1、Analysis of Security Protocols (I),John C. Mitchell Stanford University,My Second Marktoberdorf School,Fun playing volleyball, swimming, hiking Review German vocabulary Alt, Pils, Dunkel, Weizen, Dunkel Weizen wegabschneider (trail-off-cutter) Seen some 96 students at conferences What else should I
2、remember?,Computer Security,Protect information Store user passwords in a form that prevents anyone from reading them Transmit information like credit card numbers in a way that prevents others from intercepting them Protect system integrity Keep others from deleting your files Keep downloaded code
3、(such as Java applets) from modifying important data Reject mail messages that contain viruses Maintain privacy,Correctness vs Security,Program or System Correctness Program satisfies specification For reasonable input, get reasonable output Program or System Security Program resists attack For unre
4、asonable input, output not completely disastrous Secure system might not be correct Main technical differences Active interference from environment Refinement techniques may fail,Outline of these lectures,Introduction to security protocols Issues in security, protocol examples and flaws Overview of
5、cryptography Formal presentation of protocols and intruder Automated finite-state analysis A probabilistic, poly-time framework,Tractable program analysis,Goal: tools and techniques to solve useful problems Caveat: need to be realistic,program complexity,complexity of property to verify,May be possi
6、ble,Intractable,Security Protocols,Transmit information across network Keep important information secret Communicate with those you know and trustTypical handshake protocols 3-7 steps 2-5 parties client, server, key distribution service, lead to shared secret key for data transfer,Example: Secure So
7、ckets Layer,Establishing Secure Communication,Parties use SSL protocol to Choose encryption scheme, e.g. 40-bit international encryption with 2 keys 120-bit domestic encryption with 2 keys choose among versions of specific scheme Agree on shared secret key Secret key more efficient than public key A
8、void known-plaintext attack Minimize reuse of hard-to-establish public key,40,120,Some security objectives,Secrecy Info not revealed Authentication Know identity of individual or site Data integrity Msg not altered Message Authentication Know source of msg,Receipt Know msg received Access control Re
9、vocation Anonymity Non-repudiation,Example Protocols,Challenge response Mechanism for freshness Needham-Schroeder Public Key Use public-key crypto to generate shared secret Kerberos Simplified version w/o timestamps or nonces Idea of sending encrypted “tickets” SSL (briefly) Diffie-Hellman key excha
10、nge,Timeliness in Communication,Assume Alice and Bob share a private encryption key K Alice wants to know if Bob is on network Possible protocol: Alice Bob: “Hi Bob. Still there?” K Bob Alice: “I am here?” K Whats wrong with this?,Challenge-Response,Alice wants to know if Bob is still there Send “fr
11、esh” number n, Bob returns f(n) nonce = number used once This avoids reply by malicious 3rd party Protocol Alice Bob: nonce K Bob Alice: nonce+1 K Does this work?,Needham-Schroeder Key Exchange, A, Noncea Noncea, Nonceb Nonceb,Ka,Kb,Result: A and B share two private numbers not known to any observer
12、 without Ka-1, Kb -1,A,B,Kb,Anomaly in Needham-Schroeder,A,E,B, A, Na , A, Na , Na, Nb , Na, Nb , Nb ,Ke,Kb,Ka,Ka,Ke,Evil agent E tricks honest A into revealing private key Nb from B.,Evil E can then fool B.,Lowe,TMN Cell Phone Protocol,a,N,a,b,b,K,K,s,s,S,B,A,B, N ,A,B N ,A N ,TMN Replay Attack,S,B
13、,A,B, NaKs,A,A, NbKs,B, NbNa,S,D,C,D, NcKs,C,C, NbKs,D, NbNc,REPLAY,Kerberos,Client requests key from KDC C KDC : C, TGS KDC returns private key and ticket KDC C : Ks1 Kc C, Ks1 Ktgs Client sends name and ticket to TGS C TGS : CKs1, C, Ks1 Ktgs, S TCS returns private key and ticket TGS C : Ks2 Kc C,
14、 Ks2 Ks Client contacts server C S : CKs1, C, Ks1 Ks,Secure Socket Layer (SSL),Three goals Negotiate specific encryption scheme Possible “version attack” Authenticate client and server Appeal to “signature authority” Use public key to transmit secret key,Several underlying primitives: public key, si
15、gnature scheme, hash function, private key,Handshake Protocol Description,ClientHello C S C, VerC, SuiteC, NC ServerHello S C VerS, SuiteS, NS, signCA S, KS+ ClientVerify C S signCAC, VC VerC, SecretC + signC Hash( Master(NC, NS, SecretC) + Pad2 +Hash(Msgs + C + Master(NC, NS, SecretC) + Pad1) (Chan
16、ge to negotiated cipher) ServerFinished S C Hash( Master(NC, NS, SecretC) + Pad2 + Hash( Msgs + S + Master(NC, NS, SecretC) + Pad1)ClientFinished C S Hash( Master(NC, NS, SecretC) + Pad2 + Hash( Msgs + C + Master(NC, NS, SecretC) + Pad1) ,KS,Master(NC, NS, SecretC),Master(NC, NS, SecretC),Diffie-Hel
17、lman Key Exchange,Number-theoretic assumption Given three numbers p, g, ga mod p, no efficient algorithm for computing a Belief: adversary cannot find a until “too late” Protocol (assumes public prime p, generator g) Alice Bob: ga mod p Bob Alice: gb mod p Consequence Alice and Bob know gab mod p, no one else does Works on telephone, not general network. Why?,
