1. 9.2 SECURE CHANNELS
Medisetty Swathy

2. Content Introduction Authentication
Message Integrity and Confidentiality
Secure Group Communications
Example: Kerberos

3. Introduction
How to make communication between clients and servers secure ?
Authentication ?
Communication within a distributed system
Setting up a secure channel
Protection against Interception, Modification and Fabrication
Ensuring Confidentiality
Protocols for mutual Authentication and message Integrity

4. Authentication
Authentication and Message Integrity should always come together
Example of Alice & Bob
For Authentication a secure channel is set up
For Message Integrity secret-key Cryptography by means of session keys is used
Keys are securely destroyed when the channel is closed

5. Authentication Based on Shared Secret Key
Secret key is already shared between A and B
One party challenges other to a response
Response is authenticated with shared secret key
Challenge-Response protocols
Alice(A)
Bob (B) A RB
KA,B(RB) RA
KA,B(RA)

6. Authentication Based on Shared Secret Key
Designing protocols that actually works
Optimizing number of messages to three
Sending secret key along with the message
Alice(A)
Bob(B)
A, RA
RB,KA,B(RA)
KA,B(RB)

7. Authentication Based on Shared Secret Key
The reflection attack
C sends message along with the challenge
B returns challenge along with the response
Chucky (C)
Bob (B)
A,RC
First Session
RB ,KA,B(RC)
A,RB
Second Session
RB2 ,KA,B(RB)
First Session
KA,B(RB)

8. Authentication Based on Shared Secret Key
C tires to establish another session by using the challenge of B
B sends another challenge and responds with his key
C sets up First session with the key and leaves Second session
Chucky (C)
Bob (B)
A,RC
First Session
RB ,KA,B(RC)
A,RB
Second Session
RB2 ,KA,B(RB)
First Session
KA,B(RB)

9. Authentication Using Key Distribution Center
Scalability is one of the problems with shared secret key
If the distribution system has N hosts, it is difficult to manage when N is large
Key Distribution Center shares a secret key with each host; instead of pairs sharing the key
In SSK, the system needs to manage N(N-1)/2 keys; where as in KDC only N keys

10. Authentication Using Key Distribution Center
KDC hands out key to both Alice(A) and Bob(B)
A sends message to KDC and tells about B
KDC returns with a message with shared secret key KA,B
Alice (A)
KDC, generates KA,B
Bob (B)
A,B
KA,KDC(KA,B)
KB,KDC(KA,B)

11. Authentication Using Key Distribution Center
The message is encrypted with secret key KA,KDC
KDC also sends KA,B to B encrypted with secret key KB,KDC
Tinku (A)
KDC, generates KA,B
Pinky (B)
A,B
KA,KDC(KA,B)
KB,KDC(KA,B)

12. Authentication Using Key Distribution Center
There are certain drawbacks, like A wants to set up connection with B even before KDC contacts B
KDC needs to pass the key to the B before it brings in the loop
Instead KDC can just pass the keys to A and lets A to contact B with ticket

13. Authentication Using Public Key Cryptography
Does not require KDC
A and B has each others public keys with them
A sends challenge encrypted with B’s public key
B returns the decrypted challenge, along with his own challenge.
Alice (A)
Bob (B)
K+B (A,RA)
K+A(RA,RB,KA,B)
KA,B(RB)

14. Authentication Using Public Key Cryptography
B generates session key KA,B to use for further communication
B’s response to A, B’s challenge, session key are put in a message encrypted with public key of A
A returns the response using session key to acknowledge
Alice (A)
Bob (B)
K+B (A,RA)
K+A(RA,RB,KA,B)
KA,B(RB)

15. Message Integrity and Confidentiality
Message Integrity ensures protection from modification
Confidentiality ensures protection from interception
Confidentiality can be achieved by encryption through a shared secret key or public key
Message Integrity is a difficult task

16. Message Integrity and Confidentiality Digital Signatures
Message Integrity goes beyond the actual transfer through a secure channel
Example of A buying a collection item from B
In addition to authentication, digital signatures helps in improving integrity
Several ways to place digital signatures
Popular form is to use a public-key cryptosystem such as RSA

17. Message Integrity and Confidentiality Digital Signatures
A sends message m to B by encrypting with its private key
If A wants the content of the message to be secret, B’s public key is used which combines m and the signature of A
B receives the message and decrypts with A’s public key
A’s Computer
B’s Computer m m
A’s private key K-A
B’s public key K+B
B’s private key K-B
A’s public key K+A m

18. Message Integrity and Confidentiality Digital Signatures
Certain problems are associated with his method
If A’s private key is stolen
If A changes its private key
Encryption costs
A’s Computer
B’s Computer m m
A’s private key K-A
B’s public key K+B
B’s private key K-B
A’s public key K+A m

19. Message Integrity and Confidentiality Digital Signatures
Cryptographic Hash function is used to improve the situation
A uses Hash function to calculate the message digest with A’s private key
A’s message digest along with the original message is sent to B

20. Message Integrity and Confidentiality Digital Signatures
B decrypts the message digest with A’s public key
Compares the original message with the decrypted message
If both are same it understands that no modifications are done and the signature is authentic

21. Secure Group Communication
How to enable Secure communication for more than two parties ?
It is necessary to enable secure communication between more than just two parties
The server is replicated to improve the fault tolerance and performance
The replicated sever for which all the replicas exist, should be protected against modification, fabrication, interception

22. Secure Group Communication Confidential Group Communication
To ensure confidentiality, a simple scheme of letting all group members to share same secret key
This key is used to encrypt and decrypt all the messages transmitted by the members
All the members need to be trusted to keep the key a secret
This prerequisite alone makes the use of single key more vulnerable to attacks

23. Secure Group Communication Confidential Group Communication
Another solution is to maintain separate shared secret key between each pair of group members
When one attack happens others can stop sending the messages but still use their secret keys
However, instead of maintaining one key, it is necessary to maintain N(N-1)/2 keys which is a difficult problem

24. Secure Group Communication Confidential Group Communication
Using public-key cryptosystem the situation can be improved
Each member has its own (private key, public key) pair, in which public key can be used by all members for sending confidential messages
N key pairs are need for N members
Unfaithful members can be removed from group without compromising the other keys

25. Secure Group Communication Secure Replicated Servers

26. Example: Kerberos
A logs into A’s work station and its identity is sent to Authentication Server by the work station (AS)
AS authenticates the user and provides a key to set up a secure channel
This key is known only to AS and Ticket Granting System (TGS)

27. Example: Kerberos Work station asks for the password
When the correct password is entered, then the key is ready to use
A requests for the connection with B to TGS
A secure channel is established

28. References https://en.wikipedia.org/wiki/Cryptographic_hash_function
Andrew S. Tanenbaum, Maarten Van Steen, “Distributed Systems: Principles and Paradigms”, Prentice-Hall,NJ,USA.

29. Questions?

30. Thank You!!!