Actalis: Insufficient Serial Number Entropy
bugzilla.mozilla.org
bugzilla.mozilla.org
The practice of issuing certificates with a (sometimes very) long lifetime, from one year and up, results in a situation where such automation is not strictly required, and complex bureaucratic processes can be put in place to replace certs, which becomes a major issue when 'emergency' revocations are necessary. I'd argue such bureaucratic processes don't even increase 'security', because in the end they rely on people performing manual operations (often with more rights granted than strictly required), whilst an automated system can be more easily vetted, tested, and locked down.
And the defence against this stuff is curiosity - which is internal process. If you issue lots of certificates (say, more than a dozen) and you find that the "64-bit" integers in them actually only vary in 63 bits you ought to be suspicious. If Actalis (or other CAs) had declared "Hi, we found out about this after two weeks when we looked at our serial numbers more closely" instead of waiting for the problem with EJBCA to get called out explicitly I'd have _way_ more sympathy.
Likewise if you're sure you are implementing 3.2.2.4.6 Agreed‐Upon Change to Website, curiosity would suggest it's worth taking a look at some of those agreed upon changes and how they were verified, and how some failed. No failures at all? Well that's weird, let's look more closely - oh, we're counting 404 errors as success. Oops. (Yes a real public CA did this and in their case they did find it before someone else reported it).
On such networks, certificate pinning is very common and possibly even recommended, contrary to the "Basic Requirements" and recommendations of CAs.
Failing to respect such deadlines causes penalties to the local governments, and in grave cases may even be a crime: "public service interruption" which would initiate a trial, with more fines and possibly jail time.
Thus Actalis had to choose between:
1. follow the CAs "Basic Requirements" that force CAs to quickly revoke certificates when a problem is discovered. Then most of the certificates would be revoked before the public customers managed to replace them - disrupting their operativity, risking penalties for the missed deadlines and possibly trial and jail time for "public service interruption". To avoid this, they would then need to demonstrate in a public trial that the public customers were well informed that certificates could be revoked and re-issued at any time with very short warning time, and they did everything they could to avoid the "public service interruption", both pre-emptively (when negotiating the sell of certificates and educating the customers) and re-actively (when the serial numbers vulnerability was discovered). Quite a hard path.
2. contact the customers, push them to quickly replace the compromised certificates, and revoke them only afterwards, thus avoiding service disruptions.
They chose 2. Unluckily italian public organizations are very slow, which in the end caused Actalis to miss their BR deadlines by a long shot.
The argument that there might be some end-users unable to renew their prescription seems mostly used to gain sympathy. Also this will most probably not be “a large swath of internet users”.
I do hope Actalis step up their game and regain some trust. Or it may become the next symantec.
That statement "may cause significant harm" is what I expect weighed on the CA's mind. When revoking a certificate could kill someone, and there is still a high barrier to exploit (i.e. no "proven method that exposes the Subscriber's Private Key to compromise") it should be up to the CA to clearly explain the situation, and up to Ryan to accept the explanation given. ("It is our position that your CA is ultimately responsible for deciding if the harm [...] outweighs the risks")
Clearly Actalis was not in a position to articulate the harm, which is their fault.
That said, I'm fully aware of the compliance hoops that must be jumped through when providing updates to medical devices. If you have to distribute firmware to medical devices, 4 months can be a remarkably fast turnaround. But in that case, CA-issued certificates are probably inferior to self-signed certificates (on an organisational level) that are not subject to external revocation.
* The baseline requirement is 64 bits of entropy and Actalis were providing 63 bits, i.e. only short by a single bit. It would seem unusual if the baseline requirements were a mere one bit of entropy from insecurity.
* The requirement for 64 bits of entropy is to reduce the risk of hash collision attacks [1] - which have only ever been demonstrated for MD5 and SHA-1, neither of which are used to sign certificates any more.
If web security was a tightrope, this would be like hearing that the second safety net, underneath the first believed-to-be-robust safety net, was found to be strong enough to catch a 900 lbs person, when it was specified for 1000 lbs.
I believe this is due to Actalis misunderstanding the exception. Mozilla provides an exception for exceptional circumstances which Actalis's obviously were not.
I didn’t find the arguments of severity convincing then either. But the gist was that they need to be completely consistent and rigorous so it does make sense even if it is a massive inconvenience for people. Again.
- the Tuscany Region (e.g. O=Rete Telematica Regionale Toscana, etc.)
- the Piedmont Region (e.g. O=CSI Piemonte, etc.)
- central public government (eg. O=Bank of Italy, Ministry of Transports, etc.)
- major banks (e.g. O=Unicredit S.p.A., FinecoBank, etc.)
- large private companies (e.g. O=SNAM, Terna, Wind, etc.)
- chambers of commerce
On the other hand, allowing a CA to violate the BR's without pain will just encourage others to do so.
Is it so? I remember that in 2008 someone was able to create a rouge CA certificate because of the predictability of serial numbers[1]. It was a different time: we still used md5, but are you sure the limited entropy used to generate serial numbers does not pose any security risk?
This is a Brown M&M ‡. It doesn't actually matter in terms of security, 63-bits, 65-bits, it's never going to make a real difference. But we wrote 64-bits in those rules, if we can't trust you to obey that rule, who says you got the really important parts right?
