Ballot 164 – Certificate Serial Number Entropy

Voting on Ballot 164, “Certificate Serial Number Entropy” has now closed. The results are as follow:

From the CAs, we received 19 YES votes, 0 NO votes and 0 Abstentions.
From the Browsers, we received 3 YES votes, 0 NO votes and 0 Abstentions.

Therefore the ballot passes.

Detailed ballot results can be found here:

Dean Coclin

CA/B Forum Chair


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, 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 positive.

— 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 Unless the motion is withdrawn during the review period, the voting period will start immediately thereafter and will close at 2200 UTC on 8 July 2016.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:

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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