Security protocol notation

In <a href="/facts/Cryptography/e94PEVau">cryptography</a>, security (engineering) protocol notation, also known as protocol narrations and Alice & Bob notation, is a way of expressing a <a href="/facts/Cryptographic_protocol/0WuyTkFo">protocol</a> of correspondence between entities of a dynamic system, such as a <a href="/facts/Computer_network/3w5RM99p">computer network</a>. In the context of a <a href="/facts/Formal_model/crDTyP8q">formal model</a>, it allows reasoning about the properties of such a system.
The standard notation consists of a set of principals (traditionally named <a href="/facts/Alice_and_Bob/FSHudRmA">Alice, Bob</a>, Charlie, and so on) who wish to communicate. They may have access to a server S, shared keys K, timestamps T, and can generate <a href="/facts/Cryptographic_nonce/ENHRCGnw">nonces</a> N for authentication purposes.
A simple example might be the following:

A
        →
        B
        :
        {
        X
        
          }
          
            
              K
              
                A
                ,
                B
              
            
          
        
      
    
    {\displaystyle A\rightarrow B:\{X\}_{K_{A,B}}}

This states that Alice intends a message for Bob consisting of a <a href="/facts/Plaintext/tWnBDCgE">plaintext</a> X encrypted under shared key KA,B.
Another example might be the following:

B
        →
        A
        :
        {
        
          N
          
            B
          
        
        
          }
          
            
              K
              
                A
              
            
          
        
      
    
    {\displaystyle B\rightarrow A:\{N_{B}\}_{K_{A}}}

This states that Bob intends a message for Alice consisting of a <a href="/facts/Cryptographic_nonce/ENHRCGnw">nonce</a> NB encrypted using public key of Alice.
A key with two subscripts, KA,B, is a <a href="/facts/Symmetric_key/5zfe0b76">symmetric key</a> shared by the two corresponding individuals. A key with one subscript, KA, is the public key of the corresponding individual. A private key is represented as the <a href="/facts/Inverse_function/Tg5nnXlu">inverse</a> of the public key.
The notation specifies only the operation and not its semantics — for instance, private key encryption and signature are represented identically.
We can express more complicated protocols in such a fashion. See <a href="/facts/Kerberos_(protocol)/RMXs3P1w">Kerberos</a> as an example. Some sources refer to this notation as Kerberos Notation. Some authors consider the notation used by Steiner, Neuman, & Schiller as a notable reference.
Several models exist to reason about security protocols in this way, one of which is <a href="/facts/BAN_logic/fr2yd1lL">BAN logic</a>.
Security protocol notation inspired many of the programming languages used in <a href="/facts/Choreographic_programming/WxGmI2tY">choreographic programming</a>.

Security protocol notation open-in-new

Security protocol notation