802.11i Security Analysis: Can we build a secure WLAN? Changhua He Stanford University March 24 th, 2005 [email protected].

802.11i Security Analysis: Can we build a secure WLAN?

Changhua HeStanford University

March 24th, 2005

[email protected]

mailto:[email protected]

mailto:[email protected]

Outline

WLAN Security Overview Wireless Local Area Networks Wireless threats Wireless security history

IEEE 802.11i Standard Finite-state Verification of 4-Way Handshake More Insecurities and Improvements Modular Proof of Security Conclusions

802.11 Infrastructure Network

Wired Network

Can we build a secure WLAN ?

Wireless Threats

Passive Eavesdropping/Traffic Analysis Easy, most wireless NICs have promiscuous mode

Message Injection/Active Eavesdropping Easy, some techniques to gen. any packet with common

NIC, exploit MAC cooperation to interfere in a timely way

Message Deletion and Interception Possible, interfere packet reception with directional

antennas

Masquerading and Malicious AP Easy, MAC address forgeable and s/w available (HostAP)

Session Hijacking Man-in-the-Middle Denial-of-Service Attack

History of Wireless Security

802.11 (Wired Equivalent Protocol) Authentication: Open system (SSID) and Shared Key Authorization: some vendor use MAC address filtering Confidentiality/Integrity: RC4 + CRC Completely insecure

[Walker00,Arbaugh01,Wagner01,FMS01 …]

WPA: Wi-Fi Protected Access Authentication: 802.1X Confidentiality/Integrity: TKIP Reuse the legacy hardware, still problematic [Arbaugh02 …]

Availability is a big problem Frequency jamming: inevitable but expensive and

detectable Network and upper Layer: depend on protocols Link Layer DoS attack: unprotected management frames

[Arbaugh et al 01] [Bellardo et al 03] [Chen04] …

Outline

WLAN Security Overview IEEE 802.11i Standard

Data confidentiality and integrity Mutual authentication Key management protocols

Finite-state Verification of 4-Way Handshake More Insecurities and Improvements Modular Proof of Security Conclusions

IEEE 802.11i

Ratified on June 24, 2004 Data confidentiality and integrity

Encryption in Data Link Layer WEP: Wired Equivalent Privacy TKIP: Temporal Key Integrity Protocol CCMP: Counter-mode/CBC-MAC Protocol

• Long term solution, need hardware upgrade• With a fresh key, 802.11i CCMP is believed secure for data

confidentiality and integrity !

Mutual authentication RSNA: Robust Security Network Association EAP-TLS/802.1X/RADIUS

Key management 4-Way handshake, Group key handshake

RSNA Establishment RSNA Establishment Procedures

Network and Security Capability Discovery 802.11 Open System Authentication and Association EAP/802.1X/RADIUS Authentication 4-Way Handshake Group Key Handshake Secure Data Communications

Our security analysis gives: can provide satisfactory authentication and key management could be problematic in Transient Security Networks (TSN) reflection attack could be possible if not implemented

correctly Availability is still problematic

Authentica-tion Server (RADIUS)No Key

Authenticator UnAuth/UnAssoc802.1X BlockedNo Key

SupplicantUnAuth/UnAssoc802.1X BlockedNo Key

SupplicantAuth/Assoc802.1X BlockedNo Key

Authenticator Auth/Assoc802.1X BlockedNo Key


802.11 Association

EAP/802.1X/RADIUS Authentication

SupplicantAuth/Assoc802.1X BlockedMSK

Authenticator Auth/Assoc802.1X BlockedNo Key

Authentica-tion Server (RADIUS)MSK

MSK

SupplicantAuth/Assoc802.1X BlockedPMK

Authenticator Auth/Assoc802.1X BlockedPMK


4-Way Handshake

SupplicantAuth/Assoc802.1X UnBlockedPTK/GTK

Authenticator Auth/Assoc802.1X UnBlockedPTK/GTK


Group Key Handshake

SupplicantAuth/Assoc802.1X UnBlockedNew GTK

Authenticator Auth/Assoc802.1X UnBlockedNew GTK


A Complete Conversation

Data Communication

SupplicantAuth/Assoc802.1X UnBlockedPTK/GTK

Authenticator Auth/Assoc802.1X UnBlockedPTK/GTK


Outline

WLAN Security Overview IEEE 802.11i Standard Finite-state Verification of 4-Way Handshake

