Protocol Rollback and Network Security

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1 CSE 484 / CSE M 584 (Spring 2012) Protocol Rollback and Network Security Tadayoshi Kohno Thanks to Dan Boneh, Dieter Gollmann, Dan Halperin, John Manferdelli, John Mitchell, Vitaly Shmatikov, Bennet Yee, and many others for sample slides and materials...

2 Goals for Today Protocol Rollback Attacks (in SSL) Network security HW2 Lab 3 HW 3 (EC)

3 What is SSL / TLS? Transport Layer Security (TLS) protocol, version 1.2 De facto standard for Internet security The primary goal of the TLS protocol is to provide privacy and data integrity between two communicating applications In practice, used to protect information transmitted between browsers and Web servers (and mail readers and...) https://datatracker.ietf.org/wg/tls/ Based on Secure Sockets Layers (SSL) protocol, version 3.0 Same protocol design, different algorithms Ubiquitously deployed in commercial Web browsers

4 TLS Basics TLS consists of two protocols Familiar pattern for key exchange protocols Handshake protocol Use public-key cryptography to establish a shared secret key between the client and the server Record protocol Use the secret key established in the handshake protocol to protect communication between the client and the server We will focus on the handshake protocol

5 TLS Handshake Protocol Two parties: client and server Negotiate version of the protocol and the set of cryptographic algorithms to be used Interoperability between different implementations of the protocol Authenticate client and server (optional) Use digital certificates to learn each other s public keys and verify each other s identity Use public keys to establish a shared secret

6 Handshake Protocol Structure ClientHello ServerHello, [Certificate], [ServerKeyExchange], [CertificateRequest], ServerHelloDone C [Certificate], ClientKeyExchange, [CertificateVerify] S switch to negotiated cipher Finished Record of all sent and received handshake messages switch to negotiated cipher Finished

7 ClientHello ClientHello Client announces (in plaintext): Protocol version Supported Cryptographic algorithms C S

8 ServerHello C, Version c, suite c, N c ServerHello C Server responds (in plaintext) with: Highest protocol version supported by both client and server Strongest cryptographic suite selected from those offered by the client S

9 ServerKeyExchange C, Version c, suite c, N c Version s, suite s, N s, ServerKeyExchange C Server sends public-key certificate containing either RSA, or Diffie-Hellman public key (depending on chosen crypto suite) S

10 ClientKeyExchange C, Version c, suite c, N c Version s, suite s, N s, sig ca (S,K s ), ServerHelloDone C ClientKeyExchange S Client generates some secret key material and sends it to the server encrypted with the server s public key (if using RSA)

11 Core SSL 3.0 Handshake (Not TLS) C, Version c =3.0, suite c, N c Version s =3.0, suite s, N s, sig ca (S,K s ), ServerHelloDone C {Secret c } Ks S If the protocol is correct, C and S share some secret key material (secret c ) at this point switch to key derived from secret c, N c, N s switch to key derived from secret c, N c, N s

12 Version Rollback Attack C, Version c =2.0, suite c, N c Server is fooled into thinking it is communicating with a client who supports only SSL 2.0 Version s =2.0, suite s, N s, sig ca (S,K s ), ServerHelloDone C {Secret c } Ks S C and S end up communicating using SSL 2.0 (weaker earlier version of the protocol without finished message from client)

13 SSL 2.0 Weaknesses (Fixed in 3.0) Cipher suite preferences are not authenticated Cipher suite rollback attack is possible SSL 2.0 uses padding when computing MAC in block cipher modes, but padding length field is not authenticated Attacker can delete bytes from the end of messages MAC hash uses only 40 bits in export mode No support for certificate chains or non-rsa algorithms, no handshake while session is open

14 Protocol Rollback Attacks Why do people release new versions of security protocols? Because the old version got broken! New version must be backward-compatible Not everybody upgrades right away Attacker can fool someone into using the old, broken version and exploit known vulnerability Similar: fool victim into using weak crypto algorithms Defense is hard: must authenticate version in early designs Many protocols had version rollback attacks SSL, SSH, GSM (cell phones)

