Comment by steventhedev | Hacker News Reader

Comment by steventhedev | Hacker News Reader

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The number 64 is arbitrary, and was chosen because of potential birthday attacks on digest algorithms. Mozilla enforce the BR sometimes in extreme ways, and we derive security and trust from that enforcement. In many cases, the BR are confusing, unclear, or ambiguous. The confusion here arose from the BR requiring 64 bits of entropy from a CSPRNG and to always be positive, and ASN.1 integer fields using signed integers, so 1 bit is lost to the sign bit.

In this specific case, they have the option to use 160 bit fields for the serial number. They chose to use the minimum required of 64, apparently out of performance concerns for embedded devices with limited memory/compute.

The CA platform (EJBCA) they use defaulted to this behavior, and from what I was able to understand[0] are in the process of pushing out an update to all their customers to resolve the issue. It is very likely that someone will start going through the CT logs and we will see many reports of insufficient entropy in cert serial numbers.

[0]: https://groups.google.com/d/msg/mozilla.dev.security.policy/...

I don't have much knowledge about certificates, except from what I read on this mail list. But are there are other CAs active (in mozilla's root trust), using 64bit serial?

Probably.

Mozilla doesn't have a big team of investigators wandering about looking for possible problems. The CAs could dob themselves in (filing a "problem report" that says they broke this rule and what they're doing about it) or an independent researcher could write to m.d.s.policy about a problem they found. Otherwise it may go undetected forever.

Most interest is focused on problems that either directly indicate a grave problem (e.g. some researcher got a CA to issue for example.com when they shouldn't because that researcher doesn't control example.com) or suggest a management problem that could hide further grave problems from us (e.g. Symantec had paperwork that directly contradicted the operational behaviour of CrossCert, so why did we notice the problem before they did?)

The 64-bit entropy rule isn't such a problem. CAs should use random values here because it protects us against a problem with the secure hash function (SHA256 usually these days). If the hash function is slightly broken the random numbers buy us time to get a new one. But if you have 63-bits instead of 64-bits, it's not like that's the difference that makes an attack viable. It's just a technical violation, it's a Brown M&M in that sense. The rules say to do it, and they didn't, so we are more worried than otherwise that they are not obeying the rules.

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