[cabfpub] Pre-Ballot 164 - Certificate Serial Number Entropy

Ben Wilson ben.wilson at digicert.com
Thu Apr 14 19:59:30 UTC 2016


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?



From: Man Ho (Certizen) [mailto:manho at certizen.com] 
Sent: Wednesday, March 23, 2016 12:27 AM
To: Ben Wilson <ben.wilson at digicert.com>; public at cabforum.org
Subject: Re: [cabfpub] Pre-Ballot 164 - Certificate Serial Number Entropy


Hi all,

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.


On 3/1/2016 12:21 AM, Ben Wilson wrote:


"CAs SHOULD generate non-sequential Certificate serial numbers that exhibit
at least 20 bits of entropy" 


"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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