Secure Human-Computer Identification against Peeping Attacks (SecHCI): A Survey Shujun LiShujun Li, Harry ShumHarry Shum Visual Computing Group Microsoft Research Asia Sep. 2002

Outline Introduction A User Study SecHCI: General Model SecHCI: A Comprehensive Survey SecHCI: Other Related Works Our Opinions

1. Introduction Outline Human-Computer Identification Problems of Widely-Used Fixed Passwords Yet Another Danger: Peeping Attack In the real world In the theoretical world Known Solutions to Peeping Attack

1.1 Human-Computer Identification Three Identifications Knowledge-based: What do you know? Fixed (textual/visual) password / PIN Pass-phase / Pass-algorithm / word-association Challenge-response identification protocol Zero-knowledge identification protocol Token-based: What do you have? Magnetic-striped card / Smart card Hand-held one-time password generator Biometrics-based: Who are you? Face / Fingerprint / Iris / …

1.1 Human-Computer Identification Three Identifications: Comparison Knowledge-based Fixed Password: Easily understood and widely accepted, but vulnerable to dictionary attack and replay attack Challenge-response protocol: Relatively complex but secure against replay attack Token-based More secure than fixed password You must physically have it / sensitive to loss Biometrics-based Always with you / minimal user efforts Performance is not really satisfactory / privacy involved

1.2 Problems of Fixed Password Dictionary attack: A troublesome paradox between security and usability Humans always select passwords from a dramatically small subset of the password space Too random or too long passwords are hard to remember for humans Compulsive password rules are useful to avoid problems, but users always try to circumvent the rules Partial solutions: Limitations still exist Pass-phrases / Pass-algorithms / Word associations / … Visual/graphical passwords

1.3 Peeping Attack In the Real World Your friends standing behind your shoulders can observe your password Your adversaries can install hidden cameras to steal your password Your adversaries can deploy malicious programs in your computer to get your password Powerful enemies can use TEMPEST (compromising emanations) devices to monitor your computer A lot of real stories on peeping attacks to banking cards (on ATMs) were reported by R. J. Anderson in 1994.

1.3 Peeping Attack In the Theoretical World SecHCI means such a human-computer identification by which one can successfully prove its identity without any auxiliary devices and via insecure communication channel. Two kinds of peeping attacks Passive peeping attack and Active peeping attack  In passive peeping attack, adversaries can only passively observe the identification procedure  In active peeping attack, adversaries can impose the verifiers Open peeping attack and Hidden peeping attack One more requirement Human sensitivity (consciousness) to faked verifiers

1.4 Solutions to Peeping Attack Non-SecHCI Displaying “******” on the screen instead of plain-password Shielding your input from malicious “eyes”. Visual shielding / TEMPEST shielding LVSVSS – a shielding based on visual cryptography One-time passwords Challenge-response protocols Biometrics?

1.4 Solutions to Peeping Attack SecHCI Matsumoto-Imai protocol proposed at EuroCrypt’91 Not secure enough, cryptanalyzed by C.-H. Wang et al. at EuroCrypt’95 Matsumoto protocols proposed at HCI International’95 and ACM CCS’96 Security against peeping attack is not strong Hopper-Blum protocols proposed at AsiaCrypt’2001 Security against peeping attack is acceptable, but the usability is not good. PhoneOIDs proposed by M. Blum (2001) All proposed PhoneOIDs have been known insure HumanAut Project supported by CMU (2002) One implementation of a variant of Hopper-Blum protocol in AsiaCrypt’2001 paper.

2. A User Study Goals and Brief Description Goals Investigate the users’ opinions on security and usability of human-computer identification system, especially fixed passwords and SecHCI Show the significance of peeping attack and SecHCI Confirm some principles in the design and implementation of human-computer identification systems Brief description A web site is constructed 18 questions are involved About 100 volunteers attended

2. A User Study 2.1 Investigation Results (1) Fixed passwords I Almost all users ever forgot their passwords Most users ever told other of their passwords Most users think security is more important than convenience (usability) after careful consideration Many users ever encountered hesitation when they set a new password Some users even have no really secret passwords Summary: for most users, security > usability, but they always forget this principle in the real world.

2. A User Study 2.1 Investigation Results (2) Fixed passwords II All users have two or more different passwords Most users have <=6 different passwords Most users use 6~10-length passwords Most users also think 6~10 is the best password length Most users think 15 (about) is the upper bound of the password length for all security applications Summary: for most users, 6~10-length passwords are good, and >16 length is unendurable.

2. A User Study 2.2 Investigation Results (3) Peeping attack Most users think peeping attack is a real danger in the security world, especially when their money and privacy is endangered. Most users will follows at least partial warns from security experts and technical news. Summary: the significance of peeping attack is confirmed, especially for electronic financial applications.

