IS 302: Information Security and Trust Week 9: User Authentication (part II) and Introduction to Internet Security 2012.

IS 302: Information Security and TrustWeek 9: User Authentication (part II) and

Introduction to Internet Security

2012

© Yingjiu Li 2007 2

Review

• Review of weak pwd authentication• Unix pwd• Windows password (LM and NT hashes)• Attacks (dictionary and brute-force)


Strong Password Authentication

I. Lamport scheme

II. Challenge response

III. Time stamp

• To address eavesdropping and replay attacks in communications


I. Lamport Scheme

• Registration phase– H: one way hash function– Bob: choose a secret s; compute and send

w0=H^100(s) to Alice in a secure manner

Bob Alice

Mallory

s

H^100(s)

Bob, w0=H^100(s)


Lamport scheme: first-time authentication

– Bob sends one-time password w1=H^99(s) to Alice

– Alice authenticates Bob by checking if H(w1)=w0

– If match, Alice replaces w0 with w1=H^99(s)

Bob Alice

Mallory

s

w1=H^99(s)

Bob, w0=H^100(s)


Lamport scheme: second time authentication

– Bob sends one-time password w2=H^98(s) to Alice

• Alice authenticates Bob by checking if H(w2)=w1

• If match, Alice replaces w1 with w2=H^98(s)

Bob Alice

Mallory

s

w2=H^98(s)

Bob, w1=H^99(s)


Discussion

• Why is Lamport scheme secure against eavesdropping and replay attacks?

• How many times can Alice authenticate Bob in Lamport scheme? After that?


II. Challenge Response Scheme• Alice and Bob share a pwd or pwd hash S• Upon receiving Bob’s id and authentication

request, Alice sends Bob a non-repeating challenge C

• Bob responses with R=E(S,C) or H(S,C)

Bob Alice

Mallory

Bob id

S SNon-repeating challenge C

Response R=E(S,C) or H(S,C)


Discussion• Why is challenge response scheme secure

against eavesdropping and replay attacks?• What is the difference between challenge

response scheme and Lamport’s scheme?


Case Study: MS Authentication Protocol (NTLM)

• Windows 2000, NT, XP, Vista, Windows 7

client serverType 1 msg: a set of flags(enc key sizes, req for mutual auth)

client serverType 2 msg: a set of flags +random challenge (8 bytes)

client serverType 3 msg: server response = MD4/MD5 hashing or DES enc(server challenge + client pwd hash)


NTLMv1 and NTLMv2• LM-hash of pw: 16 bytes (DES+DES)• NT-hash of pw: 16 bytes (MD4)

• C= 8-byte server challenge, random• K1 | K2 | K3 = NT-hash | 5 bytes-0• R1 = DES(K1,C) | DES(K2,C) | DES(K3,C)• K1 | K2 | K3 = LM-hash | 5 bytes-0• R2 = DES(K1,C) | DES(K2,C) | DES(K3,C)• Response = R1 | R2

• CS= 8-byte server challenge, random• CC=8-byte client challenge, random• CC* =8-byte client information (X, time, CC, domain name)• v2-Hash = HMAC-MD5(NT-hash, user name, domain name)• LMv2 = HMAC-MD5(v2-hash, CS, CC)• NTv2=HMAC-MD5(v2-hash, CS, CC*)• Response = LMv2 | CC | NTv2 | CC*


III. Time Stamp Scheme

• Alice and Bob keep synchronized clocks– Time T is used as one-time “challenge”– Bob sends Alice his id, and R=E(S,T) or H(S,T)– Alice verifies R=? E(S,T) or H(S,T) on Bob’s

pwd (or pwd hash) S and her current time T

BobAlice

Mallory

Bob id, R=E(S,T) or H(S,T)

S S


Discussion

• Why is time stamp secure against eavesdropping and replay attacks?

• How do you compare time stamp scheme with challenge response scheme and Lamport scheme?


Break


Introduction to Internet Security

I. Secure Socket Layer (SSL)

II. Firewall (FW)

III. Intrusion Detection System (IDS)


I. SSLSecure socket layer (SSL) or transport layer security

(TLS)• Secure communications between clients and

server in internet• TCP SSL web applications

Application protocol SMTP,HTTP,FTP…

TCP IP

Network Interface

TCP/IP Protocol Stack

SSL


SSL functionalities– Server authentication with certificate– Optional client authentication– Encrypted channel between server and client


SSL HandShake Protocol

Web client

Web server

1.SSL Request

2.Server public key certificate

3. random number R encrypted using server public key

4. Both compute session key based on R and switch to encrypted tunnel (https)


Typical Applications with SSL

• E-commerce

• VPN


II. FW

• Filter and audit traffic between internal and external network

• Prevent unauthorized access to a private network

Internal network

External networkFirewall


FW Policies• Close policy: Default deny, allow (white list)

firewall (p: packet) {

if (allow(p)) forward(p);

else drop(p);

}

• Open policy: Default permit, deny (black list) firewall (p: packet) {

if (deny(p)) drop(p);

else forward(p);

}


Close Policy Example boolean allow(packet p){ if (match(p.srcIP, 202.161.*.*) and

match (p.dstPort, 25)) return true; elseif (match(p.srcIP, 202.161.*.*) and

match (p.dstPort, 80)) return true; else return false; }


Open Policy Example boolean deny(packet p){ if (match(p.srcIP, 137.132.*.*) and

match (p.dstPort, 25)) return true; elseif (match(p.srcIP, 155.69.*.*) and

match (p.dstPort, 80)) return true; else return false; }


Windows FW– start cmd firewall.cpl advanced

settings• Inbound rules

• Outbound rules


Limitation of FW

Internal network

External networkFirewall


III. Intrusion Detection System (IDS)

• IDS should alert an attack in progress – With high accuracy– In real time manner– With complete diagnosis– With effective recommendations on how to

react


Accuracy of IDS

• False positive (alarm) rate– #false-alerts/#alerts

• False negative (miss) rate– #missed-attacks/#attacks


Signature-Based Vs Anomaly-Based• Signature based IDS

– Signature: DB of known attack patterns

– IDS reports situations that match signatures

– Good: Low false alarm rates, instantaneous detection

– Bad: Cannot detect new attacks

• Anomaly based IDS– Profile: model of normal

behaviors

– IDS reports situations that deviate from profiles

– Good: Can detect some new attacks

– Bad: High false alarm rates, high complexity

© Yingjiu Li 2007 INFT 865 Ravi Sandhu 2000 29

Why false alarm rate is high in anomaly-based detection

• Analogy– Test for a disease is 99% accurate

• 100 disease-free people tested, 99 test negative• 100 diseased people tested, 99 test positive

– Prevalence of disease is 1 in 10,000– Alice tests positive– What is probability Alice has the disease? 1 in 101 (101=1*99%+9,999*1%)– False alarm rate: 100 in 101 99%

• Now assume that test is 99.98% accurate– What is the false alarm rate?


IDS Summary• Advantages

– Complement firewall

– Continues to improve

• Disadvantages– False alarms

– Miss some new attacks

– Require security personnel to handle alarms and monitor track records


Review• How long is NTLM (v1) response message?

1. 64 bits2. 128 bits3. 384 bits

• Which of the following must have a cert in SSL?1. Client2. Server3. Both

• To detect new attacks, you need to choose1. Firewall2. Signature-based IDS3. Anomaly-based IDS

IS 302: Information Security and Trust Week 9: User Authentication (part II) and Introduction to Internet Security 2012.

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