A Protocol for Secure Public Instant Messaging
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1 Financial Cryptography - Feb 27, 2006 A Protocol for Secure Public Instant Messaging Mohammad Mannan and Paul C. van Oorschot Digital Security Group Carleton University, Canada Mohammad Mannan Feb 27,
2 Outline IM overview and motivation Instant Messaging Key Exchange (IMKE) the protocol Security comments Mohammad Mannan Feb 27,
3 Figure 1: IM in action Mohammad Mannan Feb 27,
4 IM communication model Server Client A A s contact list B D E H Client B B s contact list A C H Client Server Communications (e.g. login, profile) Client Client Direct Communications (e.g. file data transfer) Client Client Server mediated Communications (e.g. text message) Mohammad Mannan Feb 27,
5 Do we need secure IM? IM is a popular application instant communication (home users) instant collaboration (enterprise users) Number of users : MSN 185m, Yahoo! 82m, AOL 61m a 13 of Fortune 50 companies were affected by IM-related security incidents in the last 6 months b IMlogic was bought by Symantec (Jan. 2006) a Source: ComScore Media Metrix, Aug b Source: IMlogic, Nov Mohammad Mannan Feb 27,
6 IMKE - motivation 1. Existing solutions have drawbacks SSL: relayed user messages are visible to IM server client plug-ins: client-server messages are plaintext secure protocols: not designed for integration 2. Strong password protocols do not fit efficiency simplicity Mohammad Mannan Feb 27,
7 IMKE - goals 1. Mutual assurance of identity 2. Secure communications ( C.I.A. ) 3. Forward secrecy 4. Repudiation (!) 5. Replay detection authentication phase: text message / file transfers: standard techniques Mohammad Mannan Feb 27,
8 IMKE - notation A, B, S IM users Alice and Bob, and IM server ID A P A R A {data} K {data} EA K s AS [X] AS User ID of A Password shared by A and S Random number generated by A Secret-key encryption of data using key K Public-key encryption of data using A s public key KU A Symmetric (s) session encryption key shared by A and S MAC output of X under the symmetric MAC key shared by A and S Mohammad Mannan Feb 27,
9 IMKE - features Comparing IMKE re: offline dictionary attack avoidance 1. password-only (eg. EKE): {KU A } PA 2. known server public key (eg. Halevi-Krawczyk): {P A,R} ES 3. IMKE: {K AS } ES, {P A } KAS Public key protocol independence IM server works as an online public key distribution center Secure communications between users who share no long-term secret Dynamic client public keys Mohammad Mannan Feb 27,
10 IMKE - message summary (1) Secure Public Instant Messaging Phases Authentication and Key Exchange Public Key Distribution Session Key Transport Message Labels Messages A generates a dynamic public/private key pair A, S authenticate each other using shared password A, S establish a session key A s public key is sent to and stored by S A communicates to S a desire to talk to B S forwards B s public key to A (and A s to B) A, B authenticate each other using the received public keys A, B establish a session key Mohammad Mannan Feb 27,
11 IMKE - message summary (2) Secure Public Instant Messaging Phases Authentication and Key Exchange Public Key Distribution Session Key Transport Message Messages Labels a1 A S : ID A, {K AS } ES, {KU A, f 1 (P A )} KAS a2 A S : {R S } EA, {f 2 (P A )} KAS a3 A S : f 3 (R S ) b1 A S : {KU B, ID B } K s AS, [KU B, ID B ] AS b2 B S : {KU A, ID A } K s BS, [KU A, ID A ] BS c1 A B : {K AB } EB, {R A } KAB c2 A B : {R B } EA, {f 6 (R A )} KAB c3 A B : f 7 (R A, R B ) K s AS = f(k AS, R S ), K s AB = f(k AB, R B ) Mohammad Mannan Feb 27,
12 IMKE - security Formal proofs: BAN-like analysis (outline): AVISPA protocol analysis tool: Mohammad Mannan Feb 27,
13 IMKE - attacks not addressed 1. Keyloggers can collect passwords 2. A false public key of S on client allows offline dictionary attacks 3. Malicious IM server may forward false client public keys (MIM) 4. IM worms Mohammad Mannan Feb 27,
14 IMKE - implementation 1. Integrated with Jabber 2. Usable performance authentication time doubles, but still less than 0.5 second little effect on text messaging and bulk data transfer 3. Incrementally deployable Mohammad Mannan Feb 27,
15 Concluding remarks 1. Secure IM: becoming increasingly important 2. IMKE: simple, integratable 3. Main lesson from IMKE implementation: practical today Mohammad Mannan Feb 27,
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