國立清華大學資訊工程系資訊安全實驗室

孫宏民hmsun@cs.nthu.edu.twPhone: 03-5742968

Network Security--- User Authentication Network Security--- User Authentication and Key Agreement Protocolsand Key Agreement Protocols

第 2 頁

Outline

Basic Cryptographic Concept

Symmetric Encryption

Asymmetric Encryption

Digital Signature

Encrypted Key Exchange (EKE)

Conclusions

第 3 頁

Cryptographic System

Encryption

Plaintext Ciphertext

Decryption

Key

第 4 頁

•1. Confidentiality (Secrecy): The intruder cannot read the encrypted message from the ciphertext.

•2. Authentication: It should be possible for the receiver of a message to ascertain its origin; an intruder should not be able to masquerade as someone else.

•3. Integrity: It should be possible for the receiver of a message to verify that it has not been modified in transit; an intruder cannot substitute a false message for a legitimate one.

•4. Nonrepudiation: A sender should not be able to falsely deny later that he sent a message.

Four Basic Services of Cryptography:

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Plaintext(M) M

Ciphertext(C)

K1 K2

Encryption Decryption

Cryptanalyst

Symmetric Cryptosystem: The encryption & decryption keys are

the same. (EK(M)= C & DK(C)= M).

 Asymmetric Cryptosystem: Encryption & decryption keys are

different. (EK1(M)=C & DK2(C)=M)

The encryption key is public, while the decryption key can not be calculated from the public key.

Cryptographic System

第 6 頁

Symmetric Cryptosystem

DES (1977) IDEA (1992)RC5 (1994)AES (2001)

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Security Service: Confidentiality AuthenticationIntegrity Advantage： High Speed

Disadvantages：how to obtain a common secret key between two parities,

the number of secret keys is too big, can not achieve nonrepudiation.

Symmetric Cryptosystem

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Asymmetric Cryptosystem

RSA (1978) El-Gamal (1984) McEliece (1978) Knapsack (1978)

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Security Service: Confidentiality Integrity Authentication (by Signature) Nonrepudiation (by Signature) Advantage： a pair of keys for each user

Disadvantages： Slow speed Public key need to be authenticated by CA

Asymmetric Cryptosystem

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RSA (Encryption & Decryption)

Public key: n = pq, p and q are large primes (512 bit), e ( gcd (e, (p-1)(q-1)) =1)Private key: d, where ed = 1 mod (p-1)( q-1) Encryption: C=M e mod nDecryption: M= C d mod n

p=47, q=71, => n=3337 e=79, => d = 1019 M=688 Encryption: C=M e mod n = 68879 mod 3337 = 1570 Decryption: M= C d mod n =15701019 mod 3337 = 688

第 11 頁

One-way hash function

Input: X (unlimited length) Output: Y=H(X) (fixed length, e.g., 160 bit)

Given X, it is easy to compute Y.

Given Y, and H( ), it is computational infeasible to compute X. Given X and Y, it is computational infeasible to find X’ such that Y=H(X’).

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Message Message

Signature(S)

M

Signer A Verifier B

SignatureGeneration

Private Key

SignatureVerification

Public Key

Ture or False

Digital Signature

第 13 頁

Hash Functions : SHA MD5 FFT Snefru N Hash

Hash

Signature Functions : RSA DSA El-Gamal Elliptive Curve LUC

Digital Signature

M

h(M) SS

M

第 14 頁

Public key: n = pq, p and q are large primes (512 bit), e ( gcd (e, (p-1)(q-1)) =1), h is a hash function.Private key: d, where ed = 1 mod (p-1)( q-1)

Sign: S= h(M) d mod n

Verify: h(M) = S e mod n

RSA Digital Signature

第 15 頁

Secure Communication between Client and Server

Using Symmetric Cryptosystem: Each client and the server

share a common secret key.

Disadvantages:

1. Secret key must be strong

2. If the secret key is revealed, the messages in the past will also

be revealed.

Client Server

Ek(M’)

IDc, Ek(M)

第 16 頁

Secure Communication between Client and Server

Using Asymmetric Cryptosystem: Encryption + Signature

(See next page)

Disadvantages:

1. Public keys need to be authenticated by a CA.

2. Private key must be strong.

3. If the server’s private key is revealed, the messages in the past

will also be revealed.

