1. IMAGE-BASED AUTHENTICATION
Richard E. Newman, Piyush Harsh, and Prashant Jayaraman
University of Florida

2. Human Authentication What you are (biometric) What you have (token)
What you know (password)

3. Problems with Passwords
Meaningful
Word of mouth transfer
Sticking it near workstation
Image-based authentication (IBA) can solve these

4. Definitions
Image Space (IS) –the set of all images used by the IBA system.
Individual Image Set (IISa) – the set of images that a user Alice (a) chooses to authenticate herself.
Key Image – any image in a user's IIS.
Presentation Set (PS) – the set of images presented to Alice (from which the key images must be selected) for a given authentication attempt.
PS_i – the ith subset of PS presented to Alice during a run – PS = U PS_i

5. Architecture Authentication User Agent (AUA)
Authentication Server (AS)
The communication between them is encrypted using authenticated Diffie-Hellman
The AS is assumed to be a part of the Trusted Computing Base

6. Basic Protocol - Initialize
Image Set Selection
Alice selects ‘n’ images (n is set by the administrator, Bob)
Bob stores the image set at the AS
Presentation Subsets
Bob picks one image from IISa and some other images from IS-IISa for each PS_i
Alice picks the IISa image from each PS_i

7. Basic Protocol - Authenticate
Authentication
A→B: Username=Alice
B→A: Presentation set for Round 1, PS1.
A→B: Identified image.
B→A: Presentation set for Round 2, PS2.
…...
B→A: Presentation set for Round R, PSR.
If all R steps are successful, Bob authenticates Alice

8. Attacks Image-based authentication is not foolproof
The are four points of vulnerability
information stored on the AS
information sent between the AS and the AUA
the output at the AUA
the input at the AUA.

9. Keystroke Logging: AUA Input
Eve can observe or log Alice’s keystrokes and later authenticate herself as Alice.
Counter
Display the images in random order
- keystrokes are are only meaningful for this PS in this display order

10. Shoulder Surfing: AUA Output Logging
Eve can observe Alice’s screen (during the authentication process)and later authenticate herself as Alice.
Counter
Display the image when the mouse is over it. Otherwise, gray out the image
If input is hidden, then which image is selected is not known – only get PS_i’s
More on PS-based attacks later

11. TEMPEST Attack: AUA Output
Electromagnetic emanations from the output are used to recreate the screen a distance away.
Counter
Use contrasting colors that a person can easily distinguish, but which look the same to the eavesdropper.
Blur the images.
Add random noise to the images.

12. Brute Force Attack Counters Select every possible combination.
Note that dictionary attack is impossible.
Counters
Keep IIS and IS large
Attack cannot be done offline

13. Frequency Correlation Attack: Presentation Sets
Intersection Attack
The IS is large, and PS_i’s are chosen randomly (with one image from IIS). Any image that repeats across attempts, is very likely to be a part of IIS
Logic Attack
If the PS is the same (but not PS_i’s) in every attempt, using logic, within a small number of authentication attempts the attacker can narrow down the IIS to one or a few subsets from the PS.

14. Countering Frequency Correlation Attacks
Decoy Screens
A decoy screen is image grid consisting of images none of which are part of the user’s IIS. The user has to select “none of the above” to succeed in those rounds.
Make use of x rounds of decoy screens and y (y<=n) rounds or screens with images from user image set.

15. Countering Frequency Correlation Logic Attacks
Image Buckets
The IS can be partitioned into groups of images called image buckets. When an image from the IIS is displayed, all of the other images in the image bucket to which this image belongs will also be shown.
The intersection of the images displayed will never decrease.

16. Leaking Image Set Size Randomized number of rounds
The size of the image set is equal to the number of rounds.
Correlation between the Image set size and the number of rounds may be blurred
Randomized number of rounds
The number of rounds is randomized according to a bounded normal distribution.
The mean number of rounds and the variance can be changed as necessary.

17. Implementation Issues
Image Set Storage
If the images are randomized, only the seed for each image need be stored
Otherwise, entire IS needs to be stored
Security Implications
AS must store each user’s IIS.
If the AS is compromised, the IIS of every user can be obtained.
The scheme depends on the impenetrability of the AS

18. Key Strength
If K images per display may be selected, then with R rounds and |PS_i|=N we obtain an equivalent key size of KS= R log (C(N,K)) .
If K=1 then KS= R log (N)

19. Equivalent key bits for N=16 images/round

20. Equivalent key bits per key image

21. Conclusions IBA is in its infancy IBA is more user-friendly
It is difficult to share IBA image sets without showing the person the images
IBA offers an alternative to passwords that my be attractive for some situation
Asymmetric bandwidth
Poor user input capability
Protection at AS still an issue