Murφ Model Checker Modeling the 4-way handshake Attacks and solutions

More Insecurities and Improvements Modular Proof of Security Conclusions

Finite-State Verification

Powerful methodology Used in protocol and software verifications No requirements on implementations

• Comparing to static analysis and dynamic tracing

Challenges Requires complete understanding of what you

check State space reduction techniques

Many tools available Generic model checking:

• Murφ, Spin, SMV Automatic model generation model checking:

• Pathfinder, Bandera, Verisoft

......

Correctnesscondition violated

Murφ rules for protocol participants and the intruder define a nondeterministic state transition graph

Murφ will exhaustively enumerate all graph nodes

Murφ will verify whether specified security conditions hold in every reachable node

If not, the path to the violating node will describe the attack

Murφ Model Checker

Running Murφ

Intruder Model

AnalysisTool

Formal Protocol

Informal Protocol

Description

Find error

Mur codeRFC, IEEE Std.

Mur code, similar for

all protocols

Specify security conditions and run

Mur

Diagnosing Errors

Bad abstraction Removed too much detail from the protocol when

constructing the abstract model Add the piece that fixes the bug and repeat

Genuine attack Yay! Hooray! Attacks found by formal analysis are usually quite

strong• Independent of specific cryptographic schemes• Independent of implementations, etc.

Test an implementation of the protocol, if available

The 4-Way Handshake

AA, ANonce, AA RSN IE, GTK, sn+1, msg3, MIC SPA, sn+1, msg4, MIC

PTK Derived

PTK DerivedRandom GTK

PTK and GTK802.1X Unblocked

PTK and GTK 802.1X Unblocked



AA, ANonce, sn, msg1

SPA, SNonce, SPA RSN IE, sn, msg2, MIC

Modeling the 4-Way Handshake

Authenticators/Supplicants: An association is maintained between a pair of

authenticator and supplicant Each association has a uniquely shared PMK Multiple sequential legitimate handshakes in one

association

Intruder Impersonate both supplicant and authenticator Eavesdrop, intercept and replay messages Compose messages with known nonce and MIC Forge fresh Message 1 Predict and replay nonces for pre-computation of MIC

Invariants Represent the security property, must be true for each

state

Authenticator Msg 1 (Mur)ruleset i: AuthenticatorId do ruleset j: AgentId do rule 20 "Authenticator sends Message 1 to associated Supplicants" !ismember(j, AuthenticatorId) & -- no message to Authenticators !ismember(j, IntruderId) & -- no message to Intruders

multisetcount(l:net, true) < NetworkSize & (aut[i].associations[j].session.state = A_PMKSA | aut[i].associations[j].session.state = A_DONE) ==> var outM: Message; -- outgoing message begin outM.source := i; outM.dest := j; outM.mtype := M_1; outM.nonce := freshNonce(); outM.sequence := freshSequence(); outM.address := i; multisetadd(outM, net); aut[i].associations[j].peer := j; aut[i].associations[j].pmk := usePMK(i, j); aut[i].associations[j].sequence := outM.sequence; aut[i].associations[j].nonce := outM.nonce; aut[i].associations[j].session.state:= A_WAITFORNS; end; end;end;

