Cryptography Related Question [Not Bitcoins]

Joel_Jantsen

legendary

Activity: 1876

Merit: 1308

Get your game girl

Quote from: shorena on February 26, 2016, 08:51:54 PM

Sounds like Im doing your homework here. Oh well. We are using symmetric keys so each pair of people that wants to communicate need 1 key. If n is 3 and the names are Alice, Bob and Claire we get the following pairs.

A-B
A-C
B-C

All other possible pairs (like B-A) are doubles. I will leave it for you to find the general formular for any n. If you have trouble finding a solution start from 3 and ask yourself how many additional(!) keys you need to add for the 4th person. Repeat adding more people until you see a pattern and test for a higher number.

Problem solved .Thanks for tip sho.Yes it was a homework, exactly 5 marks you have helped me gain. Tongue

shorena

copper member

Activity: 1498

Merit: 1499

No I dont escrow anymore.

Quote from: Joel_Jantsen on February 26, 2016, 08:01:49 PM

Not sure if this belongs here but could not find the other closest section to my query.
The question is :

Quote

Suppose that everyone in a group of N people wants to communicate secretly with the N–1 others using symmetric key cryptographic system. The communication between any two persons should not be decodable by the others in the group. The number of keys required in the system as a whole to satisfy the confidentiality requirement is

The options are :

A) 2N
B) N(N – 1)
C) N(N – 1)/2
D) (N – 1)2

Need this for a project I'm working on.Logical explainations appreciated.

Sounds like Im doing your homework here. Oh well. We are using symmetric keys so each pair of people that wants to communicate need 1 key. If n is 3 and the names are Alice, Bob and Claire we get the following pairs.

A-B
A-C
B-C

All other possible pairs (like B-A) are doubles. I will leave it for you to find the general formular for any n. If you have trouble finding a solution start from 3 and ask yourself how many additional(!) keys you need to add for the 4th person. Repeat adding more people until you see a pattern and test for a higher number.