The document explores the relationship between security and usability, highlighting that humans are often the weakest link in security processes. It discusses examples such as passwords and CAPTCHAs to outline the usability-security dilemma, emphasizing the challenges in balancing secure yet user-friendly solutions. Possible solutions include graphical passwords and hardware-based security measures, although the search for effective replacements for traditional textual passwords remains ongoing.

1. Usable Security:
When Security Meets Usability
Shujun LI (李树钧)
Senior Lecturer (Associate Professor)
Department of Computing
University of Surrey
http://www.hooklee.com

2. 2
Outline
- Where is University of Surrey? 
- Humans = The Weakest Link?
- Security vs. Usability
- Example 1: Passwords
- Usability-security dilemma: textual passwords
- Graphical passwords: a better solution?
- Example 2: CAPTCHAs
- A brief introduction to the term
- Usability-security dilemma
- Some Selected Topics for Research

3. 3
London  Guildford, Surrey
Highway A3: 33 miles
Fastest train: 33 mins

4. 4
University of Surrey @ Guildford
Guildford Railway
Station 
University of Surrey
Campus (by walk):
17 mins

5. 5
Dept. Computing @ University of Surrey

6. Usable Security
Humans = The Weakest Link?

7. 7
Security is a process, NOT a product.
- A product is secure A process is secure.
(Bruce Schneier)
6)

8. 8
Social engineering attacks do work well!
- Hackers only need to break the weakest link in a
process – humans!

9. 9
A real hacker’s testimony
Testifying before Congress not long ago, I explained
that I could often get passwords and other pieces of
sensitive information from companies by pretending
to be someone else and just asking for it.
Kevin D. Mitnick and William L. Simon
The Art of Deception: Controlling the Human Element of
Security (New York: John Wiley & Sons Inc., 2003).

10. 10
Social engineering everywhere:
Phishing, SMiShing, vishing, …
- Getting your password from you.

11. 11
Different kinds of weak humans
- Weak designers
- Weak programmers
- Weak assemblers
- Weak distributors
- Weak deployers
- Weak maintainers
- Weak users
- Weak …
 Security holes in the
delivered products
 Security holes in
the deployed system

12. 12
Are you a weak link of your system?
- Have you installed PGP or any other encryption software
for your email client?
- How often do you use the above encryption software to
protect your personal emails?
- Have you ever written some of your passwords down (on
paper, on mobile phone, …) to avoid forgetting them?
- Are you sharing the same passwords over multiple web
sites?
- How often do you click the detail of a digital certificate
shown in your web browser and check its content?
- Have you changed the default password of your home
router?

13. Usable Security
Security vs. Usability

14. 14
What does security mean?
- Confidentiality
- Information/Systems should be protected from unauthorized
access.
- Tools: Data encryption, user authentication, privacy enhancing
tools, …
- Integrity
- Information/Systems should be protected from unauthorized
manipulation.
- Tools: Cryptographic hashing, digital signature, …
- Availability
- Information should be protected from attacks making it unavailable
to legitimate users (e.g. DoS attacks).
- Tools: intrusion detection, distributed service, …

15. 15
What does usability mean?
- There is no widely accepted explanation. My personal
summary is the following.
- Psychological Acceptability
- A computer system (its functionalities and especially its computer-
human interface) should be designed for easy and correct use
without error by any human user.
- Economic Acceptability
- A computer system should be acceptable to the target human users
with reasonable costs.
- Reconfigurability/Scalability/Sustainability/Manageability
- A computer system should be easily
reconfigured/maintained/managed to adapt to different/new
requirements of end users.

16. 16
Security-usability dilemma
- Security is NOT what users want – users want their work to
be done and they don’t know what security really mean!
- Security often requires users to make HARD decisions, but
they do NOT have enough time or experience!
- Higher security often requires more computation  Higher
costs, slower process, more difficult to understand and use,
user’s tendency to misuse (intentional or unintentional), …
- Large systems involve many components and different
groups of users  requirements of different components
and users may conflict.
- Different aspects (C, I, A) of security may conflict with each
other as well, which further complicate the problem.
- …

17. 17
Security-usability dilemma: examples!
- For passwords the dilemma is:
- If a password is very strong (secure), then it
is not usable (hard to remember).
- If a password is usable (easy to remember),
then it is very weak (insecure).
- If I have to use a strong password but cannot
remember it, I will write it down!
- For CAPCTAHs the dilemma is:
- If a CAPTCHA is strong (hard for machines),
then it is hard to solve by humans.
- If a CAPTCHA is easy for humans to solve, it
is often weak (i.e., easy for machine as well).

