Question 1

Question 2

Question 1

State the major differences between symmetric-key and asymmetric-key cryptosystems.         
Symmetric-key cryptosystem:
Uses only one private key in both encryption and 
decryption processes. It does not ensure integrity and non-repudiation properties. 
That is, it does not protect the sender from the receiver forging the message (or signature) or claiming the message (or signature) sent by the sender. 

Asymmetric-key cryptosystem:
Uses a pair of public and private keys, with the public key used in encryption and the private key used in decryption processes. 
It ensures integrity and non-repudiation in communication.             

Question 2

With the help of a diagram, explain how the public-key cryptosystem can be used to provide authentication/digital signature.
 

Question 3

With the help of a diagram, explain how the public-key cryptosystem can be used to provide both confidentiality and authentication.
 

Both confidentiality and authentication can be achieved using a hybrid public-key cryptosystem, in which two pairs of public-private keys are used. 
A pair of public-private keys from the sender A, (PrvA, PubA) will be used in signing-verifying the message, while a pair of public-private keys from the receiver B. 
(PrvB, PubB) will be used in encrypting-decrypting the message.  

The message is firstly signed using PrvA and next encrypted using PubB.
When the message is received, it will be decrypted using PrvB and finally verify using PubA.

Question 4

Briefly explain the One-way and Trap-door one-way functions.
One-way function:
A function maps every object in a domain into its corresponding unique image in the range, 
such that the function is easily computable but infeasible (or difficult) to invert it.

Trap-door one-way function:
A function maps every object in a domain into its corresponding unique image in the range, 
such that the function is easily computable but infeasible (or difficult) to invert it, except when trap-door information is given.