secure human-computer identification against peeping attacks (sechci): a survey shujun lishujun li,...

Secure Human-Computer Identification against Peeping Attac

ks (SecHCI): A Survey

Shujun Li, Harry ShumVisual Computing GroupMicrosoft Research Asia

Sep. 2002

Outline

Introduction

A User Study

SecHCI: General Model

SecHCI: A Comprehensive Survey

SecHCI: Other Related Works

Our Opinions

1. IntroductionOutline

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 IdentificationThree 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 IdentificationThree 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 attackToken-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 rulesPartial solutions: Limitations still exist Pass-phrases / Pass-algorithms / Word associations / … Visual/graphical passwords

1.3 Peeping AttackIn the Real World

Your friends standing behind your shoulders can observe your passwordYour adversaries can install hidden cameras to steal your passwordYour adversaries can deploy malicious programs in your computer to get your passwordPowerful enemies can use TEMPEST (compromising emanations) devices to monitor your computerA lot of real stories on peeping attacks to banking cards (on ATMs) were reported by R. J. Anderson in 1994.

1.3 Peeping AttackIn 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 AttackNon-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 AttackSecHCI

Matsumoto-Imai protocol proposed at EuroCrypt’91 Not secure enough, cryptanalyzed by C.-H. Wang et al. at EuroCry

pt’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 n

ot 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 Asia

Crypt’2001 paper.

2. A User StudyGoals 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 implementat

ion of human-computer identification systemsBrief 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 Study2.1 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.


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.


SecHCI Most users wish the identification procedure can be fini

shed within 1 minute Most users think security and usability should be balan

ced 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 <H(x), C(y)>, which is in the set {accept, reject, }, where means H find C is a fake verifier.

3. SecHCI: General Model3.2 What is SecHCI?

Completeness A HCIP is complete if Pr[<H(z),C(z)>=accept]1-Pc.

Soundness A HCIP is sound if Pr[<H(x),C(y)>=accept]Ps.

(, , )-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 Model3.3 Definitions of Security

(p, k)-security against passive peeping attack Pr[<AA(Tk(H(z),C(z))), C(z)>=accept]p, where AA denot

es adversaries observe k random sampled identifications.

(p, k)-security against active peeping attack Pr[<AA(Tk(H(z),C(z))), C(z)>=accept]p, where AA denot

es adversaries observe k chosen identifications.(q, k)-human sensitivity (consciousness) to fake verifiers Pr[<H(z),C(z,AA(Tk(H(z),C(z))))>=]1-q, where C(z,AA

(Tk(H(z),C(z)))) denotes the fake verifier by AA.

3. SecHCI: General Model3.4 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 Survey4.1 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=(a1,…,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=(f1,…,f), then i=1~, af(i)=W(i).

4. A Comprehensive Survey4.1 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 Survey4.2 Matsumoto Protocols

Matsumoto Protocol 0 [ACM CCS’96] Fs is a finite field of order s.

The password is u vectors k1~ku, where ki is v-dimensional vector in Fsv.

The challenge is a non-zero v-dimensional vector qi in Fsv-{0}; the response ai is a element in Fs.

If i=1~u, ai=qiki, the user is accepted.

Matsumoto Protocol 1 and 2 [ACM CCS’96] Non-essential variants of Protocol 0.

4. A Comprehensive Survey4.2 Matsumoto Protocols

Usability Issues Protocol 1 can make implementations easier. Protocol 2 can provide a better trade-off between securi

ty and usability. Some graphical implementations of Protocol 1 and 2 ar

e given in Matsumoto’s paper.

Security Issues To break the password, only O(u) observations are need

ed for both passive and active peeping attack.

4. A Comprehensive Survey4.3 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. T

he response r is 0 or 1. For total m challenge, if r=cx holds for at least

(1-)m challenges, the user is accepted.


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.


Hopper-Blum Protocol 2 [AsiaCrypt’2001] Basically, Protocol 2 is similar to Protocol 1 wit

h two chief modifications. Modification 1: the response is calculated with

sum of k mins. Modification 2: the linear error-correcting mech

anism is introduced to avoid malicious change of legal challenges.


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 Survey4.4 HumanOIDs@CMU

HumanAut@CMU An image-based SecHCI, n images are involved and n/

2 images compose the password. A non-essential variant of Hopper-Blum Protocol 1. Th

e 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 Survey4.4 HumanOIDs@CMU

PhoneOIDs@CMU PhoneOIDs is “challenge-response protocols fo

r 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 Works5.1 Visual/Graphical Passwords

Selective pictures based passwords PassfaceTM: In each round, select your pass-face from 9

candidate faces. Déjà Vu: Select m portfolio images from n candidate i

mages.Point-and-click passwords PassPic: Click your pass-positions with your pass-order Graphical Password Windows in Passlogix v-GOTM SS

O: 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 Works5.2 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 1997. 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 2000. Now a CAPTCHA project is supported by Aladdin Center of CMU.


Distorted texts based CAPTCHAs Gimpy@CMU Another Gimpy-like CAPTCHA@AltaVista Pessimal print

Visual pattern based CAPTCHAs Bongo@CMU

Image based CAPTCHAs PIX@CMU CAPTCHAs based on image search problem More image processing techniques can be used to distor

t involved images


Sound/Speech based CAPTCHAs Sounds@CMU Byan@CityUHK

Text-only CAPTCHAs Impossibility of text-only CAPTCHAs under si

x assumptions “Find the Bogus Word”

Chinese CAPTCHAs?

5. Other Related Works5.3 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 Works5.3 More Topics on HIPs

Formal Studies on Security and Complexity of HIPsComputer Vision and HIPsBiometricsVisual CryptographyHuman-Error-Tolerant Passwords (or Fuzzy Commitment)Other Sides?

5. Other Related Works5.4 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 SecHCI6.1 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 SecHCI6.2 Our Opinion

Three principles Intentional errors Redundancies Balance

Two desired requirements The password length <= 16 The identification time <= 1 minute.

6. Our Opinion on SecHCI6.3 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 c1,…,c2m {0,

1}n. The response is 2m bits r1,…r2m. If i=1~m, (r2i-1-c2i-1x)+(r2i-c2ix)=1 (mod 2), t

hen the user is accepted. Such a protocol may be OK as a new solution o

f SecHCI.

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secure human-computer identification against peeping attacks (sechci): a survey shujun lishujun li,...

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