1. Analyzing Key Distribution and Authentication Protocols
Lecture slides by Lawrie Brown for “Cryptography and Network Security”, 5/e, by William Stallings, Chapter 14 – “Key Management and Distribution”.
© 2015 Richard Newman 1

2. Analysis of Key Distribution Protocols
Ways to break cryptographic controls:
The cipher algorithm is broken
The keyspace is too small
The protocol is broken
The implementation is broken
The system does not protect the keys adequately
Opening quote. 2

3. Analysis of Key Distribution Protocols
The cipher algorithm is broken
Use good, tested algorithms
The keyspace is too small
Use large enough keys
Make sure key selection has sufficient entropy
Opening quote. 3

4. Analysis of Key Distribution Protocols
The implementation is broken
Take care about random numbers
Take care about key gen & rotation
Take care about leaks
The system does not protect the keys adequately
Enforce adequate security policies and use good controls in rest of system
Opening quote. 4

5. Analysis of Key Distribution Protocols
The protocol is broken
Analyze the protocol
Prove its properties
Determine its assumptions
Identify the obligations it places on the environment
Make sure requirements are satisfied!
Opening quote. 5

6. Analysis of Key Distribution Protocols
If the protocol is broken, no amount of patching will fix it
What does it expect/require of underlying cryptographic functions?
What does it require of generated variables (e.g., nonces, random numbers)
What does it expect of the system
Opening quote. 6

7. Analysis of Key Distribution Protocols
If the protocol is broken, no amount of patching will fix it
By definition, protocols are used by multiple parties
Require parties to use same protocol
Protocol change requires all parties using it to change also – how to coordinate?
Need to make new protocol (version)
Opening quote. 7

8. How do We Analyze Protocols?
Burroughs, Abadi, and Needham introduced BAN Logic
Many other logics since then
Many other analysis techniques also
But BAN logic is relatively simple and suffices to expose much of what we want:
assumptions
vulnerabilities
flaws
The topics of cryptographic key management and cryptographic key distribution are complex, involving cryptographic, protocol, and management considerations. The purpose of this chapter is to give the reader a feel for the issues involved and a broad survey of the various aspects of key management and distribution. 8

9. BAN Logic See DES SRC-039 Tech Report Steps:
Transform protocol to idealized protocol
State initial assumptions
Associate assertions with messages
Apply postulates to beliefs and assertions to derive additional beliefs
Opening quote. 9

10. BAN Logic Monotonic logic: Only add, never subtract belief
Questions it helps answer:
What does this protocol achieve?
Does this protocol require more assumptions than another one?
Does this protocol do anything unnecessary (e.g., encryption)?
Opening quote. 10

11. BAN Logic Notation P, Q, S – Principals X, Y – formulas or statements
Ka – A’s public key
Ka-1 – A’s corresponding private key
Kab – Symmetric key for A and B
{X}K – encryption (or signing) of X with K
<X>Y – authentication of X using secret Y
P → Q: M – P sends Q message M
Opening quote. 11