18. Usable Security
Example 1:
Passwords (for User Authentication)

19. 19
How many passwords are there?
- 4 digits (PINs): 104 ≈213.3
- 6 digits (PINs): 106 ≈220
- Lowercase letters only, 7 characters:
267=8,031,810,176≈233
- Lowercase letters plus digits, 7 characters:
367=78,364,164,096 ≈236
- Lowercase and uppercase letters plus digits, 7
characters: 627=10,030,613,004,288 ≈242
- Lowercase and uppercase letters plus digits, 10
characters: 6210=839,299,365,868,340,224≈259.5

20. 20
How fast are today’s computers?
255

21. 21
What passwords are being used?
- Dinei Florêncio and Cormac Herley, A Large-Scale
Study of Web Password Habits, in Proc. WWW
2007, ACM/W3C
- Real passwords collected from 544,960 web users in
three months in 2006.

22. 22
What passwords are being used?
- DataGenetics, PIN analysis, 3rd September 2012
- 3.4 million leaked passwords composed of 4 digits.
xy00
9999
00xy 19xy
mmdd
xyxy

23. 23
Password cracking: 1979
- R. Morris and K. Thomson, “Password security: A
case history,” Communications of the ACM, vol.
22, no.11, 1979
- In a collection of 3,289 passwords…
- 15 were a single ASCII character
- 72 were strings of two ASCII characters
- 464 were strings of three ASCII characters
- 477 were strings of four alphamerics
- 706 were five letters, all upper-case or all lower-case
- 605 were six letters, all lower-case
- 492 appeared in dictionaries, name lists, and the like
2,831
passwords

24. 24
Password cracking: 1990
- Daniel V. Klein, “Foiling the Cracker: A Survey
of, and Improvements to, Password Security,” in
Proc. USENIX Workshop on Security, 1990
- In a set of 15,000 passwords
- 25% were cracked within 12 CPU months
- 21% were cracked in the first week
- 2.7% were cracked within the first 15 minutes

25. 25
Password cracking: 2005
- Arvind Narayanan and Vitaly Shmatikov, “Fast
dictionary attacks on passwords using time-
space tradeoff,” in Proc. CCS’2005, ACM
- In a collection of 142 real user passwords
- 67.6% (96) were cracked with a searching complexity
2.17×109≈231
25

26. 26
Password cracking: 2013
- Dan Goodin, “Anatomy of a hack: How crackers
ransack passwords like ‘qeadzcwrsfxv1331’,” ars
technica, 28 May 2013
- Three crackers were given 16,449 hashed passwords
and the best of them was able to crack 90% of the
passwords.
- Remark 1: All the passwords are considered harder
ones because they are what remained uncracked in a
much larger database of leaked passwords.
- Remark 2: Nate Anderson, Ars deputy editor and a self-
admitted newbie to password cracking, was able to
crack around 50% of the passwords within a few hours.

27. Usable Security
Password Security-Usability
Dilemma: Solutions?

28. 28
First the so-called XKCD method

29. 29
More solutions?
- Passphrases
- Strong password policy
- Frequently changed passwords
- Dynamic passwords (such as iTANs)
- Hardware-based solutions
- One-time password generators (such as mTANs and RSA®
SecurID)
- Physical tokens (such as smart cards)
- Challenge-response protocols
- Biometrics (finger/face/iris/palm/… recognition, …)
- Graphical passwords…

30. 30
Why may graphical passwords help?
- An old saying: “A picture is worth a thousand
words.”

31. 31
Why may graphical passwords help?
- 一图胜千言。

32. 32
Why may graphical passwords help?
- Graphics and images contain richer information
than texts, and harder to be exactly described by
both humans and computers.
-  Larger password space
-  Less weak passwords
-  More difficult to construct dictionary
-  Easier to remember and harder to forget
-  Harder to tell them to others (at least via phone )
-  A better balance between usability and security?

33. 33
Yet another advantage
- Graphical passwords are more secure against two
new attacks:
- Martin Vuagnoux and Sylvain Pasini, Compromising
Electromagnetic Emanations of Wired and Wireless
Keyboards, in Proc. USENIX Security Symposium 2009
- Kehuan Zhang and XiaoFeng Wang, Peeping Tom in
the Neighborhood: Keystroke Eavesdropping on Multi-
User Systems, Proc. USENIX Security Symposium 2009

34. 34
A classification of graphical passwords
- Class 1: Drawing-based passwords
- Class 2: Location-based graphical passwords
- Class 3: Recognition-based graphical passwords
- Class X: Hybrid graphical passwords?

35. 35
Class 1: DAS (Draw-A-Secret)
- I. Jermyn, A. Mayer, F. Monrose, M. K. Reiter and A. D.
Rubin, “The Design and Analysis of Graphical Passwords,”
in Proc. USENIX Security Symposium 1999 (Best paper
and best student paper awards!)

