Modelling and Analysing of Security Protocol: Lecture 1. Introductions to Modelling Protocols. Tom Chothia CWI

Transcription

1 Modelling and Analysing of Security Protocol: Lecture 1 Introductions to Modelling Protocols Tom Chothia CWI

2 This Course This course will primarily teaching you: How to design your own secure communication protocols. How to analyse protocols and look for faults. How to use automatic tools to help you do this. Secondary skills: Know which protocols to use for which jobs. Improve your system design skills.

3 Course Outline This Lecture: How we model protocols Types of encryption used. Lecture 2: Types of attacks on protocols Good protocol design Homework ( 1/6 of total score).

4 Lecture 3: Verifying protocols using BAN logic. Lecture 4: Automatically verifying protocols. Homework ( 1/6 of total score) Lecture 5: Course Outline Anonymity protocols.

5 Course Outline Lecture 6: Verifying probabilistic protocols in PRISM Lecture 7: Fair exchange & Zero knowledge Lecture 8 to Lecture 10 Short students presentations ( 2/3 of total score ) Lecture 11 Summary

6 Sources Take notes if you want but you will get handouts with all the important details and the slides, handouts, papers, homework and links will be available at:

7 This Lecture Part 1: Simple notation for protocols Modelling rules Needham-Schroeder and Kerberos protocols Part 2: A high level overview the to cryptography Symmetric key encryption, public key encryptions and signing Abstract equation for modelling encryption

8 A Simple Protocol A sends message M to B : A M B written as: A B : M

9 Rules We write down protocols as a list of messages sent between principals, e.g. 1. A B : Hello 2. B A : Offer 3. A B : Accept

10 A Simple Protocol A M B Message M can be read by the attacker

11 A Simple Protocol A B M Even now!

12 Rule The attacker can read all the messages sent across the network.

13 Encryption We can keep our data safe by using encryption: A { M } Kab B A B : { M } Kab

14 Rule We can use Encryption {M} K, E K (M) Signing Sign K (M), S K (M), MAC K (M) Hashing #(M), Hash(M) We assume that these are prefect cannot be broken by brute force.

15 Encryption M is now secret A { M } Kab B but the protocol is not safe

16 Replay Attack A 1: { Pay Elvis 5 } Kab B 1) A B : { Pay Eve 5 } Kab

17 Replay Attack A 1: { Pay Elvis 5 } Kab B E 2: { Pay Elvis 5 } Kab 1) A B : { Pay Eve 5 } Kab 2) E B : { Pay Eve 5 } Kab

18 Rule The attacker can repeat any message it see.

19 A Nonce 1. A A 2. { N a } Kab 3. {N a + 1} Kab, { Pay Elvis 5 } Kab B 1. A B : A 2. B A : { N a } Kab 3. A B : { N a + 1 } Kab, { Pay Elvis 5 } Kab

20 Rule We can generate nonces. This is a new random values. If you generate a new nonce for a session you know that all future messages with that include that nonce are part of the same session.

21 A Nonce 1. A A 2. { N a } Kab 3. {N a + 1} Kab, { Pay Elvis 5 } Kab 4. A B 5. { N a2 } Kab 6. {N a2 + 1} Kab, { Pay Bob 5 } Kab

22 A Nonce 1. A A 2. { N a } Kab 3. {N a + 1} Kab, { Pay Elvis 5 } Kab 4. A B 5. { N a2 } Kab 6. {N a2 + 1} Kab, { Pay Bob 5 } Kab E 6. {N a2 + 1} Kab, { Pay Elvis 5 } Kab

23 Rule The attacker can run multiple rounds of the protocol. The attacker can break up messages, invent new values, keys, nonces,.. combine any of these into new message.

24 A Better Protocol 1. A A 2. { N a } Kab 3. {N a, Pay Elvis 5 } Kab B 1. A B : A, N a 2. B A : { N a } Kab 3. A B : {N a, Pay Elvis 5 } Kab

25 Key Establishment Protocol This was easy because A and B shared a key. Often the principals do not share a key, in which case we need a Key Establishment Protocol. This usually involves a Trust Third Party who has a shared key with each party.

26 The Needham-Schroeder Public Key Protocol A famous authentication protocol 1. A B : E B ( N a, A ) 2. B A : E A ( N a, N b ) 3. A B : E B ( N b ) N a and N b can then be used to generate a symmetric key

27 An Attack Against the Needham-Schroeder Protocol The attack acts as a man-in-the-middle: 1. A C : E C ( N a, A ) 1`. C(A) B : E A ( N a, A ) 2`. B C(A) : E A ( N a, N b ) 2. C A : E A ( N a, N b ) 3. A C : E C ( N b ) 3`. C(A) B : E B ( N b )

28 The Corrected Version A very simple fix: 1. A B : E B ( N a, A ) 2. B A : E A ( N a, N b ) 3. A B : E B ( N b )

29 The Corrected Version A very simple fix: 1. A B : E B ( N a, A ) 2. B A : E A ( N a, N b, B) 3. A B : E B ( N b )

30 Rule The attacker can act as a participant of the protocol.... (sometimes)

31 Kerberos A protocol for key establishment and authentication used in Windows, MacOS, Apache, OpenSSH, A S : A,B,N A 2. S A : {K AB,B,L,N A,..} K AS,{K AB,A,L,..} KBS 3. A B : {A,T A } K AB,{K AB,A,L,..} KBS 4. B A : {T A +1} K AB

32 Kerberos A and S share the key K AS and B and S share K AS Both A and B trust S to generate a new key for them: K AB N is a nonce, T is a timestamp and L is an expiration time. 1. A S : A,B,N A 2. S A : {K AB,B,L,N A,..} K AS,{K AB,A,L,..} KBS 3. A B : {A,T A } K AB,{K AB,A,L,..} KBS 4. B A : {T A +1} K AB

33 Sources For lectures 1 & 2 the the primary reference material is the handouts. This information is covered in more depth in Paper: Prudent Engineering Practices for Cryptographic Protocols (by Abadi & Needham) Book: Protocols for Authentication and Key Establishment (by Boyd & Mathuria) there are copies in the library.

34 This Lecture Part 1: Simple notation for protocols Modelling rules Needham-Schroeder and Kerberos protocols Part 2: A high level overview of cryptography Symmetric key encryption, public key encryptions and signing Abstract equation for modelling encryption