Invariant: Security Properties

invariant "PTKs are consistent and fresh"

forall i: AuthenticatorId do

forall j: SupplicantId do

aut[i].associations[j].session.state = A_DONE

->

(sup[j].associations[i].session.state = S_DONE &

ptkEqual(aut[i].associations[j].session.ptk,

sup[j].associations[i].session.ptk) &

aut[i].associations[j].sid = sup[j].associations[i].sid) |

(sup[j].associations[i].session.state = S_PTKSA &

aut[i].associations[j].sid <= sup[j].associations[i].sid)

end;

end;

Forged Message 1 Attack


PTK Derived










4-Way Handshake Blocking


PTK Derived








AA, ANonce[1], sn, msg1

AA, ANonce[n], sn, msg1

4-Way Blocking: Solution 1

Random-Drop Queue:

Randomly drop a stored entry to adopt the state for the incoming Message 1 if the queue is filled.

Not so effective


Authenticate Message 1 To reuse the algorithms/hardware, set nonces to

special values, e.g., 0, and derive PTK. Calculate MIC for Msg 1 using the derived PTK Good solution if PMK is fresh

If PSK and cached PMK, replay attacks ! Add a monotonically increasing global sequence

counter Use local time in authenticator side Sufficient space in Message 1 ( 8-octet sequence field ) No worry about time synchronization

Modifications on packet format


Re-Use Nonce Supplicant re-use SNonce until one 4-way handshake

completes successfully Derive correct PTK from Message 3 Authenticator may (or may not) re-use ANonce

Solve the problem, but More computations in the supplicant Attacker might gather more infomation about PMK by

playing with Message 1, ok if PMK is strongPTK=PRF{PMK, AA||SPA||ANonce||SNonce}

Performance Degradation


Combined solution Supplicant re-use SNonce Store one entry of ANonce and PTK for the first Message 1 If nonce in Message 3 matches the entry, use PTK

directly; otherwise derive PTK again and use it.

Advantages Eliminate the memory DoS attack Ensure performance in “friendly” scenarios Only minor modifications on the algorithm in the

Supplicant• No modifications on the packet format

Adopted by TGi Simple solution, but not immediate Practical considerations, not designing a new protocol

Reflection Attack

{A2, Nonce2, RSN IE, sn, msg2, MIC}{A1, Nonce1, RSN IE, GTK, sn+1, msg3, MIC}

{A1, sn+1, msg4, MIC}

Bogus Authentication Peers Authenticated

{A1, Nonce1, sn, msg1}{A2, Nonce1, sn, msg1}

{A1, Nonce2, RSN IE, sn, msg2, MIC}

{A2, Nonce1, RSN IE, GTK, sn+1, msg3, MIC}

{SPA, sn+1, msg4, MIC}

AdversaryImpersonates CommunicatingPeers

Legitimate Devices Authenticator and Supplicant

Reflection Attack: Solutions

Possible in ad hoc networks Each participant plays the role of both authenticator and

supplicant

Less damage if strong confidentiality adopted Adversary fool the peers to send packets Cannot decrypt the packet and generate response

Violate the mutual authentication concept Solutions:

Restrict each participant to play only one role: ok for WLAN, but inappropriate for ad hoc networks

Each participant play both roles, but under different PMK

4-Way Handshake Summary

Finite-state verification Find subtle bugs, suitable for verifying “small” protocols State-space explosion is a problem

Message 1 vulnerability Forge first message to block the protocol TPTK/PTK to prevent this, but only do partial work Store all states, but queue size is a problem Final solution adopted by Standards Committee

Reflection attack Possible in ad hoc scenario Each entity plays only one role, or both roles under

different PMK

Outline

WLAN Security Overview IEEE 802.11i Standard Finite-state Verification of 4-Way Handshake More Insecurities and Improvements

Poisoning and rollback attacks TKIP Michael Countermeasure problem Failure recovery

