<remark><p>Modified order of explanation to ease understanding; removed discussion of alternate algorithms, which is better left to a more in-depth security analysis.</p>
<remark><p>Recommended hashing the secret to satisfy length requirement; hostnames and Stream ID should be separated by spaces to avoid ambiguity; updated example to match revisions to RFC 3920.</p>
<p>Clarified and corrected roles of originating and receiving servers; updated recommendation and main example to use HMAC-SHA256 for key generation.</p>
<p>&rfc3920; does not specify in detail a recommended algorithm for generating the keys used in the server dialback protocol. This document provides such a recommendation as an aid to implementors of XMPP servers. This document is not meant to supersede any text in <cite>RFC 3920</cite>; however, the recommendations in this document have been incorporated into &xep0220;.</p>
<li>The resulting dialback key is a Keyed-Hash Message Authentication Code (see &nistfips198a;) generated using the SHA256 hashing algorithm (see &nistfips180-2;).</li>
<li>The Secret is used as a "key" within the HMAC generation process; because <cite>HMAC</cite> recommends that the length of the HMAC key should be at least half the size of the hash function output, the Secret SHOULD be hashed via SHA256 prior to use in the in HMAC generation process.</li>
<li>The Secret MUST either be set up in a configuration option for each host or process, or generated as a random string when starting the XMPP server. Creation of the Secret MUST NOT require communication between the Originating Server, the Authoritative Server, and optionally a third party (such as a database).</li>
<li>The output of the SHA256 hashing algorithm MUST be provided in the hexadecimal representation; this helps to avoid encoding problems.</li>
<li>The hostname of the Receiving Server, the hostname of the Originating Server, and the Stream ID SHOULD be concatenated with a Unicode space character (U+0020) as the delimiter; this helps to avoid ambiguity in concatenation. <note>For example, the string "example.inform.example.org" could be construed as a concatenation of "example.info" and "rm.example.org" or of "example.in" and "form.example.org".</note></li>
<p>This document closely follows the description of the dialback protocol in <cite>RFC 3920</cite> and <cite>XEP-0220</cite>, but omits steps that are not important for the generation and validation of the dialback keys. For ease of comparison the numbering of the steps is the same as in section 8.3 of <cite>RFC 3920</cite> and Appendix C.3 of <cite>XEP-0220</cite>. Any line breaks in the examples are included for the purpose of readability only.</p>
<p>The Authoritative Server calculates the valid key for this verify request, using data supplied in the packet and the secret shared with the Originating Server.</p>
<p>9. The Authoritative Server compares this value to the key contained in the verification requests and informs the Originating Server of the result, in our example a success:</p>
<p>This document introduces no security considerations or concerns above and beyond those discussed in <cite>RFC 3920</cite> and <cite>XEP-0220</cite>.</p>
