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

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.

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).

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

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:

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

“A” sends message “M” to “B”: written as: A  B : M A Simple Protocol AB M

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”

A Simple Protocol AB M Message “M” can be read by the attacker

A Simple Protocol AB M Even now!

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

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

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.

Encryption “M” is now secret AB { M } Kab but the protocol is not safe

Replay Attack AB 1: { Pay Elvis €5 } Kab 1) A  B : { Pay Eve €5 } Kab

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

Rule The attacker can repeat any message it see.

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

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.

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

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

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.

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

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.

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

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 )

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 )

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 )

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

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

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,..} K BS 3.A  B : {A,T A } K AB,{K AB,A,L,..} K BS 4.B  A : {T A +1} K AB

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.

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