Codes & Ciphers Ltd12 Duncan RoadRichmond, SurreyTW9 2JD

Information Security GroupRoyal Holloway, University of London

Egham, SurreyTW20 0EX

Impersonation Attacks

Fred Piper

Crete - 2007 2

Outline

• Information security

• User recognition– Use of passwords/PINs/cryptogrphic keys– Use of ‘tokens’

• Phishing

• Protection against man-in-the-middle attacks

• Multi factor or multi channel?

• The use of mobile phones as a ‘token’ for user recognition?

Crete - 2007 3

What is Information Security?

Some features include:

• Confidentiality

– Protecting information from unauthorised disclosure

• Integrity

– Protecting information from unauthorised modification, and ensuring that information can be relied upon and is accurate and complete

• Availability– Ensuring information is available to authorised users when

they need it

Crete - 2007 4

Defences and Attacks

Defences:

• Introduce security mechanism to protect data– Technical– Procedural– Contractual

• Introduce strong authentication mechanism

Attacks:

• Break the technical security mechanism

• Impersonate an authorised entity by breaking procedural mechanism

Crete - 2007 5

User Recognition (1)

3 factors:

1) Something you know (Password/PIN/Cryptographic Key)

2) Something you own (Token)3) Personal characteristic (Biometrics)

NOTE: Usually one-way authentication Tokens and biometrics often require ‘readers’‘Danger’ of false ‘readers’Cost issues

Crete - 2007 6

User Recognition (2)

• Many systems rely on more than 1 factor

• For multi-factor systems compromise of 1 factor should not enable impersonation

• The PIN/magnetic stripe card for ATM networks is an example of a 2-factor system where each individual factor is ‘weak’

Crete - 2007 7

Something You Know

• Password

• PIN

• Cryptographic key

Obvious observations:– A PIN is a password with limited alphabet– A cryptographic key may be regarded as a (secret) password

which the user may use but probably not know– Policies for the management of PINs and Passwords are

inconsistent

Crete - 2007 8

Password Policy

It is often recommended that:

• Users should adopt a large alphabet (at least alpha- numeric with upper and lower case letters)

• Passwords should be long (at least 8 characters?)

• Passwords should be randomly generated

• Passwords should be different for each system

• Passwords should be changed frequently

• Passwords should not be written down

Crete - 2007 9

PINs

• Personal identification number

• Usually 4 digits (sometimes 6)– Reason: users will not be able to remember longer PINs!

NOTE: This is inconsistent with general password policy. Undoubtedly a weak password

Crete - 2007 10

Cryptographic Keys

• It is the use of a cryptographic key, rather than revealing its value, that identifies a user

Crete - 2007 11

Cipher System

Cryptogramc

Encryption Key

EncryptionAlgorithm

Messagem

DecryptionAlgorithm

Decryption Key

Messagem

Interceptor

Key establishment channel

(secure)

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Two Types of Cipher System

• Conventional or Symmetric– Decryption Key easily obtained from Encryption Key

• Public or Asymmetric– Computationally infeasible to determine Decryption Key from

Encryption Key

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Keys as Identifiers

• Asymmetric System– Use of the private key acts as an identifier to ‘everyone’

• Symmetric System– Use of a key identifies users only to those (trusted) people who

share that key

NOTE: If an asymmetric system is used, an impersonator may either

1. obtain the use of the user’s private key

2. substitute their public key for that of the user

Crete - 2007 14

Authentication Using Smart Tokens

• Static Password Tokens– Owner authenticates himself to token– Token identified owner to system

• Dynamic Password Tokens– Token generates new password– (Owner activates token with PIN)– Owner enters ID plus dynamic password– System knows which dynamic password to accept

• Challenge-Response Tokens– System generates challenges– Owner activates token with PIN and enters challenge– Token generates response (probably challenge encrypted with key that

is unique to token)– System knows which response to accept

Crete - 2007 15

Dynamic Passwords

User’s password changes frequently (possibly at each

log-in)

Change influenced by at least one of:

• Secret information known to user

• Intelligent device which is unique to user

Crete - 2007 16

Challenge/Response for Dynamic Password

Given an unpredictable challenge, user’s token produces a response which is:

• Appropriate to the challenge

• Dependent on a user’s token

• Dependent on user’s knowledge

Crete - 2007 17

The Challenge / Response Principle for hand-held token

Key Key

Random number

Challenge PIN-Controlled

A A

Response

A - encrypt or OWFY/N = ?

