In cryptography, security (engineering) protocol notation, also known as protocol narrations and Alice & Bob notation, is a way of expressing a protocol of correspondence between entities of a dynamic system, such as a computer network. In the context of a formal model, it allows reasoning about the properties of such a system.

The standard notation consists of a set of principals (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 plaintext X encrypted under shared key K_A,B.

Another example might be the following:

This states that Bob intends a message for Alice consisting of a nonce N_B encrypted using public key of Alice.

A key with two subscripts, K_A,B, is a symmetric key shared by the two corresponding individuals. A key with one subscript, K_A, 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, private key encryption and signature are represented identically.