Lecture 1: Computer Security

Lecture 1

What is a security protocol?

Sequence of interactions between entities designed to achieve a certain end

Provide security services across a distributed system.

Who else does it involve? 

- The environment- considered to be hostile and intrusive, think intruders!

- A trusted third party- ttp server such as paypal

The building blocks of a message

M means message

[M]K means message is encrypted- K means key

[|M|] K means digital signature

H(M) means hash function

Note that also have tuples (n-generalisation of a pair)

(M1, M2, …, Mn) means its an N-Tuple

Nonce

Is a fresh and random value, usually a number which is unique and unpredictable, usually written with symbol: Ns

Timestamps
 data representing current time and date, usually symbol T

Strings- 
words, letters etc.

Alternative to the crypto jargon is below!

Encrypt(M, K) means encryption

Sign(M, K) means digital signature

Hash(M) means hash function

Reverse operations- although never appearing in messages would be

Decrypt (M, K) decryption

Validate (M, K) digital signature validate

Example message

Notice P means public key and Pr means private!

May want to see more messages to try and get the hang of it
in this case- message is being sent, along with nonce A-
this message is encrypred with the public key of B, alongside a timestamp- it is all signed with the private key of A as it is outside the brackets

Secrecy

Note that in 1 the message is secret in transferal of A to B

Known as I/O secrecy, input output

Whereas 2, the message is created WITHIN the protocol but not passed as I/O

Example

Kab is the session key, as it is used straight after the end of a protocol to encrypt and communicate secure messages

The key is used to encrypt and decrypt the message

Also can trust the key was produced by A as it is the same message and signed by A

Authentication

Of origin- being able to authenticate messages origin is who it is supposed to be

Entity- verifiy it REALLY is that person.

Non-Repudiation

Can produce some evidence that certain interactions in the protocol have occurred

In the picture above can see that server accepted and was happy A was creator of message

Encrypting messages means third party will be able to see some even has occurred but not know exactly what occurred

Message encrypted wit Kab but attacker doesn’t know that key!
so therefore, by knowing the private