1. Authentication in Networks Advanced Network Security Peter Reiher August, 2014

2. Outline The basic authentication problem
Authentication options for networks
Practical authentication in the Internet

3. Authentication in a Network
I’m Bill!
Subject
The network
Authenticator
How can the authenticator be sure that the subject really is Bill?

4. Issues to Consider The parties can only use the network to communicate
Implying that authentication works using bit patterns
Bits are easy to copy
Networks can be eavesdropped upon
No inherent guarantee that next packet is related to last packet
Must we authenticate each packet?

5. Authentication Options
Authentication usually performed in one of three ways:
Authenticate by what you know
Authenticate by what you have
Authenticate by what you are
How well do these work in network settings?

6. Authentication By What You Know
Passwords
Cryptographic keys
Security question responses
Usually, the authenticating entity asks for some knowledge
The subject must provide the right knowledge

7. How It Works in a Network
OK, here’s Bill’s secret
I’m Bill!
Prove it!
BILL!
Subject
The network
Authenticator
If it’s the right secret,

8. Attackers might guess the secret
Potential Problem #1
Attackers might guess the secret
I’m Bill!
I wonder what Bill’s Secret might be . . .
Maybe it’s . . .
And here’s my secret
BILL!

9. What Does This Mean? The secret must be unguessable
Not either simple or obvious
Bad examples:
Short passwords
Something related openly to subject’s identity (like his name)

10. Eavesdroppers can overhear and replay the secret
Potential Problem #2
OK, here’s Bill’s secret
I’m Bill!
Prove it!
Eavesdroppers can overhear and replay the secret
I’m Bill!
BILL!
And here’s my secret
Subject
The network
Authenticator
BILL!
Bill’s secret

11. What Does This Mean? Either the attacker must be unable to eavesdrop
Which may be true, but can be impossible to guaranteed
Or he must be unable to use what he hears
How to achieve the latter?
Proper crypto

12. What Do We Mean By “Proper Crypto”?
Not just a strong cipher (e.g., AES)
But also something that cannot be replayed
If the attacker can copy and replay the encrypted secret, crypto didn’t help

13. OK, here’s Bill’s secret
Improper Crypto
OK, here’s Bill’s secret
I’m Bill!
Prove it!
The stolen encrypted authentication information decrypts to the secret!
I’m Bill!
BILL!
And here’s my secret
Subject
The network
Authenticator
BILL!
Bill’s secret

14. How Do We Solve the Problem?
Use a different crypto key each time
Making sure only the real Bill could have it
Or use the same key, but include a different nonce
Either way, require “Bill” to encrypt his secret differently each time

15. The General Problem for Network Authentication
If you authenticate by what you know
You’d better make sure no one else knows it
Which means asking for something different each time
Different piece of knowledge
Different encryption of same piece

16. Authentication by What You Have
Certificates
Security tokens of various sorts
Challenge is you must prove possession across a network
Unlike in person, when you can just show the item (e.g., passport)

17. How It Works in a Network
OK, here’s proof that I have the special item
I’m Bill!
Prove it!
BILL!
Note the similarity to the previous approach!
Subject
The network
Authenticator
If the proof is sufficiently convincing . . .

18. What’s The Same? What’s Different
The authenticator gets a bunch of bits over the network
If they’re right, he authenticates
What’s different?
How the bits get created
That’s where we can improve things

19. How Should This Work?
The weakness of authentication by what you know was the secret
If the secret got out, the authentication failed
What if it’s a different secret every time?
No problem with eavesdropping
No replay issues
Authenticating by what you have helps if the item generates new bits every time

20. Generating New Bits Typically requires an active computing device
Something with memory and processing capability
On each request, it generates a fresh response
The authenticator must be able to check the response for correctness

21. How To Generate the New Bits?
Challenge/response
The authenticator sends a random number
The device encrypts it with its secret key
The authenticator checks the encryption
Hash chains
The device generates new bits using a cryptographic hash of the last set of bits
The authenticator determines if the bits are the next in the chain

22. Some Difficulties
The authenticator must share a secret with the device
Unless you use PK
But still requires pre-arrangement
Problems if hash chain gets out of sync with authenticator
Solvable using clocks, instead of sequence
Requires rough clock synchronization

23. Weakness of the Approach
Loss of special device makes it impossible to authenticate
Theft of device may allow thief to improperly authenticate
Must pre-arrange to have authentication device in users’ hands

24. An “Engineering” Approach
Use a smart phone instead of security token
Most people have smart phones
They have compute, storage, and communications capabilities
They also have a unique number (telephone number) that contacts them (maybe . . .)
Authenticate by asking app on the smart phone to handle challenge/response
Solves some problems, adds others

25. Authentication By What You Are
Prove your identity with biometrics
Fingerprints, face recognition, retinal scans, etc.
Provide that information to the authenticator
He checks against a stored version

26. How It Works in a Network
OK, here’s my fingerprint information
I’m Bill!
Prove it!
BILL!
Note the similarity to the previous approaches!
Subject
The network
Authenticator
If it’s a good match for Bill’s known fingerprint information . . .

27. Biometrics and Networks
Not a particularly good match
The biometric information is converted to bits and sent across the network
The receiver has no idea how the bits were created
Fresh biometric reading?
Saved version of previous reading?
Stolen copy of a reading?

28. Biometrics and Non-Human Authentication
Biometrics can’t be used to authenticate computers or programs
Only people (or perhaps animals)
Maybe there are some characteristics of computers that are similar
Maybe not
A question for research

29. A Common Issue for Network Authentication
Ultimately, you’re getting a bundle of bits packaged in one or more packets
It’s hard to guarantee how the bits were created
It’s easy to copy bundles of bits
Whatever authentication mechanism is used, it must handle that problem