1. One-way Hash Function
Network Security

2. Objectives of the Topic
One-way Hash Function
Objectives of the Topic
After completing this topic, a student will be able to
explain working of one-way hash function.

3. Figures and material in this topic have been adapted from
One-way Hash Function
Figures and material in this topic have been adapted from
“Network Security Essentials : Applications and Standards”, 2014, by William Stallings.

4. One-way Hash Function
Message authentication is a procedure that allows communicating parties to verify that received messages, file, document, or other collection of data are authentic.

5. One-way Hash Function There are two important aspects:
to verify that the contents of the message have not been altered, and
to verify that the source is authentic.

6. One-way Hash Function
Also, we would like to verify a message’s timeliness (it has not been artificially delayed and replayed) and sequence relative to other messages flowing between two parties.
These are related to data integrity.

7. One-way Hash Function:
Is an alternative to the message authentication code (MAC).

8. One-way Hash Function
A hash function accepts a variable-size message M as input and produces a fixed-size hash value h = H(M).

9. One-way Hash Function
When a hash function is used to provide message authentication, the hash function value is often referred to as a message digest.

10. One-way Hash Function
A hash function does not take a secret key as input.
To authenticate a message, the message digest is sent with the message in such a way that the message digest is authentic.

11. One-way Hash Function
There are three ways in which the message can be authenticated.
A) The message digest can be encrypted using encryption if it is assumed that only the sender and receiver share the encryption key, then authenticity is assured.

12. One-way Hash Function

13. One-way Hash Function
B) The message digest can be encrypted using public-key encryption.
This approach has two advantages:
(1) It provides a digital signature as well as message authentication.

14. One-way Hash Function
(2) It does not require the distribution of keys to communicating parties.

15. One-way Hash Function

16. One-way Hash Function
These two approaches require less computations over approaches that encrypt the entire message.
There has been interest in developing a technique that avoids encryption altogether.

17. One-way Hash Function
C) uses a hash function but no encryption for message authentication.
This technique assumes that two communicating parties, say A and B, share a common secret value SAB.

18. One-way Hash Function
When A has a message to send to B, it calculates the hash function over the concatenation of the secret value and the message: MDM = H(SAB||M).
It then sends [M||MDM] to B.

19. One-way Hash Function
Because B possesses SAB, it can recompute H(SAB||M) and verify MDM.
Because the secret value itself is not sent, it is not possible for an attacker to modify an intercepted message.

20. One-way Hash Function
As long as the secret value remains secret, it is also not possible for an attacker to generate a false message.

21. One-way Hash Function

22. One-way Hash Function
A variation on the third technique is the one adopted for IP security
It also has been specified for Simple Network Management Protocol (SNMP)v3.
End