第 17 頁

Secure Communication between Client and Server

第 18 頁

User Authentication in general

Based on one or more of: something a user has (smart card/token card) something a user is (fingerprint/voiceprint/retinal scan) something a user knows (password/short secret)

What’s a popular user authentication system based on three of these?

第 19 頁

Secure Password Authentication

Remote user access Goal: to be secure without requiring the user to

carry/remember anything except password

Remote client Firewall

protecteddomain

VPN traffic

(authenticated using password)

第 20 頁

Dictionary Attacks (Password Guessing Attacks)

An off- line, brute force guessing attack conducted by

an attacker on the network.

Attacker usually has a “dictionary” of commonly-used

passwords to try

People pick easily remembered passwords

“Easy- to- remember” is also “easy- to- guess”

第 21 頁

Passwords in the Real World

Entropy is less than most people think

Dictionary words, e. g. “pudding”, “plan9”

– Entropy: 20 bits or less

Word pairs or phrases, e. g. “hate2die”

– Represents average password quality

– Entropy: around 30 bits

Random printable text, e. g. “nDz2\ u> O”

– Entropy: slightly over 50 bits

第 22 頁

Password-based protocols

Telnet, FTP are insecure

Client ServerIDc , Password

Client ServerIDc , h(Password)

Hash function is still insecure due to dictionary attacks.

第 23 頁

Password-based Protocol with Challenge

Insecure against the dictionary attacks.

Client Server

h(Cha,Password)

Cha

ID

第 24 頁

We need ..............

a password-based authentication protocol whichis secure against dictionary attacks.

第 25 頁

What to do after authentication?

We need a common session key to protect our communication.

Diffie-Hellman key agreement provides two parties to share a common session key.

Secure Communication between Client and Server

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Diffie-Hellman Key Agreement

Goal : to provide two parties share a common session key

p : large prime, g : generator

gRa mod p

gRb mod p

K= (gRb)Ra mod p K= (gRa)Rb mod p

Client Server

第 27 頁

Man-in-the-middle attack

pg a mod pg a mod'

pg b mod' pg b mod

pg ab mod)( ' pg ba mod)( 'pg ba mod)( '

pg ab mod)( '

第 28 頁

Diffie-Hellman Key Agreement

Diffie-Hellman key agreement is vulnerable to the man-in-the-middle attack; it does not reach authentication

How about Diffie-Hellman key agreement using public key?

Problem: (1) does not provide forward secrecy,

(2) hard to remember (not a password).

pgY aXa mod

aaa CertYID ,,

bbb CertYID ,,

pgY bXb mod

pgYK abab mod)( pgYK bab

a mod)(

Client Server

第 29 頁

Forward Secrecy

Prevents one compromise from causing further damage

Compromising Should Not Compromise

Current password Future passwords

Old password Current password

Current password Current or past session keys

Current session key Current password

第 30 頁

Research Goal

To design a user authentication and key agreement protocol via password.

The protocol must satisfy the following requirements:

1. based on password only,

2. password may be weak,

3. be secure against the dictionary attack,

4. can provide perfect forward secrecy.

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Encrypted Key Exchange (EKE)[Bellovin and Merritt, 1992]

Two parties share a common password EKE can withstand dictionary attack

)( AEP

))(( REP A

))))(((( 1 REPPDR AA

P P

Generate encrypt/decrypt keys

AA DE ,

))((1AA EPPE

Generate R

R(Cha)

R(Cha||Chb)

R(Chb)

ServerClient

第 32 頁

DHEKE [Bellovin and Merritt, 1992]

)mod(, pgPA aR

pg bR mod

pg aR mod

pgK ba RR mod)()(),mod( b

R ChKpgP b

pgK ab RR mod)()||( ba ChChK

)( aChK

第 33 頁

Three-Party Key Exchange Protocol

Each client shares an easy-to-remember password with the server.

The protocol is responsible for establishing secure communication between two clients via the help of the server.

Application: E.g., ICQ, or mobile users

第 34 頁

STW-3PEKE [Steiner, Tsudik, and Waidner, 1995]

A S B

APA BR }{

BA PBPA ARBRA }{,}{,

SS NB

NA RR ,

KN

B flowR S ]1[,

KKflow ]]1[[

pRK BS NNA mod)(

pRK AS NNB mod)(

ANA gR

BNB gR

第 35 頁

Undetectable On-line guessing attack (I)[Ding and Horster, 1995]

A S B

APA BR }{

BA PAPA ARBRA }~

{,}{,

SS NA

NA RR

~,

SS NA

NA RR

~check

?

