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

[Brown, Wendy (10421)]

Why does the entropy have to be in the serial number vs a combination of serial number and date/time bits ?

From: public-bounces at cabforum.org [mailto:public-bounces at cabforum.org] On Behalf Of Richard Barnes
Sent: Friday, February 26, 2016 6:26 PM
To: Ryan Sleevi <sleevi at google.com>
Cc: CABFPub <public at cabforum.org>
Subject: Re: [cabfpub] Pre-Ballot 164 - Certificate Serial Number Entropy



On Fri, Feb 26, 2016 at 6:03 PM, Ryan Sleevi <sleevi at google.com<mailto:sleevi at google.com>> wrote:
Is there a reason for the change from "entropy" to "unpredictable bits"

Would you be opposed to "64 bits of random data from a cryptographically strong random number generator"?

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"

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)

I would prefer this proposal.  It provides a specific thing that can be verified (whereas "entropy" and "unpredictable" are vague statistical properties).

--Richard


On Fri, Feb 26, 2016 at 1:49 PM, Ben Wilson <ben.wilson at digicert.com<mailto:ben.wilson at digicert.com>> wrote:

For discussion:

Pre-Ballot 164 - Certificate Serial Number Entropy

-- Motion Begins --

In Section 7.1 of the Baseline Requirements,

REPLACE

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

WITH

"Effective April 1, 2016, CAs SHALL use a Certificate serialNumber greater than zero (0) that contains at least 64 unpredictable bits."

-- Motion Ends --


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