<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div>Because it's way simpler? Even if you lie about hours, minutes, and seconds, that only gets you like more 16 bits of entropy per date, at the cost of degrading the semantic utility of the dates. </div><div id="AppleMailSignature"><br></div><div id="AppleMailSignature">On the other hand, you can put pretty much all the entropy you want in the serial number with no consequence; they just have to be unique with a CA's scope. There are a whole lot of certs out there with 128-bit serial numbers, which gives plenty of room for both randomness and whatever semantics the CA wants to put in. <br><br>Sent from my iPhone. Please excuse brevity. </div><div><br>On Feb 26, 2016, at 20:43, Brown, Wendy (10421) <<a href="mailto:wendy.brown@protiviti.com">wendy.brown@protiviti.com</a>> wrote:<br><br></div><blockquote type="cite"><div>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">Why does the entropy have to be in the serial number vs a combination of serial number and date/time bits ?</span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> <a href="mailto:public-bounces@cabforum.org">public-bounces@cabforum.org</a> [<a href="mailto:public-bounces@cabforum.org">mailto:public-bounces@cabforum.org</a>]
<b>On Behalf Of </b>Richard Barnes<br>
<b>Sent:</b> Friday, February 26, 2016 6:26 PM<br>
<b>To:</b> Ryan Sleevi <<a href="mailto:sleevi@google.com">sleevi@google.com</a>><br>
<b>Cc:</b> CABFPub <<a href="mailto:public@cabforum.org">public@cabforum.org</a>><br>
<b>Subject:</b> Re: [cabfpub] Pre-Ballot 164 - Certificate Serial Number Entropy</span></p>
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<p class="MsoNormal">On Fri, Feb 26, 2016 at 6:03 PM, Ryan Sleevi <<a href="mailto:sleevi@google.com" target="_blank">sleevi@google.com</a>> wrote:</p>
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<p class="MsoNormal">Is there a reason for the change from "entropy" to "unpredictable bits"</p>
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<p class="MsoNormal">Would you be opposed to "64 bits of random data from a cryptographically strong random number generator"?</p>
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<p class="MsoNormal">The concern I have with the language change is that while "entropy" is arguably less ambiguous, I fear "unpredictable bits" will create a situation where a CA says "No one knows our [deterministic] algorithm, therefore it's unpredictable"</p>
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<p class="MsoNormal">I admit, I'm not terribly thrilled with my rewrite either, because I don't think it should be required to use an RNG on an HSM, for example (that's arguably overkill), but I do want to make sure that the source of entropy is cryptographically
strong (thus ruling out Microsoft's GUIDs, crappy RNGs, etc)</p>
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<p class="MsoNormal">I would prefer this proposal. It provides a specific thing that can be verified (whereas "entropy" and "unpredictable" are vague statistical properties).
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<p class="MsoNormal">--Richard</p>
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<p class="MsoNormal">On Fri, Feb 26, 2016 at 1:49 PM, Ben Wilson <<a href="mailto:ben.wilson@digicert.com" target="_blank">ben.wilson@digicert.com</a>> wrote:</p>
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<p>For discussion:</p>
<p><b>Pre-Ballot 164 - Certificate Serial Number Entropy</b> </p>
<p>-- Motion Begins -- </p>
<p>In Section 7.1 of the Baseline Requirements, </p>
<p>REPLACE </p>
<p>"CAs SHOULD generate non-sequential Certificate serial numbers that exhibit at least 20 bits of entropy"
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<p>WITH </p>
<p>"Effective April 1, 2016, CAs SHALL use a Certificate serialNumber greater than zero (0) that contains at least 64 unpredictable bits."
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<p>-- Motion Ends -- </p>
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