ANA gR

recordAPA BR }{

guess AP

~ get AR

~

AB RR~

第 36 頁

Undetectable On-line guessing attack (II) [Ding and Horster, 1995]

S B

BAPBPA ARBRA }{,}

~{, ~

SS NB

NA RR ,

ASBS NNB

NNA RR

~?

)()(check

guess AP

~

compute pgR ANA mod

~ ~

pgR BN

B mod

第 37 頁

Off-line Guessing Attack on STW-3PEKE [Lin, Sun, and Hwang, 2000]

A* S* B

X

BPB ARXA }{,,

YR SNA ,

~ ~

KflowY ]1[, pRK BS NNA mod)

~(

~

BP~

guess

BNB gR

SN

A NgR A~

,~ ~

BPB ARR }{ from ~

get B AS NN

BRK~~

)~

( ~

compute

Xflow1check ,]1[decrypt?

Kflow

第 38 頁

LSH-3PEKE (with server’s public key)[Lin, Sun, and Hwang, 2000]

A S B

SKAA PRraA },,{,

SS KBKAA BPRrbPRraA },,{,},,{,

rbAraB RARB ],[,],[

KBraB CflowhRB ]),1([,],[

BC

BNARK

ANBRK

ANA gR

BNB gR

第 39 頁

LSSH-3PEKE (without server’s public key)[Lin, Sun, Steiner, and Hwang, 2001]

A S BA,B

B

S

A

SP

NP

N gg ][,][ 21

B

SS

SA

AP

NNK

NA ggBAfgRA ][),,,(,, 21

,

AS NNSA gK )( 1

,

),,(,

),,,(,

2

,

1

,

S

SB

B

S

SA

NK

NB

NKA

gBAfgR

gBAfR

BS NN

SB gK )( 2,

),,,(),,,,(,, ABKBAK RRBAfRRBAfSASB

),,(),,,,(, ', AKABKB RBAfRRBAfRSA

),,(' BK RBAf

)(1 BNARHK

)(2' BNARHK

)(1 ANBRHK

)(2' ANBRHK

第 40 頁

Performance Comparison

第 41 頁

Conclusions

Password authentication and key agreement protocols are widely used.

EX： Electronic Commerce, Electronic Stock Trading

Two-parties protocols are suitable for client/server environment.

EX： Telnet, FTP

Three-Party protocols are suit for single server and multiple clients environment. Any two clients can authenticate each other and reach secure communication.

第 42 頁

Verifier-based Protocol

A server does not store plain password directly. Instead of storing a plain password, a server stores a

verifiable text (called verifier). It provides higher security level: an attacker must perform

dictionary attack when the server is corrupted. Furthermore, a verifier-based protocol can withstand the

stolen verifier attack.

第 43 頁

1. Bellovin, S. Merritt, M., 1992, “Encrypted key Exchange: Password-based Protocols Secure against Dictionary Attacks. Proceedings of IEEE Symposium on Research in Security and privacy, Oakland. 2. Steiner, M. Tsudik G. and Waidner, M., 1995, “Refinement and Extension of Encrypted Key Exchange,” ACM Operating Systems Review, Vol.29, Issue 3, pp. 22-30. 3. Ding, Y. and Horster, P., 1995, “Undetectable On-line Password Guessing Attacks”, Technical Report, TR-95-13-F, July. 4. C. L. Lin, H. M. Sun, and T. Hwang, 2000, Three-Party Encrypted Key Exchange: Attacks and a Solution,” ACM Operating Systems Review, Vol. 34, No. 4, pp. 12-20 . 5. C. L. Lin, H. M. Sun, M. Steiner, and T. Hwang, 2001, "Three-party Encrypted Key Exchange Without Server Public-Keys," IEEE Communications Letters, Vol. 5, No. 12, pp. 497-499. 6. C. L. Lin, H. M. Sun, and T. Hwang, 2001, “Efficient and Practical DHEKE Protocols ,” ACM Operating Systems Review, Vol. 35, No. 1, pp. 41-47.

References