FIT2093 Lecture Notes - Lecture 8: Public Key Infrastructure, Ciphertext, Digital Signature
31 May 2018
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FIT2093 - W8&9 Notes!
1. What are the essential ingredients of an asymmetric cipher?!
-Plaintext: original message or data!
-Encryption algorithm: the performs substitutions and transformations of the plaintext!
-Public key!
-Secret/Private Key: put into the encryption algorithm!
-Cipher text: the scrambled messaged!
-Decryption algorithm: reverse of the encryption algorithm, uses key and cipher text to produce
original plaintext!
2. List and define three uses of a public key cryptosystem!
-Encryption/decryption: the sender encrypts the message with the recipients public key!
-Digital signature: sender essentially signs the message with their own private key!
-Key distribution: two sides cooperate to exchange a session key!
3. What is a public-key certificate!
Also known as a digital certificate is an electronic document used to prove ownership of a public
key!
A certificate consists of a public key plus a user ID of the key owner, with the whole block signed by
a trusted third party (= certificate authority CA). The user can then publish the certificate and
anyone needing this user's public key can obtain the certificate and verify that it is valid by means
of the attached signature.
Public key infrastructure
4. Briefly explain Diffie-Hellman key exchange!
Diffie-Hellman key exchange (DH) is a method of securely exchanging cryptographic keys over a
public channel!
5. a) How many different keys are required for two people to securely communicate with each
other using an asymmetric cipher?!
4 keys, each user will have 2 keys!
b) How many different keys are required for n people to securely communicate with each other
using an asymmetric cipher?!
2^n!
6. Assume that Alice and Bob live in the 21st century and have access to encryption technology.
Alice wants to send a secret recipe to Bob that will shape the future of food in the world. What
are Alice’s options to securely send the recipe to Bob? Discuss the potential security threats
for each of the options
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Encryption algorithm: the performs substitutions and transformations of the plaintext. Secret/private key: put into the encryption algorithm. Decryption algorithm: reverse of the encryption algorithm, uses key and cipher text to produce original plaintext: list and de ne three uses of a public key cryptosystem. Encryption/decryption: the sender encrypts the message with the recipients public key. Digital signature: sender essentially signs the message with their own private key. Key distribution: two sides cooperate to exchange a session key: what is a public-key certi cate. Also known as a digital certi cate is an electronic document used to prove ownership of a public key. A certi cate consists of a public key plus a user id of the key owner, with the whole block signed by a trusted third party (= certi cate authority ca).