Modular Proof of Security Conclusions

RSN IE Poisoning

(2) Probe Request(3) Probe Response + AA RSN IE

(18) {AA, ANonce, AA RSN IE, GTK, sn+1, msg3, MIC}

RSN IE confirmation failed, Disassociation

Disassociate the Supplicant

(1) Beacon + AA RSN IEBogus Beacon + Modified RSN IE

Bogus Probe Response + Modified RSN IE

Legitimate Message Exchanges

SupplicantUnauthenticatedUnassociated802.1X Blocked

Authenticator UnauthenticatedUnassociated802.1X Blocked

RSN IE Poisoning: Solutions

Easy to launch the attack Legitimate participants unaware of it

Continue message exchanges, waste resources Adversary have more time to repeat the attack

Solutions Authenticate management frames

• Difficult to authenticate Beacon and Probe Response frame

Confirm RSN IE as soon as possible (EAP-TLS)• Necessary modifications on the standard

Relax the condition of RSN IE confirmation• Ignore insignificant bits, only confirm authentication suite• If authentication suite modified, probably fails at the

beginning of associations and retry new APs

Security Level Rollback Attack

Probe Request

Bogus Beacon (Pre-RSNA only)

Bogus Probe Response (Pre-RSNA only)

802.11 Authentication Request802.11 Authentication Response

Bogus Association Request (Pre-RSNA only)

802.11 Association ResponsePre-RSNA Connections

Beacon + AA RSN IE

Probe Response + AA RSN IE

Association Request + SPA RSN IE

SupplicantRSNA enabledPre-RSNA enabled

Authenticator RSNA enabledPre-RSNA enabled

Security Rollback: solutions

Security Level Rollback Attack Similar to general version-rollback attack Destroy the security since WEP is completely insecure Not a real vulnerability of 802.11i standard, but an

implementation problem of TSN Very possible mistake if requires transparency in

hybrid configurations

Solutions Allow only RSNA connections: secure, but too strict

for common network systems, where TSN is more convenient

Adopt both, supplicant manually choose to deny or accept a connection, authenticator restrict pre-RSNA (WEP) connections to only insensitive data

Michael Countermeasure

TKIP Michael algorithm and countermeasures Michael algorithm provides 20-bit security for MIC one successful forgery / 2 min., need countermeasures 802.11i documentation proposes:

• Cease communication for 60 sec. if two Michael MIC failures detected in one minute, re-key & deauthentication

• Limit to one successful forgery / 6 month• Verify in strict order: FCS < ICV < TSC < MIC• Update TSC unless MIC is validated

MAC IV/KeyID

TKIP MPDU Format

Ext. IV Data/MSDU MIC ICV FCS

EncryptedContains TSCMIC: Message Integrity CodeICV: Integrity Check Value (from WEP)FCS: Frame Check SequenceTSC: TKIP Sequence Counter

Michael DoS and Solutions

Still DoS attack through MIC failures ! Intercept a packet with valid TSC (possible) Modify packet and corresponding values of FCS, ICV to

get a packet with valid FCS, ICV, TSC but invalid MIC (easy)

Send modified packet twice in one minute (easy) MIC always invalid, TSC always valid

Solutions When MIC failure, cease communication only, no re-

keying and deauthentication Update TSC before MIC is validated What happens if modify TSC to extremely large number?

• Change TSC also change encryption key, wrong decryption• Some confidence on TKIP key schedule algorithm

Mitigation but not elimination

Management Frame att. & Sol. DoS attacks on plain 802.11 networks

Forge unprotected management frames, like Deauthentication/Disassociation [Bellardo et al 03]

Exploit virtual carrier sense mechanism by forging unprotected control frames, like RTS/CTS etc. [Bellardo 03]

802.11i still has these problems, solutions could be• Authenticate management frames• Validate virtual carrier sense in control frames

DoS attacks on EAP messages Forge EAPOL-Start, EAPOL-Success, EAPOL-Logoff, EAPOL-

Failure 802.11i can eliminate these by simply ignoring them ! Send more than 255 association request to exhaust the

EAP identifier space (8 bits) Adopt separate EAP identifier counter for each association