36. 36
Class 2: PassPoints
- S. Wiedenbeck, J. Waters, J.-C. Birget, A. Brodskiy and N.
Memon, PassPoints: Design and longitudinal evaluation of
a graphical password system, Int. J. Human-Computer
Studies, Vol. 63, pp. 102-127, 2005, Elsevier

37. 37
Class 3: Passfaces and Déjà Vu
- PassfacesTM
- Déjà Vu (Dhamija & Perrig, USENIX Security’2000)
Random art
http://www.random-
art.org

38. 38
Alert: Users’ choices are not random!
- Darren Davis, Fabian Monrose and Michael K. Reiter, “On
User Choice in Graphical Password Schemes,” in Proc.
USENIX Security Symposium 2004
Users tend to choose faces of
beautiful women and/or of
people in their own race.

39. 39
Alert: dictionary attack comes back!
- Julie Thorpe and P.C. van Oorschot, “Human-Seeded Attacks and
Exploiting Hot-Spots in Graphical Passwords,” in Proc. USENIX
Security Symposium 2007
A dictionary of click points (hotspots) can be harvested from a set
of human users (at the attacker’s disposal), or automatically
determined by some image processing algorithms.  For
automated attack, 8% passwords were cracked within 232 guesses.

40. 40
Alert: dictionary attack comes back!
- Amirali Salehi-Abari, Julie Thorpe, and P.C. van Oorschot, “On Purely
Automated Attacks and Click-Based Graphical Passwords,” in Proc.
ACSAC’2008, IEEE Computer Society
An improved dictionary attack: 16% passwords cracked using a
dictionary of less than 231.4 entries.

41. 41
Alert: dictionary attack comes back!
- P.C. van Oorschot, Amirali Salehi-Abari and Julie Thorpe, “Purely
Automated Attacks on PassPoints-Style Graphical Passwords,” IEEE
Trans. Information Forensics and Security, 5(3), 2010
Improved dictionary attacks: 7-16% passwords cracked using a dictionary
of 226 entries, 48-54% passwords using a dictionary of 235 entries.

42. 42
Alert: dictionary attack comes back!
- Krzysztof Golofit, “Click Passwords Under Investigation,” in
Proc. ESORICS’2007, Springer

43. 43
Alert: dictionary attack comes back!
- Julie Thorpe, P.C. van Oorschot, “Graphical Dictionaries
and the Memorable Space of Graphical Passwords,” in
Proc. USENIX Security Symposium 2004
- Mirror symmetric DAS passwords are used to construct a dictionary
The sub-password-space is
exponentially smaller than the
full space.

44. 44
Alert: dictionary attack comes back!
- Ziming Zhao, Gail-Joon Ahn, Jeong-Jin Seo, Hongxin Hu,
“On the Security of Picture Gesture Authentication,” in
Proc. USENIX Security Symposium 2013
- 10K Windows 8 Picture passwords were collected from 800 users.
- A training based approach: 24% of passwords cracked in one
database with a dictionary of size is 219 (total password space 231).

45. 45
Alert: usability problems!
- Karen Renauda and Antonella De Angeli, “My password is
here! An investigation into visuo-spatial authentication
mechanisms,” Interacting with Computers, vol. 16, pp.
1017-1041, Elsevier, 2004
- Problem 1: the incredible difficulty related to choosing the
background image.
- Problem 2: the user’s difficulty in pin-pointing a good pass-
point.
-  “The cognitive aspects of visual information processing
would appear to make the use of spatial position untenable
for authentication systems.”
45

46. 46
What have we learned?
- Textual passwords are bad.
- Graphical passwords haven’t been proven as a
(much) better replacement.
- There is still a long way ahead before we find a
real replacement of the current bad textual
passwords.
- For serious applications, moving to hardware
seems to the be most sensible choice.

47. Usable Security
Example 2: CAPTCHAs

48. 48
Starter 1: SONY CAPTCHA
- CAPTCHA @ SONY web forum (2011)
- In Google Chrome 21.0.1180.75 m:
- In Mozilla Firefox 15.0.1:
- In MSIE 9.0.8.112.16421:
- It is obviously weak, but…

49. 49
Starter 2: an e-banking CAPTCHA
- CAPTCHA @ a Chinese bank’s e-banking login
Web page
- In all web browsers:
- It seems to be better than the previous one, but is not
really strong. However, the simplest way of breaking it is
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50. 50
Starter 3: CAPTCHA @ a Chinese site
- “Input the result of executing the above code
________ refresh the page to get other code”.

51. 51
More starters: top 10 worst CAPTCHAs
- No. 1:
- No. 2:
- No. 3:

52. Usable Security
Captchas (or CAPTCHAs):
A Brief Introduction

53. 53
What are Captchas (or CAPTCHAs)?
- CAPTCHA
- Completely Automated Public Turing test to tell
Computers and Humans Apart
- It was proposed to fight against automated programs
abusing web resources (e.g. spamming).
I am human!
Then solve this!

