Security (engineering) protocol notation is a way of expressing a protocol of correspondence between entities of a dynamic system, such as a computer network. It allows reasoning about the properties of such a system, and provides a formal model , to which the BAN logic can be applied.

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:

This states that Alice intends a message for Bob consisting of a plain text X₁ encrypted under shared key K_AB.

Another example might be the following:

This states that Bob intends a message for Alice consisting of a Nonce encrypted uusing public key of Bob.

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.