2. A User Study 2.2 Investigation Results (4) SecHCI Most users wish the identification procedure can be finished within 1 minute Most users think security and usability should be balanced in the design of secure human-computer identification Summary: a good SecHCI must balance security and usability, and the consuming time for one identification should be <= 1 minute.

3. SecHCI: General Model 3.1 Fundamentals SecHCI should be a challenge-response protocol with time-variant parameters like the following one. Define SecHCI as a HCIP – human-computer interactive protocol (H,C) with auxiliary input. The transcript between H and C is T(H(x), C(y)), and the output of the protocol is, which is in the set {accept, reject,  }, where  means H find C is a fake verifier.

3. SecHCI: General Model 3.2 What is SecHCI? Completeness A HCIP is complete if Pr[ =accept]  1-P c. Soundness A HCIP is sound if Pr[ =accept]  P s. ( , ,  )-Human-Only Executability (HOE) A HCIP is ( , ,  )-human-only executable if any T(H(x),C(y)) can be carried by (1-  ) population with the error probability , and can be finished within  seconds. A SecHCI is a HCIP satisfying completeness and soundness, and ( , ,  )-HOE with acceptable parameters.

3. SecHCI: General Model 3.3 Definitions of Security (p, k)-security against passive peeping attack AA Pr[ =accept]  p, where A denotes adversaries observe k random sampled identifications. (p, k)-security against active peeping attack AA Pr[ =accept]  p, where A denotes adversaries observe k chosen identifications. (q, k)-human sensitivity (consciousness) to fake verifiers A AA Pr[ =  ]  1-q, where C(z,A(T k (H(z),C(z)))) denotes the fake verifier by A.

3. SecHCI: General Model 3.4 Security in the Real World Basic Attacks Random response attack (soundness) Brute force (exhaustive) attack Dictionary attack Peeping Attacks Store-and-replay attack Intelligent off-line password attack  Differential attack / Deduction-based attack / Intersecting attack Multi-onlooker peeping attack Advanced Attacks Partially-known password attack Malicious administrator attack Denial-of-Logon attack

4. A Comprehensive Survey 4.1 Matsumoto-Imai Protocol Matsumoto-Imai protocol [EuroCrypt’91] An simple example to show the basic idea:  ={1,2,…,9,0},  ={1,2,…,8} , the password is  ={1,2,4,6} ,  ={1,2,3,4} , W=3124. Assume  =#(  )=8 and =#(  )=4, the challenge q is a bijection from  to , and the response is a  -length word a=(a 1,…,a  ) whose characters are all in . The accepted responses should satisfy the following requirement: extract all characters in q and also in , and record their order in q to compose a list f=(f 1,…,f ), then  i=1~, a f(i) =W(i).

4. A Comprehensive Survey 4.1 Matsumoto-Imai Protocol Security problems Only one observation is enough to know . This protocol cannot resist “replay challenge attack” (an active peeping attack). Only several observations is needed to decrypt  and then find W. [C.-H. Wang et al. EuroCrypt’95] In passive peeping attack, the number of observations is also rather small. C.-H. Wang et al. proposed a modified version, but whose usability is too poor.

4. A Comprehensive Survey 4.2 Matsumoto Protocols Matsumoto Protocol 0 [ACM CCS’96] F s is a finite field of order s. The password is u vectors k 1 ~k u, where k i is v- dimensional vector in F s v. The challenge is a non-zero v-dimensional vector q i in F s v -{0}; the response a i is a element in F s. If  i=1~u, a i =q i  k i, the user is accepted. Matsumoto Protocol 1 and 2 [ACM CCS’96] Non-essential variants of Protocol 0.

4. A Comprehensive Survey 4.2 Matsumoto Protocols Usability Issues Protocol 1 can make implementations easier. Protocol 2 can provide a better trade-off between security and usability. Some graphical implementations of Protocol 1 and 2 are given in Matsumoto’s paper. Security Issues To break the password, only O(u) observations are needed for both passive and active peeping attack.

4. A Comprehensive Survey 4.3 Hopper-Blum Protocols Hopper-Blum Protocol 1 [AsiaCrypt’2001] The password is a (0,1)-vector x  {0,1} n whose weight is k. The challenge is also a (0,1)-vector c  {0,1} n. The response r is 0 or 1. For total m challenge, if r=c  x holds for at least (1-  )m challenges, the user is accepted.

4. A Comprehensive Survey 4.3 Hopper-Blum Protocols Security Issues Hopper-Blum Protocol 1 cannot resist replay challenge attack (active peeping attack). Some Errors and More Problems The result of Theorem 1 is wrong. The masquerading probability of random response attack is slightly overestimated. Paradox exists between security and usability, especially on the value of k.

4. A Comprehensive Survey 4.3 Hopper-Blum Protocols Hopper-Blum Protocol 2 [AsiaCrypt’2001] Basically, Protocol 2 is similar to Protocol 1 with two chief modifications. Modification 1: the response is calculated with sum of k mins. Modification 2: the linear error-correcting mechanism is introduced to avoid malicious change of legal challenges.