[Arbaugh et al 01] [Lynn02] [Moore02] [Bellardo et al 03] [Chen04] …

Failure Recovery Important for large protocols like 802.11i

Not affect protocol correctness, but efficiency Not eliminate DoS vulnerabilities, but make DoS more

difficult

802.11i adopts a simple scheme Whenever failure, restart from the beginning, inefficient ! What about restart from nearest point ? Tradeoffs

• Defensive DoS attack vs Captured DoS attack• Assumptions on adversary’s capability and network scenario

A better failure recovery for 802.11i If failure before 802.1X finishes, restart everything Otherwise restart components from nearest point Confidence on 802.1X authentication, mobile user?

• channel scanning time >> protocol execution time

Summarize Insecurities

Components insecurities The 4-Way Handshake: blocking and reflection TKIP Michael Countermeasure attacks

Combine 802.11i as a whole RSN IE Poisoning Attack: waste resources Security Level Rollback Attack: for hybrid configuration Attacks on unprotected management frames Inefficient failure recovery

Improved variant of 802.11i Mitigate all discussed vulnerabilitites Need more modifications on existing codes

Improved 802.11i ArchitectureStage 1: Network and Security Capability Discovery

Stage 2: 802.1X Authentication (mutual authentication, shared secret, cipher suite)

Stage 3: Secure Association (management frames protected)

Stage 4: 4-Way Handshake(PMK confirmation, PTK derivation, and GTK distribution)

Stage 5: Group Key Handshake

Stage 6: Secure Data Communications

Michael MIC Failure or Other Security Failures

Group Key Handshake Timout

4-Way Handshake Timout

Association Failure

802.1X Failure

Outline

WLAN Security Overview IEEE 802.11i Standard Finite-state Verification of 4-Way Handshake More Insecurities and Improvements Modular Proof of Security

Methodology Protocol compositional logic Proved theorems

Conclusions

Proof of security is necessary and useful “Head-scratching” approach is inconvincible Finite-state verification is incomplete Prove security vs find bugs

Difficulties How to model the protocol? How to define security? How to prove? There are already some formal methods to do these!

• [BAN90], [Paulson97], [Strand98], [Spi98], … …

IEEE 802.11i a large protocol, complicated control flows Compositional Logic, originated from [DMP01]

Motivations & Difficulties

Divide-and-conquer paradigm in security Divide the large protocol to several components Use protocol logic to reason the security properties of

each subprotocol Study the compositionality of all components

802.11i components 802.11 Association: a physical connection assumed EAP/802.1X/RADIUS Authentication 4-Way Handshake Group Key Handshake Secure Data Communications: CCMP believed secure

Methodology

Protocol Logic: Intuition

Alice’s information Protocol Private data Sends and receives

Honest Principals,Attacker

Protocol

Private Data

Formalizing the Approach

Abstraction based on Dolev-Yao Model [1983] “Black-box” cryptography No partial knowledge, no statistical tests

Language for protocol description Arrows-and-messages are informal “Cord” program for each protocol role: terms & actions

Protocol Semantics How does the protocol execute? Protocol, initial configuration, run

Protocol logic Stating security properties

Proof system Formally proving security properties

Cords: 4-Way Handshake

Authenticator =

(X, Y, PMKXY)[

new x; send X, Y, x, m1;receive Y, X, z;match z/y,m2,Hash(PTKXY,y,m2);

send X,Y,x,m3,Hash(PTKXY,x,m3);

receive Y, X, z;match z/m4, Hash(PTKXY, m4)

]X

Supplicant =

(Y, PMKXY)[

receive X, Y, z; match z/x, m1;new y; send Y, X, y, m2,Hash(PTKXY,y,m2);

receive X, Y, z;match z/x,m3,Hash(PTKXY,x,m3);

send Y, X, m4, Hash(PTKXY, m4)

