Sunlight, a Certificate Transparency log implementation

https://letsencrypt.org/2024/03/14/introducing-sunlight.html

by ben_s • 2 years ago

217 43 2 years ago

letsencrypt.org

Dowwie• 2 years ago

https://github.com/FiloSottile/sunlight

mcpherrinm6• 2 years ago

I’m excited for others to see the work we’ve been doing to make CT easier to run.

Also see the discussion on the ct-policy mailing list: https://groups.google.com/a/chromium.org/g/ct-policy/c/v9Jzl...

(I am the author of the blog post)

FiloSottile5• 2 years ago

This is one of the projects I've been most excited about in the last few years. It let me backport to Certificate Transparency some of the modern designs ideas that came after it.

Beyond the Let's Encrypt announcement and the ct-policy thread (which includes a technical and advantages summary), here are a few resources that might be interesting.

- Design document, including architecture and tradeoffs: https://filippo.io/a-different-CT-log

- Implementation: https://github.com/FiloSottile/sunlight

- API specification: https://c2sp.org/sunlight

- Website, including test logs and feedback channels: https://sunlight.dev/

If you’re thinking “oh we could use something similar” please reach out! Sunlight is retrofitting some of the modern tlog designs on a legacy system. With a greenfield deployment you can do even better! I’m working with the Sigsum project on specs, tooling, and a support ecosystem to make deploying tlogs easier and safer.

cpach• 2 years ago

Cool!

tlog = transparency log, but not neccessarily for X509 certificates?

simonw2• 2 years ago

A lot of the benefits I see people proposing for blockchain systems - things like "an immutable log of real estate ownership that no-one can ever tamper with" - look to me like they would be better handled by the certificate transparency log mechanism rather than by a proof-of-work/proof-of-stake blockchain.

ulrikrasmussen• 2 years ago

I agree. I have been toying with the idea of a system where you have a set of participants who each have responsibilities for maintaining some state that users and other participants rely on. For example, a network of banks including a central bank, but the network could also include merchants who rely on the banks for payments and trusted third parties who keep track of agreements such as loan contracts. Instead of putting everything on a blockchain, make each participant be the sole writer for one or more ledgers (e.g. one ledger per account they maintain) and use a non-repudiable two-phase commit protocol to coordinate multiple transactions performed atomically across ledgers.

Using the techniques from certificate transparency, as soon as a participant has published that they have performed a transaction on their ledger, then they cannot take that back again without an auditor finding out. They also cannot branch their ledgers (i.e. double spend) as long as all participants periodically gossip received transactions to the auditors. Or rather, they CAN, but the auditor will detect it eventually and have proof of it. What you get is a sort of multi-party SWIFT-like messaging system which doesn't require banks to change their basic accounting infrastructure and tokenize everything.

kfreds6• 2 years ago

Absolutely. The vast majority of such use cases are way better served by a transparency log than a blockchain.

Imagine an embedded system that only accepts firmware updates that are digitally signed and discoverable in a public append-only log. Such a proof would have to be attached to the firmware update just like the digital signature, with a size constraint on the order of kilobytes. To be fair I haven't kept up-to-date with blockchains, but I find it hard to believe that any existing blockchain could solve this. Transparency logs can.

I wonder when businesses that currently use blockchains as immutable logs will realise that there is a more cost-effective solution. One of the reasons it hasn't happened yet is because blockchains have magnitudes more mindshare than transparency logging. In fact, there doesn't seem to be that many people who deeply understand the design parameter space of transparency logs.

Shameless plug: My colleagues and I have spent a few years designing one - Sigsum. As Filippo mentions he has contributed to its design as well.

eudoxus• 2 years ago

To be fair I haven't kept up-to-date with blockchains, but I find it hard to believe that any existing blockchain could solve this. Transparency logs can.

Any insight as to why you think blockchains can't handle this?