HOST USER

Crete - 2007 18

Impersonation Attacks

• 1-way authentication– Steal and/or copy token– Guess or ‘observe’ password/PIN/cryptographic key– Con the user into divulging password/PIN eg phishing– Gain access to device using key

• 2-way authentication– Man-in-the-middle

Crete - 2007 19

Phishing Attacks (1)

• Social engineering

• Attacker discovers secret ‘information known’

• Banking customers have been ‘prime’ targets via email messages and fake websites

Crete - 2007 20

Phishing Attacks (2)

Countermeasures

• User education/awareness

• Use of 2 or 3 factor systems so that compromise of 1 factor has limited impact

Crete - 2007 21

Identification over the Internet

• Many applications use 2-factor systems that allow ‘card not present’ transactions

• Effectively a physical token is replaced by a virtual token which is nothing more than a card number

• This is a 1-factor system

• In Singapore the FA mandates use of genuine 2-factor authentication

• In UK banks are starting to issue customers with Chip and PIN ‘readers’

Crete - 2007 22

Phishing

• Browsers starting to try to detect fake websites– Google Safe Browsing for Firefox browser– Microsoft’s Internet Explorer 7

• Anti-Phishing War Group (APWG)– Forum to discuss phishing issues and share best practices– www.antiphishing.org

Crete - 2007 23

Phishing Attacks

Summary

• Enable attacks to discover secret ‘information known’

• ‘Best’ countermeasure is user education/awareness

• Effectiveness of attack decreases for 2 or 3 factor systems

• Use of conventional 2 or 3 factors often expensive and needs special hardware

• Introduction of ‘one-time’ PINs such as ITANs for some German e-banking systems helps

Crete - 2007 24

D-H Man-in-the-Middle Attack

B

FraudsterF

The Fraudster has agreed keys with both A and BA and B believe they have agreed a common key

A

F’s public key

F’s public key B’s public key

A’s public key

Crete - 2007 25

Protection Against Man-in-the-Middle Attack

• Rely on TTP to establish key management infrastructure (eg PKI)

• Use second (independent) communications channel to confirm key between A and B

(Over) Simple Example:– D-H protocol establishes 1024 shared bits– AES key is 128 bits from agreed positions– Users exchange different 32-bit sequences from the 1024 bits

over second (possibly insecure) channel

Crete - 2007 26

OOB (Out of Band) Authentication

Requirement

• A user claims an identity over a computer network

• Host wants to use a second channel to confirm it is the genuine user

• Neither party is willing to pay for ‘extra hardware’

Crete - 2007 27

Use of Two Channels

• Prior to PK crypto, most cryptographic systems needed a second (secure) channel for initial key establishment

• One motivation for introduction of PK crypto

Crete - 2007 28

OOB Communications

• Symmetric cryptography– Use secure second channel to enable secure communications

over an insecure primary channel

• Authentication– Use second (possibly insecure) channel to confirm that the

communications over the primary channel are secure

NOTE: In this context an insecure channel is one where interception is possible.

Crete - 2007 29

Communications

How does ‘token’ communicate with host:

• One channel or two channel system?

• One-way or two-way authentication?

• What is the interface?– The user?– A reader that is part of the network?– Other?

Crete - 2007 30

Mobile Phones

• There is a move towards systems where the mobile phone is ‘something you own’

• No reader required

• No extra cost (in the sense that most people have them)

• Use their own channel

• Security implications?

Crete - 2007 31

Question

• Are there situations where using the mobile phone as a token in a 2-factor system and using a second communication channel can decrease the chance of successful impersonation attacks?