]Y

Authenticator: 4-Way Handshake

Pre-conditionHas(X, PMKXY) Has(Y, PMKXY) (Has(Z, PMKXY) (Z = X Z = Y))

Secret Key Agreement Honest(X) Honest(Y)

Has(X, PTKXY) Has(Y, PTKXY) (Has(Z, PMKXY) (Z = X Z = Y))

Session Authentication Y. Honest(X) Honest(Y) Send(X, X, Y, x, m1) Receive(Y, X, Y, x, m1) Send(Y, Y, X, y, m2, Hash(PTKXY,y,m2))

Receive(X, Y, X, y, m2, Hash(PTKXY,y,m2))

Send(X, X, Y, x, m3, Hash(PTKXY,x,m3))

Receive(Y, X, Y, x, m3, Hash(PTKXY,x,m3))

Send(Y, Y, X, m4, Hash(PTKXY, m4))

Receive(X, Y, X, m4, Hash(PTKXY, m4))

Protocol Composition Composition

Convenient for analyzing large protocols and systems Not only for proving things, but also for building systems

Non-destructive combination Ensure that the combined parts do not degrade each

other’s security Assumptions about the environment: invariance

assertions

Additive combination: Accumulate security properties of combined parts,

assuming they do not interfere Properties achieved by individual protocol roles

Invariants of one component must be satisfied by any other components

802.11i Proved Properties

EAP-TLS Mutual authentication: actions matched in order Secret key achieved: known and only known to

peers

The 4-Way Handshake Session authentication: actions matched in order Secret key agreement: fresh PTK derived

Group Key Handshake Session authentication: actions matched in order Secret group key: group key distributed

802.11i components compose safely

Answering the question

Can we build a secure WLAN? (with 802.11i) Yes !

Limitations “Security” means “mutual authentication” and “secret

key” If CCMP secure, also have data confidentiality and integrity Assume perfect “black-box” cryptography Assume legitimate entities are honest

Only from perspective of the protocol No consideration on implementations

• Software bug, buffer overflow … No consideration on the whole system

• IPsec or firewall implemented? • SSL application?

Conclusions

Finite-state verification of 802.11i components Very useful methodology to find bugs Vulnerabilities found in the 4-Way Handshake

Attacks and solutions on 802.11i On the 4-Way Handshake: blocking and reflection On RSN IE: poisoning and rollback On the TKIP Michael Algorithm Failure recovery Improved 802.11i

It is really secure Prove “mutual authentication” and “secret key” properties We CAN build a secure WLAN !

Questions?

[http://www.pacwireless.com]

Temporal Key Integrity Protocol

Re-use legacy hardware: RC4 for encryption Michael algorithm for MIC

“Dolev-Yao” Model

Inspired by their 1983 paper D. Dolev and A. Yao. “On the security of public key protocols”.

IEEE Transactions on Information Theory, 29(2):198-208.

Adversary is a nondeterministic process Can read any message, decompose it into parts and re-assemble Cannot gain partial knowledge, perform statistical tests, …

“Black-box” cryptography Adversary can decrypt if and only if he knows the correct key Assumes that cryptographic functions have no special properties

Most mechanized formal methods for security analysis use some version of this model

Typical Dolev-Yao Term Algebra

Attacker’s term algebra is a set of derivation rules

Tu Tv T[u,v]

Tu Tv

Tcryptu[v] vT Tu

T[u,v]

Tu

T[u,v]

Tv

Tcryptu[v] Tu

Tv

if u=v for some

In the real world, there is no guarantee that attacker is restricted to these operations! He may perform probabilistic operations, learn partial information, etc.

Cords: 4-Way Handshake

Security properties

Sample Proof

802.11i Security Analysis: Can we build a secure WLAN? Changhua He Stanford University March 24 th, 2005 [email protected].

Documents

group key handshake

fresh key

xradius key management

security analysis

key supplicant authassoc

key authenticator authassoc

history of wireless

key supplicant unauthunassoc