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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>You’re right, given a randomly generated 20-byte serial number, you have 159 unpredictable bits. <o:p></o:p></span></p><p class=MsoNormal><a name="_MailEndCompose"><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p> </o:p></span></a></p><span style='mso-bookmark:_MailEndCompose'></span><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'> Ryan Sleevi [mailto:sleevi@google.com] <br><b>Sent:</b> Thursday, April 14, 2016 2:03 PM<br><b>To:</b> Ben Wilson <ben.wilson@digicert.com><br><b>Cc:</b> Man Ho (Certizen) <manho@certizen.com>; public@cabforum.org<br><b>Subject:</b> Re: [cabfpub] Pre-Ballot 164 - Certificate Serial Number Entropy<o:p></o:p></span></p><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal>Ben:<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Are you sure your math is correct? A serial number is 20 bytes, with the high bit needing to be 1 (for the encoding of positive INTEGERS within DER). This leaves 159 bits for entropy. So you certainly can't have more unpredictable bits than that :)<o:p></o:p></p></div></div><div><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal>On Thu, Apr 14, 2016 at 12:59 PM, Ben Wilson <<a href="mailto:ben.wilson@digicert.com" target="_blank">ben.wilson@digicert.com</a>> wrote:<o:p></o:p></p><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0in;margin-bottom:5.0pt'><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>Man,</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>Have you had a chance to do further research on the capabilities of your system? Our CA issues certificates with 32 hexadecimal characters for the serial number. There are 4 bits of entropy for each hexadecimal character. Therefore, our serial numbers have 128 bits of entropy and 16*32= 512 unpredictable bits. An 8-hexadecimal character serial number would have 32 bits of entropy and 128 unpredictable bits. A 20-bit entropy would be equal to 5 hexadecimal characters, or 80 unpredictable bits, so this seems like this is a downgrade to go to 64 unpredictable bits. Am I right?</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>Ben</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><a name="m_216582708830614260__MailEndCompose"><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'> </span></a><o:p></o:p></p><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><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'> Man Ho (Certizen) [mailto:</span><a href="mailto:manho@certizen.com" target="_blank"><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>manho@certizen.com</span></a><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>] <br><b>Sent:</b> Wednesday, March 23, 2016 12:27 AM<br><b>To:</b> Ben Wilson <</span><a href="mailto:ben.wilson@digicert.com" target="_blank"><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>ben.wilson@digicert.com</span></a><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>>; </span><a href="mailto:public@cabforum.org" target="_blank"><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>public@cabforum.org</span></a><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><br><b>Subject:</b> Re: [cabfpub] Pre-Ballot 164 - Certificate Serial Number Entropy</span><o:p></o:p></p></div></div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;margin-bottom:12.0pt'>Hi all,<br><br>Is the meaning of "at least 64 unpredictable bits" setting the same or a higher requirement than "at least 20 bits of entropy" ? I'm not quite sure whether our certificate generation software has this setting in itself.<br><br>Cheers<br>Man<o:p></o:p></p><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>On 3/1/2016 12:21 AM, Ben Wilson wrote:<o:p></o:p></p></div><blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'><p>REPLACE <o:p></o:p></p><p>"CAs SHOULD generate non-sequential Certificate serial numbers that exhibit at least 20 bits of entropy" <o:p></o:p></p><p>WITH <o:p></o:p></p><p>"Effective April 1, 2016, CAs SHALL use a Certificate serialNumber greater than zero (0) that contains at least 64 unpredictable bits." <o:p></o:p></p></blockquote><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p></div></div><p class=MsoNormal style='margin-bottom:12.0pt'><br>_______________________________________________<br>Public mailing list<br><a href="mailto:Public@cabforum.org">Public@cabforum.org</a><br><a href="https://cabforum.org/mailman/listinfo/public" target="_blank">https://cabforum.org/mailman/listinfo/public</a><o:p></o:p></p></blockquote></div><p class=MsoNormal><o:p> </o:p></p></div></div></body></html>