<li>Publishing public keys to a set of subscribers.</li>
<li>Querying another entity for its public keys.</li>
</ul>
<p>An entity MAY have multiple public keys with different formats, signatures, algorithms, strengths and expiry dates. Each client used by a user may use different keys.</p>
</section1>
<section1topic='Public Key Publication and Retrieval via PEP'anchor='usecases-pubsub'>
<p>If the pubkeys PEP node does not exist already then the entity MUST create it first. In this case, the entity SHOULD specify that the keys will only be pushed to subscribers whenever new keys are published (i.e. not when subscribers become newly available or when a new subscription is created). If the user wants to control access to his/her identity (see <linkurl='#security'>Security Considerations</link>) then the entity MUST also specify an appropriate access model other than "Open".</p>
<examplecaption='Entity Creates Public Keys Publishing Node'><![CDATA[
<p>Once the publishing node has been created, the entity can update the keys at any time. Each public key MUST be wrapped in a <KeyInfo/> element as specified in &w3xmlsig;. Each <KeyInfo/> element MUST contain a <KeyName/> element with a unique (for the user) name to allow the key to be referenced by other protocols. All the <KeyInfo/> elements MUST be wrapped in a single <pubkeys/> element.</p>
<p> Note: Any change to the list of keys requires all keys to be republished. So clients SHOULD take care to request (see third example below) and then include all keys for the JID that were published previously by other clients (unless the keys have expired). <note>The keys are long-term so they will only be republished infrequently.</note></p>
<examplecaption='Entity Publishes Keys to its Server'><![CDATA[
<p>Note: The stanza containing the event notification (see example above) MAY also include 'replyto' data (as specified by the &xep0033; protocol) to provide an explicit association between the published data and the <em>resource</em> that published it.</p>
<section1topic='Requesting Public Keys Directly From Another Entity'anchor='usecases-request'>
<p>If an entity wishes to request the public keys of another entity and it cannot access the keys via <cite>Personal Eventing via Pubsub</cite>, then the entity MAY send an IQ to the other entity:</p>
<p>The other entity MUST make a careful access control decision before returning only those public keys for which it holds the corresponding private key (not necessarily the full list of keys being published via <cite>Personal Eventing via Pubsub</cite>):</p>
<examplecaption='Successful public keys response'><![CDATA[
<p>If the receiving entity decides not to return the public keys, it MUST return an IQ error, which SHOULD be &unavailable; (to avoid divulging presence to unauthorized entities), but MAY be some other appropriate error, such as &forbidden; or ¬allowed;:</p>
<examplecaption='Access to public keys denied'><![CDATA[
<p>The reliable association between a user or entity and its public keys is beyond the scope of this document. However, it is RECOMMENDED that each client maintains its own secure library of the public keys (or the "fingerprints" of the keys) it associates with other users (not necessarily JIDs).</p>
<p>Whenever public keys are published an identity is typically associated with a JID. Although the public keys are public information, it may be critically important for the user of the JID to keep his identity secret from all but a few specified people. Implementors MUST take great care to ensure the identity of the user of a JID is never divulged to anyone except the entities who have been permitted by the user to access the public key.</p>