Would Van Halen abort a concert if there was a single brown M&M in a bowl of 1000? Probably not because even though it's a violation of their contract, they got their point across, it still means the organisers had read through the full contract and tried to obey.
Reading through the discussion, I wish I could be as strict as Ryan Sleevi is in demanding that browsers fix their incompatibilities with the web's BR (ehm.. standards). Chrome, there's this bug where this element is placed one pixel off from where it should be (it's by no means critical and does not impact users of any website in any meaningful way, but according to the CSS Box Model Module Level 3 spec, paragraph suchandsuch it's wrong). How about you fix it by next week or I'll uninstall you from all systems on the world.
In the 2008 attack, the CA was using sequential serial numbers. They weren't randomized at all.
The attackers had to do a large amount of computation to produce colliding certificates even when they knew exactly what the rest of the content of the certificate would be.
In the aftermath of that, we got MD5 deprecation and also a requirement that certificates include randomness that wouldn't be predictable to the subscriber, so that the subscriber doesn't know what the collision target is.
It's a little complicated to foresee the exact size of the benefit from this in different threat models, but in the model where the attacker has the capability to produce two related texts with the same SHA-256 hash, the current precaution means that the attacker has only a 1/2⁶⁴ probability that using that capability in conjunction with a certificate issuance will yield a matching certificate.
In 2008, certificate issuance usually cost money for the subscriber, where now it needn't, but there are still issuance rate limits and there's now Certificate Transparency, so all of the attempts will become public.
A bigger risk is presumably an n-way collision capability where an attacker can produce, not just 2, but n related plaintexts that all have the same SHA-256. In that case the attacker has an (n-1)/2⁶⁴ probability per certificate issuance that the issued certificate has the desired hash, assuming nothing unexpected or uncontrollable happens during the certificate issuance. (Another tricky problem, for example, is the time of issuance, which can be specified accurate to the second by the CA and appears in the certificate.)
Especially when nobody has demonstrated a SHA-256 collision or research that's close to producing one, and all attempts would be public in Certificate Transparency, and all CA issuance is rate-limited in some way, it doesn't seem like even 1/2⁶³ is that bad. Just five or ten bits of entropy in the certificate would probably have been enough to stop the 2008 researchers' attack from succeeding at all.
The attack would also have to have been carried out while the existing certificates were being issued (if there was no successful attack during certificate issuance, there won't be an attack after-the-fact).
I like tialaramex's brown M&M analogy: browser vendors are concerned with ensuring that CAs take rules and policies very seriously, even if there's no conceivable way that a particular problem could be related to an attack or vulnerability.
If the majority of outstanding certificates were held by the Italian government, major banks and hospitals, what are the CA supposed to do if they're just told "No, you won't revoke the certificates until we're ready, we don't think the risk is worth it"? Further, reading a comment below on the usage of these certificates by the Italian state for mandatory reporting: it sounds like revoking could be considered a criminal offense...
This very much reads like a private entity mandating that tens if not hundreds of thousands of Euros are spent by the Italian state over a very minor security risk.
The best part here too is that Mozilla's link on revocation basically says "we understand sometimes it's more risky to revoke according to our policy than to take a little longer, we just don't care and will utter our disappointment in you either way".
The CA is irrelevant, they're just a middle-man.
If you think somebody else should decide - maybe the Government of Italy, or the Queen of England, or Donald Trump, or you personally, then here's a few questions for your new Root Trust Programme:
1. Why? At least Mozilla's rationale is related to a fact, they make Firefox, so it trusts whatever they decide, what would be the rationale for why the Pope gets to decide?
2. Are they actually doing it? This is largely a tedious responsibility. But, if you decide to slack off, every Firefox user gets screwed. So, you know, you're going to need to put those hours in. Forever. I've lost count of how many people or organisations decided they could do better and didn't last a year.
3. Where's the transparency? The main way Mozilla stands out from the other big trust store operators (Apple, Microsoft, Google, and arguably Oracle) is that they're a not-for-profit and so they operate transparently. Your contributions are welcome at m.d.s.policy https://groups.google.com/forum/#!forum/mozilla.dev.security... where we are currently discussing the minutiae of Certificate Policy documentation. If your alternative is less transparent, how is that not worse?
I don't know how involved you are, but to a lay observer this story seems like Mozilla's policies are entirely black and white, to the benefit of nobody (except perhaps to reduce work I suppose, which is reasonable, but not really a valid reason in terms of security)
Is there no tiered approach to risks? Hell, in this situation it seems like more harm and risk will have been created by the rush to reissue certificates that would have been caused by this theoretical security vulnerability.
Edit: Actually, on further reading, it seems like the issue is more that Actalis didn't correctly invoke their right to this discretionary power?
> we have met a lot of resistances and compliances from the enterprise customers, mainly public entities, to whom we have wrongly responded by giving more time. Four months to replace some certificates is more than enough.
> As managing director I have just decided some organization and role changes with immediate effect: I have removed the SSL infrastructure and Operation managers
If you have a good sales pitch and a customer who knows little about the product you're selling you can sell all kinds of nonsense.
"Cheap DV is good enough for private webpages, but for a serious webpage you need at least OV" makes little sense, but sounds good enough for many.