<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Jun 9, 2015, at 8:29 AM, Thompson, Jeff <<a href="mailto:jefft0@remap.ucla.edu" class="">jefft0@remap.ucla.edu</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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<div class="">> I think the purpose is to allow both ends to uniquely identify the key. Is there any particular reason of using the first 4 bytes of the digest?</div>
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<div class="">Using a short identifier (4 bytes) is to keep the packet short for low-power devices. Making a short identifier from the digest is an easy way to get a unique identifier instead of maintaining a separate list of sequential ID numbers. Does that answer
your question?</div></div></div></blockquote><div><br class=""></div><div>I do not think the first 4-bytes can provide a “unique” identifier. If uniqueness is the major concern, we should use digest, given collision in sha256 has not been found yet. </div><div><br class=""></div><div>I did not quite understand what you mean by “a separate list of sequential ID numbers”. </div><div><br class=""></div><div>My concern is about using digest or something related alone. Given HMAC is about authenticity, one might not be able to tell from a digest or even first 4 bytes that the key can be trusted for a particular data packet. We have to maintaining an additional mapping between the privilege of a key (which is usually the key name) and the key anyway. So using a digest does not save too much at the device side. </div><div><br class=""></div><div>For length of the key locator, given the packet size is already 1500-byte, I am not sure if it is a good tradeoff to sacrifice some good security property for just tens of bytes.</div><div><br class=""></div><div class=""><span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px;"><span class="Apple-style-span" style="border-collapse: separate; orphans: 2; text-align: -webkit-auto; widows: 2; border-spacing: 0px;"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">Yingdi</div><div class=""><br class=""></div></div></span><br class="Apple-interchange-newline"></span><br class="Apple-interchange-newline"></div></div><br class=""></body></html>