- Security protocol notation
**Security (engineering) protocol notation**is a way of expressing a protocol of correspondence between entities of a dynamic system, such as acomputer network . In the context of aformal model , it allows reasoning about the properties of such a system.The standard notation consists of a set of individuals (traditionally named Alice, Bob, Charlie, and so on) who wish to communicate. They may have access to a server S, shared keys K, timestamps T, and can generate nonces N for authentication purposes.

A simple example might be the following:

:$A\; ightarrow\; B:\{X\_1\}\_\{K\_\{AB$

This states that

**A**lice intends a message for**B**ob consisting of a plain text**$X\_1$**encrypted under shared key**K**._{AB}Another example might be the following:

:$B\; ightarrow\; A:\{N\_B\}\_\{PK(A)\}$

This states that

**B**ob intends a message for**A**lice consisting of a**N**once encrypted using public key of Alice.A key with two subscripts is a

symmetric key shared by the two corresponding individuals. A key with one subscript is the public key of the corresponding individual. A private key is represented as the inverse of the public key.The notation specifies only the operation and not its semantics — for instance, public key encryption and signature are represented identically.

We can express more complicated protocols in such a fashion. See Kerberos as an example.

Several models exist to reason about security protocols in this way, one of which is

BAN logic .

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