[cabfpub] Ballot 164 - Certificate Serial Number Entropy

Ben Wilson ben.wilson at digicert.com
Fri Jun 24 08:17:44 MST 2016

Ballot 164 - Certificate Serial Number Entropy


This ballot has been proposed by Jacob Hoffman-Andrews of Let's Encrypt and
endorsed by Ben Wilson of DigiCert and Tim Hollebeek of Trustwave:


Statement of intent:


As demonstrated in
-1.0.pdf, hash collisions can allow an attacker to forge a signature on the
certificate of their choosing. The birthday paradox means that, in the
absence of random bits, the security level of a hash function is half what
it should be. Adding random bits to issued certificates mitigates collision
attacks and means that an attacker must be capable of a much harder preimage
attack. For a long time the Baseline Requirements have encouraged adding
random bits to the serial number of a certificate, and it is now common
practice. This ballot makes that best practice required, which will make the
Web PKI much more robust against all future weaknesses in hash functions.
Additionally, it replaces "entropy" with "CSPRNG" to make the requirement
clearer and easier to audit, and clarifies that the serial number must be


-- Motion Begins --


In Section 1.6.1 of the Baseline Requirements,




CSPRNG: A random number generator intended for use in cryptographic system.



In Section 7.1 of the Baseline Requirements,




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




"Effective September 30, 2016, CAs SHALL generate Certificate serial numbers
greater than zero (0) containing at least 64 bits of output from a CSPRNG."


-- Motion Ends --


The review period for this ballot shall commence immediately, and will close
at 2200 UTC on 1 July 2016. Unless the motion is withdrawn during the review
period, the voting period will start immediately thereafter and will close
at 2200 UTC on 8 July 2016. Votes must be cast by posting an on-list reply
to this thread.


A vote in favor of the motion must indicate a clear 'yes' in the response. A
vote against must indicate a clear 'no' in the response. A vote to abstain
must indicate a clear 'abstain' in the response. Unclear responses will not
be counted. The latest vote received from any representative of a voting
member before the close of the voting period will be counted. Voting members
are listed here: https://cabforum.org/members/


In order for the motion to be adopted, two thirds or more of the votes cast
by members in the CA category and greater than 50% of the votes cast by
members in the browser category must be in favor. Quorum is currently ten
(10) members- at least ten members must participate in the ballot, either by
voting in favor, voting against, or abstaining.

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