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<title>Security protocol notation</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Security protocol notation</span></span>

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<p>In <a href="Cryptography" title="Cryptography">cryptography</a>, <b>security (engineering) protocol notation</b>, also known as <b>protocol narrations</b><sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> and <b>Alice & Bob notation</b>, is a way of expressing a <a href="Cryptographic_protocol" title="Cryptographic protocol">protocol</a> of correspondence between entities of a dynamic system, such as a <a href="Computer_network" title="Computer network">computer network</a>. In the context of a <a href="Formal_model" class="mw-redirect" title="Formal model">formal model</a>, it allows reasoning about the properties of such a system.

</p><p>The standard notation consists of a set of principals (traditionally named <a href="Alice_and_Bob" title="Alice and Bob">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="Cryptographic_nonce" title="Cryptographic nonce">nonces</a> N for authentication purposes.

</p><p>A simple example might be the following:

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<annotation encoding="application/x-tex">{\displaystyle A\rightarrow B:\{X\}_{K_{A,B}}}</annotation>

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</math></span><img src="./8aea7612a5a95fd14f7e994131d0e1d236afe876.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.338ex; width:17.54ex; height:3.343ex;" alt="{\displaystyle A\rightarrow B:\{X\}_{K_{A,B}}}" loading="lazy"></span></dd></dl>

<p>This states that <b>A</b>lice intends a message for <b>B</b>ob consisting of a <a href="Plaintext" title="Plaintext">plaintext</a> <b>X</b> encrypted under shared key <b>K<sub>A,B</sub></b>.

</p><p>Another example might be the following:

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<annotation encoding="application/x-tex">{\displaystyle B\rightarrow A:\{N_{B}\}_{K_{A}}}</annotation>

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</math></span><img src="./39926388bedf1da3cd100d1ce76bd22536795407.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:17.521ex; height:3.009ex;" alt="{\displaystyle B\rightarrow A:\{N_{B}\}_{K_{A}}}" loading="lazy"></span></dd></dl>

<p>This states that <b>B</b>ob intends a message for <b>A</b>lice consisting of a <a href="Cryptographic_nonce" title="Cryptographic nonce"><b>n</b>once</a> <b>N<sub>B</sub></b> encrypted using public key of Alice.

</p><p>A key with two subscripts, <b>K<sub>A,B</sub></b>, is a <a href="Symmetric_key" class="mw-redirect" title="Symmetric key">symmetric key</a> shared by the two corresponding individuals. A key with one subscript, <b>K<sub>A</sub></b>, is the public key of the corresponding individual. A private key is represented as the <a href="Inverse_function#Notation" title="Inverse function">inverse</a> of the public key.

</p><p>The notation specifies only the operation and not its semantics — for instance, private key encryption and signature are represented identically.

</p><p>We can express more complicated protocols in such a fashion. See <a href="Kerberos_(protocol)" title="Kerberos (protocol)">Kerberos</a> as an example. Some sources refer to this notation as <i>Kerberos Notation</i>.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> Some authors consider the notation used by Steiner, Neuman, & Schiller<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> as a notable reference.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>

</p><p>Several models exist to reason about security protocols in this way, one of which is <a href="BAN_logic" class="mw-redirect" title="BAN logic">BAN logic</a>.

</p><p>Security protocol notation inspired many of the programming languages used in <a href="Choreographic_programming" title="Choreographic programming">choreographic programming</a>.

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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFBriaisNestmann2005" class="citation book cs1">Briais, Sébastien; Nestmann, Uwe (2005). <a rel="nofollow" class="external text" href="http://sbriais.online.fr/papers/A_Formal_Semantics_For_Protocol_Narrations_TGC05-final.pdf">"A Formal Semantics for Protocol Narrations"</a> <span class="cs1-format">(PDF)</span>. <i>Trustworthy Global Computing</i>. Lecture Notes in Computer Science. Vol. 3705. pp. <span class="nowrap">163–</span>181. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2005LNCS.3705..163B">2005LNCS.3705..163B</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1007%2F11580850_10">10.1007/11580850_10</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-3-540-30007-6</bdi>.</cite></span>

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<cite id="CITEREFChappell1999" class="citation journal cs1">Chappell, David (1999). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20170815043157/https://www.microsoft.com/msj/0899/kerberos/kerberos.aspx">"Exploring Kerberos, the Protocol for Distributed Security in Windows 2000"</a>. <i>Microsoft Systems Journal</i>. Archived from <a rel="nofollow" class="external text" href="https://www.microsoft.com/msj/0899/kerberos/kerberos.aspx">the original</a> on 2017-08-15.</cite></span>

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<cite id="CITEREFSteinerNeumanSchiller1988" class="citation conference cs1">Steiner, J. G.; Neuman, B. C.; Schiller, J. I. (February 1988). <a rel="nofollow" class="external text" href="http://clifford.neuman.name/publications/1988/198802-Usenix-Kerberos/198802-Usenix-Steiner-Neuman-Schiller-Kerberos.pdf">"Kerberos: An Authentication Service for Open Network Systems"</a> <span class="cs1-format">(PDF)</span>. <i>Proceedings of the Winter 1988 Usenix Conference</i>. Usenix. Berkeley, CA: USENIX Association. pp. <span class="nowrap">191–</span>201<span class="reference-accessdate">. Retrieved <span class="nowrap">2009-06-10</span></span>.</cite></span>

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<cite id="CITEREFDavis,_DonSwick,_Ralph1989" class="citation book cs1">Davis, Don; Swick, Ralph (1989-03-17). <a rel="nofollow" class="external text" href="ftp://athena-dist.mit.edu/pub/ATHENA/kerberos/doc/user2user.ps"><i>Workstation Services and Kerberos Authentication at Project Athena</i></a> <span class="cs1-format">(PS)</span>. p. 1<span class="reference-accessdate">. Retrieved <span class="nowrap">2009-06-10</span></span>. <q>…our notation follows Steiner, Neuman, & Schiller,…</q></cite></span>

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