54. 54
CAPTCHA has many names!
- CAPTCHA: A Turing test?
- Automated Turing Test? – The human interrogator in a
Turing test is automated by a computer.
- Reversed Turing Test? – The role of something (human
interrogator) is reversed in a Turing test.
- CAPTCHA = HIP (Human Interactive Proof)?
- Historically, Blum et al. coined the term HIP to cover
many human-involved security systems including
CAPTCHA and HumanOID.
- So, CAPTCHA  HIP.
- CAPTCHA = Authentication code?
- …

55. 55
CAPTCHA: before the term was coined
- Moni Naor, Verification of a human in the
loop or identification via the Turing test, 1996
- , “Add-URL” web page, protected by
a scheme later known as CAPTCHA, 1997
- US Patent 6195698, Method for
selectively restricting access to computer
systems, filed on 13 April, 1998, issued on 27
February, 2001
- Jun Xu, Richard Lipton and Irfan Essa, Hello,
Are You Human? Georgia Institute of
Technology College of Computing Technical
Report, GIT-CC-00-28, 13 November 2000

56. 56
CAPTCHA: after the term was coined
- 2000: Udi Manber from described the
“chat room problem” to Manuel Blum at the UC
Berkeley (who later moved to the CMU).
- 2000-2003: Blum and his collaborators coined the
term “CAPTCHA” and proposed some early
designs at www.captcha.net.
- 2002: the first report on
breaking CAPTCHAs appeared.
- 2002 onwards: a new kind of
cat-and-mouse game…

57. 57
CAPTCHAs everywhere
- Many (most?) user registration web pages are
protected by CAPTCHAs.
- Many login pages and web forms as well.

58. 58
CAPTCHAs everywhere
- CAPTCHA (reCAPTCHA) has been used for
digitizing books by Google.

59. 59
CAPTCHAs everywhere
- CAPTCHA has been used as a new advertising
platform as well!

60. 60
CAPTCHAs everywhere
- SweetCaptcha
- PlayThru
- MotionCAPTCHA

61. 61
Multi-CAPTCHA engines
- One example

62. Usable Security
CAPTCHA:
Security-Usability Dilemma

63. 63
Insecure but usable CAPTCHAs
- Almost all (if not all) e-banking CAPTCHAs [S. Li
et al. ACSAC 2010]

64. 64
Strong but less usable CAPTCHAs
- Google CAPTCHA (not reCAPTCHA)
- Simplest are not very hard to solve
- Averagely OK?
- Some are very hard (if not impossible) to solve
- Google has replaced this CAPTCHA by reCAPTCHA for
user registration, but still keep it for login (only after
three continuous login errors occur).

65. 65
CAPTCHA security mixed with usability
- Attackers are recruiting humans to do the job!

66. 66
CAPTCHA security mixed with usability
- Attackers also know how to recruit humans without
even paying them a penny (since 2007)!

67. 67
Questions about CAPTCHAs
- Can we finally find a CAPTCHA scheme with a
better balance between security and usability?
- Can security and usability be measured
automatically?
- Do we have any alternative solutions to the
problem?
- Cost-based proof-of-work (PoW) protocols?
- CAPTCHA + (Behavioural) Biometrics?
- CAPTCHA + BMI (brain-machine interface)?
- …

68. Usable Security
Selected Topics for Research

69. 69
Usable security research
- New forms of graphical passwords
- Pass-Maps: passwords on world maps
- New hardware based user authentication schemes
- Lower costs, simpler HCI, less system requirements, …
- New user authentication scheme secure against
observers
- Observers = shoulder-surfers, hidden cameras,
keyloggers, screen scrapers, malware, …
- Automated security and usability evaluation
- Human simulators, crowdsourcing, formal methods, …

70. 70
Usable security research
- New password management frameworks
- Password policies: Organization vs. Individual
- Human factor vs. Trust management
- Why should users trust a piece of software?
- Password cracking
- Discovery of new rules
- Modeling of human behaviour
- Password strength measurement
- Security visualization
- Better visualization of passwords?

71. 71
Usable security research
- Privacy management
- Privacy vs. Security
- User privacy vs. Digital forensics
- Economic modeling of computer security systems
and related human behaviour
- Business model vs. Mental model
- End users vs. Cyber criminals
- Underground economy
- Human factors and their impact on security of e-
payment systems
- Does NFC based banking bring new security problems?

72. 72
Usable security research
- Impact of mobile computing on usable security
- Enhanced mobility = Better usability = Worse security?
- Usability and security of mobile banking systems
- Usability and security issues in smart homes
- Smart grid and meters
- Smart TV (e.g. TV banking)
- Usability and security of physical-cyber systems
- Internet of Things
- Car security
- Medical and health devices
- …

73. Usable Security
Thanks for your attention!
Questions + Answers 
Collaborations?