Network systems security by mort anvari lecture2

Network Protocols Abstractions of communication between two processes over a network  Define message formats  Define legitimate sequence of messages  Take care of physical details of

Network Protocols

Network Systems Security

Mort Anvari

Network Protocols

 Abstractions of communication between two processes over a network

 Define message formats

 Define legitimate sequence of messages

 Take care of physical details of different network hardware and machines

 Separate tasks in complex

Protocol Layering

 Many problems need to be solved in a communication network

 These problems can be divided into

smaller sets and different protocols

are designed for each set of problem

 Protocols can be organized into layers

to keep them easy to manage

Properties of Protocol

Layer

independent of functions of other

Protocol Stack: OSI Model

Application Presentation Session Transport Network Data link Physical

Communicating End Hosts

Network Data link Physical

Route r

 It is desirable to verify that a

protocol can correctly perform

functions that it was designed for

 Particularly important for security protocols

Shortcomings of Plain English

 Ambiguity

 Different words can have similar meanings

process p sends message m to process q

process p transmits message m to process q process p forwards message m to process q process p delivers message m to process q

 Same word can have different meanings

process p sends message m to process q

process p sends file f to process q

Shortcoming of Time Chart

 Not scalable

 Many legitimate sequences of messages

 Cannot list all possible legitimate sequences when the number of sequences grows

exponentially

Shortcoming of Using

Programming Language

 Hard to prove correctness of protocol

specification

 For example, protocol specified in C

language may involve overlap, and may involve transmission delay

authentication protocols and

uncover various protocol

weaknesses

Logical Constructs

 Assume P and Q are network agents, X is a message,

and K is an encryption key

 P believes X: P acts as if X is true, and may assert X in

other messages

 P has jurisdiction over X: P's beliefs about X should

be trusted

 P said X: At one time, P transmitted (and believed)

message X, although P might no longer believe X

 P sees X: P receives message X, and can read and

repeat X

 If P believes that Q said <X, Y>, the

concatenation of X and Y, then P also believes that Q said X, and P also

Shortcomings of BAN Logic

 High level of abstraction

 Need for a protocol idealization step, in which user is required to transform each message in a

protocol into formulas

Can only verify a round everytime

Abstract Protocol Notation

 Presented by Mohamed Gouda in the

book Elements of Network Protocol

Design

 Formal and scalable

 Proof of correctness of protocol

specification can be easily done using state transition diagram

<name of input> : <type of input>

var <name of variable> : <type of variable>

Action Execution

 Specified as <guard> -> <statement>

 Satisfy three conditions

executed one at a time; one action cannot start while another action execution is in progress

State Transition Diagram

 Define semantic of a protocol

 State is defined by a value for each

variable in protocol and by a message set for each channel in protocol

 Transition is movement from current state to next state triggered by an

action execution

Adversary Model

 Adversary can change contents of protocol channels by executing the following actions a finite number of times

 Message loss: lose an original message

 Message modification: modify the field of an

original message to cause a modified message

 Message replay: replace an original message by

another original message to cause a replayed

Prove Correctness of Secure

Protocol

 Execution of adversary actions may lead the protocol to a bad state

 Protocol is said to be correct if it

converges to its good cycle in a finite

number of steps after adversary finishes executing its actions