Logical Analysis and Verification of Cryptographic Protocols - Loria

2 CHAPTER 1. INTRODUCTION
Figure 1.1 Example of protocol
� A ⇒ B : “Hello, I am A.”
B ⇒ A : “Hi, I am B. Nice to meet you.”
absence of the intruder. A simple example of protocol is given in Figure 1.1.
This protocol describes the first meeting between two people. In this protocol,
we have two roles “A” (abbreviation of “Alice”) and “B” (abbreviation of
“Bob”).
In the scenario, the roles represent abstract participants. In the protocol described
in Figure 1.1, we have two roles “A” (the initiator role) and “B” (the
responder role).
We call an execution of the protocol any coherent, with respect to the description
of that protocol, set of exchanges of messages between its participants. In
the execution of a protocol, the roles are instantiated by concrete participants, also
called agents. When an agent a instantiates a role R, we say that “the agent a plays
the role R”. We remark that a role can be played by many agents and any agent
can play many roles or the same role many times. We call session of a protocol a
set of exchanges of messages between the participants of the protocol which is
(1) coherent with respect to the description of that protocol, (2) can be repeated,
and (3) where each role is instantiated only once.
We remark that it is important to take into consideration many instances
of the same protocol, and thus we talk about execution of one session of the
protocol, also called run of the protocol, and execution of many sessions of the
protocol.
Example 1 The following set of exchanges of messages
� (1).1 Alice(A) ⇒ Bob(B) : “Hello, I am Alice.”
(1).2 Bob(B) ⇒ Alice(A) : “Hi, I am Bob. Nice to meet you.”
represents an execution of one session of the protocol described in Example 1.1.
In this execution, we have two concrete participants, “Alice” playing the role “A”
and “Bob” playing the role “B”.
Example 2 The following set of exchanges of messages
⎧
⎪⎨
⎪⎩
(1).1 Alice(A) ⇒ Bob(B) : “Hello, I am Alice.”
(2).1 Bob(A) ⇒ Marlie(B) : “Hello, I am Bob.”
(1).2 Bob(B) ⇒ Alice(A) : “Hi, I am Bob. Nice to meet you.”
(2).2 Marlie(B) ⇒ Bob(A) : “Hi, I am Marlie. Nice to meet you.”
represents an execution of two sessions of the protocol described in Example 1.1. In this
execution, we have three concrete participants, “Alice”, “Bob” and “Marlie”: “Alice”