4. A Comprehensive Survey 4.3 Hopper-Blum Protocols Merits Protocol 2 can resist active peeping attack. Protocol 2 has 0.1-human sensitive to fake verifiers. Problems Usability of Protocol 2 is even more poor than Protocol 1. Some problems in Protocol 1 still exist in Protocol 2.

4. A Comprehensive Survey 4.4 An image-based SecHCI, n images are involved and n/2 images compose the password. A non-essential variant of Hopper-Blum Protocol 1. The challenge is always a vector with fixed weight. Usability is poor when n is too large. Pass-Rules You can freely change all n images. Then you can use some meaningful features of the n/2 pass-images to remember so many pictures.

4. A Comprehensive Survey 4.4 PhoneOIDs is “challenge-response protocols for use over the phone”, which means SecHCI protocols of two parties with limited computation capabilities. Many PhoneOIDs have been proposed, but all are insecure.

5. Other Related Works 5.1 Visual/Graphical Passwords Selective pictures based passwords Passface TM : In each round, select your pass-face from 9 candidate faces. Déjà Vu: Select m portfolio images from n candidate images. Point-and-click passwords PassPic: Click your pass-positions with your pass-order Graphical Password Windows in Passlogix v-GO TM SSO: Click several things to construct your password. Drawing-based passwords Draw-a-Secret (DAS): Draw your pass-strokes on a m  n grid.

5. Other Related Works 5.2 CAPTCHAs CAPTCHA stands for “Completely Automated Public Turing Test to Tell Computers and Humans Apart”, also called Reverse Turing Test by some researchers. The chief application of CAPTCHA is to foil malicious online robots, and can also be used to relax the security against random response attack in SecHCI protocols. The first paper on CAPTCHA occurred in 1996 (by M. Naor). The first implementation of CAPTCHA is designed in The initial booming of interests on CAPTCHAs is promoted by the occurrence of Gimpy, a CAPTCHA designed by M. Blum et al. at CMU in Now a CAPTCHA project is supported by Aladdin Center of CMU.

5. Other Related Works 5.2 CAPTCHAs Distorted texts based CAPTCHAs Another Gimpy-like Pessimal print Visual pattern based CAPTCHAs Image based CAPTCHAs CAPTCHAs based on image search problem More image processing techniques can be used to distort involved images

5. Other Related Works 5.2 CAPTCHAs Sound/Speech based CAPTCHAs Text-only CAPTCHAs Impossibility of text-only CAPTCHAs under six assumptions “Find the Bogus Word” Chinese CAPTCHAs?

5. Other Related Works 5.3 More Topics on HIPs HIP means “Human Interactive Proof”, which covers many topics, such as SecHCI protocol, CAPTCHA, and visual/graphical password. There is a HIP project at Aladdin Center of CMU to support research and product transfer of theoretical results.

5. Other Related Works 5.3 More Topics on HIPs Formal Studies on Security and Complexity of HIPs Computer Vision and HIPs Biometrics Visual Cryptography Human-Error-Tolerant Passwords (or Fuzzy Commitment) Other Sides?

5. Other Related Works 5.4 ZK Identification Protocol Many Zero-Knowledge based identification protocols have been proposed. The basic idea used in ZK protocols may be useful for the design of SecHCI protocols. The general model of ZK identification protocols: 1) P=>V: a public (random) witness; 2) V=>P: a (random) challenge; 3) P=>V: a response (dependent on the witness and the challenge).

6. Our Opinion on SecHCI 6.1 A Comparison By security against passive peeping attack Matsumoto-Imai Protocol < Matsumoto Protocols < Hopper-Blum Protocol 2 < Hopper-Blum Protocol 1; By security against active peeping attack Matsumoto-Imai Protocol < Matsumoto Protocols < Hopper-Blum Protocol 1 < Hopper-Blum Protocol 2; By usability Hopper-Blum Protocol 2 < Matsumoto-Imai Protocol < (0,1)-version of Hopper-Blum Protocol 1  decimal version of Hopper-Blum Protocol 1  Matsumoto Protocols.

6. Our Opinion on SecHCI 6.2 Our Opinion Three principles Intentional errors Redundancies Balance Two desired requirements The password length <= 16 The identification time <= 1 minute.

6. Our Opinion on SecHCI 6.3 A Prototype Protocol Following our opinions on SecHCI, we can give a prototype protocol as follows The password is a (0,1)-vector x  {0,1} n whose weight is k. The challenge is 2m (0,1)-vectors c 1,…,c 2m {0,1} n. The response is 2m bits r 1,…r 2m. If  i=1~m, (r 2i-1 -c 2i-1  x)+(r 2i -c 2i  x)=1 (mod 2), then the user is accepted. Such a protocol may be OK as a new solution of SecHCI.

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