Computer Security

Computer Security- lecture 4

Non-repudiation

May be useful to look in comp sec book

“the ability to deny a false rejection or refusal of an obligation, with irrefutable evidence”

Protocols

Aim to collect evidence to prove that the other party did send or receive evidence

Note that the environment is passive, but participants may not be

Reliance on TTP server to verify information

Aims of non-repudiation

Produce evidence that is required

Evidence must be suitable

Agent behave in accordance?

B wants to prove a sent message happened

Would send evidence to A who sends message, to which B responds

Works for A in that A can show B received as B responded

So can see that A is sending a message which is signed to B

B is replying with signed message to say it has been received

Note both are encrypted- only the two know their own private keys

A then sends to the server, a signed message destined for B- which is a signed message with the Key K for B

The server sends this to B, including details that it came from A and was accepted by server

Server also bounces the message back to A- SAME message

So therefore there is proof that the message has been both sent A to B and received from B to A

Non-repudiation of origin

A sent message needs proof from B

So B has a signed message which is encrypted and a signature

This is the evidence B should produce to the environment to prove that A sent message

So B is sending the signed message which was sent to b and encrypted with private key by A

But also sends the servers a b message

So overall evidence from b is data 1 and 2 which shows sent message from A

After lecture: Can see the first sent information which shows that A sent a message which was encrypted with key K, we know that this message was sent by A due to A’s digital signature

Data two contains the key- from A to B and signed by Server who verifies it came from A

So B uses the key on the first message- and if it works then B knows that it was sent by A

So using data 1 to show the message is encrypted with key K

And then data 2 to decrypt the message with the details of key K sent by A

Then B can show the message must have come from A!

Explained again-

B receives signature and encrypted message

B  also receieves the key to decrypt- signature

B uses key which is signed by A to decrypt message signed by A

Knows its by A as only A has key to encrypted message and has sent key to B

But what if A wants to prove B received

Would also have message which is encrypted FROM A

As well as digital signature

So A provides both of these showing that the message was sent by A and then signed by the server to say that B had received it

How to make this fair? Either bit of evidence shouldn’t be producible before a certain minimum amount of time

In essence;  when the server has given its signature to both A and B, evidence can be produced

For showing that B received the message

A has the message from B which is signed by B with Kprb

Also has the signed message from server which is saying the key was sent to B and accepted by server

Can prove B received by showing the signature that b received encrypted message

Server response that B received the key

So therefore B received key and message!

Anonymity

Note that events that refer to identity of agents

Events’s that the environment observes

Second does not imply first- can observe event without knowing who is involved

The three cryptographers

Three cryptographers share a meal

End of each meal each cryptographer is informed in secret, whether they are to pay or not

Either at most one pays or else organisation will pay for all 3

Main goal is cryptographers to know who is paying or whether organisation is

But paying crypto to be anonymous

So they want to know whether one of them is paying or organisation, but keep identity of the one paying anonymous

So A sends message to C

A sends to B

C sends to B

So if 1 and 0s in diner setting

If number of 1s is even, organisation has paid

If odd, cryptographer

Thus observes the event but not the agent!