15 Version Check in SSL 3.0 (Approximate) 2 C, Version c =3.0, suite c, N c Embed eight 3s into left side of this secret if server said Versions=2.0 2 Version s =3.0, suite s, N s, sig ca (S,K s ), ServerHelloDone C {Version c,secret c } Ks If embedded version information includes eight 3s but server supports version 3, issue error. S If the protocol is correct, C and S share some secret key material secret c at this point switch to key derived from secret c, N c, N s switch to key derived from secret c, N c, N s

16 SSL/TLS Record Protection Use symmetric keys established in handshake protocol

17 Internet Infrastructure backbone local network local network Internet service provider (ISP) ISP TCP/IP for packet routing and connections Border Gateway Protocol (BGP) for route discovery Domain Name System (DNS) for IP address discovery

18 (Some) Entities User Network Admin Intermediate ISPs Server

19 (Some) Goals Service (can get to Internet) Privacy (middle-entities shouldn t know what communicating or with whom) User Fairness (e.g., get service I paid for) Integrity (can t impersonate me, modify my data) Safety (network shouldn t attack me)

20 (Some) Goals Network Admin Service (clients can get to Internet) Performance (network works well) Identity (know what s on network) Safety (no one launching attacks) Accountability (can find bad users)

21 (Some) Goals Service (deliver traffic -> earn $$) Reliability & Performance (network works well) Intermediate ISPs Integrity of delivered traffic (can bill customers properly, you re not overcharged by providers)

22 (Some) Goals Service (deliver traffic -> earn $$) Reliability & Performance (network works well) Server Analytics (better delivery) Accounting (can bill customers properly) Safety (not being attacked)

23 (Some) Malicious Goals User Launch undetectable attacks Probe for vulnerabilities Network Admin Intermediate ISPs Spy on/tamper with traffic Impersonate servers/users Server Spy on users Identify anonymous users

24 OSI Protocol Stack application presentation session transport network data link , Web, NFS RPC TCP, UDP, ICMP IP Ethernet physical

25 Data Formats application layer Application data message transport layer TCP header data TCP header data TCP header data segment network layer IP header TCP header data packet data link layer Ethernet header IP header TCP header data Ethernet trailer frame

26 IP (Internet Protocol) Connectionless Unreliable, best-effort protocol Uses numeric addresses for routing Typically several hops in the route Alice s computer Bob s ISP Alice s ISP Packet Source Dest Bob s computer

27 TCP (Transmission Control Protocol) Sender: break data into packets Sequence number is attached to every packet Receiver: reassemble packets in correct order Acknowledge receipt; lost packets are re-sent Connection state maintained on both sides book mail each page remember received pages and reassemble

28 UDP (User Datagram Protocol) Sender: break data into packets Sequence number - maybe? If Application wants them Receiver: receive packets No acknowledgement Dropped packets are skipped - no retransmission video mail each frame stream frames to application

29 ICMP (Control Message Protocol) Provides feedback about network operation Out-of-band messages carried in IP packets Error reporting, congestion control, reachability, etc. Example messages: Destination unreachable Time exceeded Parameter problem Redirect to better gateway Reachability test (echo / echo reply) Message transit delay (timestamp request / reply)

30 (Some) Malicious Goals User Launch undetectable attacks Probe for vulnerabilities Network Admin Intermediate ISPs Spy on/tamper with traffic Impersonate servers/users Server Identify anonymous users

31 Detecting attacks User Launch undetectable attacks Problem: IP packets contain source IP address Solution: Spoof IP address

32 Inferring DDOS (Moore, Voelker, Savage 01) SYN packets B of the D V B V C V Victim Attacker where backs tack r 3.2 C D Attack Backscatter The e spoof acros samp

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