPSec and no other encryption. c. Answer the same question again, but for traffic that is encrypted with only Wi-Fi encryption and sniffed from the air. d. Answer the same question again, but for traffic that is encrypted with only Wi-Fi encryption and is sniffed after it is forwarded onto the wired network. 7. (3 pts) What distinguishes a Key Derivation Function from the secure hash functions (such as the SHA family) that we studied previously? Give as much detail as necessary to be clear. Page 2 of 2
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Name _________________________

Score: ____ / 24

CPSC 42500 Homework 9: Certificates and PKI, Applications
of Encryption
1. (3 pts) Translate the following expression for a simple digitally-signed certificate to a
formula, using the same notation and variable names used in the lecture.
“Bob’s public key pkB is concatenated with the string ‘Bob, Inc.’ and the date string ‘202004-27’. The resulting string is hashed, and the result is encrypted with the certificate
authority’s secret key skCA to produce signature s.”
C = H (pkB + ‘Bob, Inc’ + ‘2020-04-27’)
s = EskCA (C)
2. (2 pts) Explain why the use of Certificate Authorities does not eliminate the need for trust on
the web. Since it does not do this, what is it good for?
Certificate authorities provide root certificates for authentication and security of software on
the internet. The use of certificate authorities does not eliminate the...

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