The only fundemental difference between blockchains and transparency logs is the trust assumptions and censorship resistence. Its fairly easy to reason about a transparency log as a permissioned blockchain. Conversely, a blockchain could be seen as a permissionless transparency log.

benlivengood• 2 years ago

Yep; the only current advantage that blockchains have over current CT logs is trustable global transactions. This is a pretty powerful advantage, but the probabilistic certainty can be shifted from confirmation height to fraction of log inclusion and cost-of-revocation, e.g. transactions can be proposed in one or two logs and then confirmed by a supermajority of acceptable logs (to the transaction) once the ordering of potentially conflicting transactions is established within at least one log.

E.g. double-spends are prevented by announcing a transaction with a TTL that is included in at least one log, such that no conflicting transaction appears within any other log before the TTL expires, at which point most/all logs accept the permanent transaction.

Inclusion in a supermajority of logs provides the probabilistic likelihood of the transaction being durable (e.g. irreversible), and individual CT logs become invalid if they sign a later conflicting transaction.

Economic incentives could be as simple as charging customers for inclusion of their transaction in the log with escrow to refund all customers if the CT log becomes invalid. This can be implemented as its own CT-log transaction because logs have an incentive to cooperate with each other on signing off on the successful completion of a valid CT log (after a year or whatever) so they all keep getting paid.

kardianos• 2 years ago

Great call out to the Go checksum database. Go walks a great line between distributed system and central verification.

JanisErdmanis5• 2 years ago

Is there a specification one can follow to implement a verifier for certificate transparency logs? I looked up RFC 6962, but it does not contain test vectors. I am particularly interested in history tree inclusion and consistency proofs and don’t care how tree root is signed.

FiloSottile5• 2 years ago

We're working on it! RFC 6962 specifies inclusion and consistency proofs, but indeed it's missing test vectors. Keep an eye on https://c2sp.org and https://c2sp.org/CCTV.

phasmantistes• 2 years ago

I'm super excited about Sunlight. The CT ecosystem is really fragile right now, with current log implemetations being expensive to operate and very difficult to operate correctly, as evidenced by the recent failures of multiple logs[1][2]. And if too many logs fall over, it becomes infeasible to include the requisite number of SCTs in certificates, or worse, already-issued certificates can become effectively untrusted.

With Sunlight reducing costs by a couple orders or magnitude and significantly easing deployment complexity, it will be a huge boon to the whole ecosystem. I really hope log monitors begin crawling sunlight logs and browsers accept them as trusted in the near future.

[1] https://groups.google.com/a/chromium.org/g/ct-policy/c/6mvSo...

[2] https://groups.google.com/a/chromium.org/g/ct-policy/c/_dhkS...

chtitux• 2 years ago

[1]: a delete command was wrongly issued to all Cassandra servers during a planned maintenance

[2]: a database server had its disk full that lead to a corrupted database

SRE is tough.

hedora2• 2 years ago

This is probably a step forward for HTTPS, but it still bugs me that most of the internet is running on a cryptography system that doesn't provide confidentiality or integrity.

Instead, it makes it possible for you to detect that someone's been using a rogue certificate after the fact. It bugs me even more that many security certification folks insist on using this junk to manage SSH keys (out of the box, SSH provides stronger security guarantees than X.509 certificates).

The whole situation reminds me of Crypto AG:

https://en.wikipedia.org/wiki/Crypto_AG

mcpherrinm6• 2 years ago

I'm biased, of course, because I work for Let's Encrypt, and spend a lot of time thinking about the trade-offs and how to make this ecosystem better.

I think claiming that "SSH provides stronger security guarantees than X.509 certificates" is pretty outlandish.

SSH is trust-on-first-use. How many people actually verify the key fingerprints? Maybe you do when you're sshing to your single corporate bastion, or to Github.com, but could you imagine if you had to do that for every website? It simply would not work. It only barely does for SSH because of the (relatively) low amount of use SSH gets. When at a previous company I set up an SSH CA, we found an SSH MITM within 3 days (though benign; an ISP-provided modem/router was resolving all hostnames to itself when the WAN was down), and the user was planning on just accepting the new SSH host key.

Certificate Transparency is an auditing system, so of course it's after-the-fact. It has helped to improve the CA system immensely. While there's still work to do, the TLS ecosystem is unrivalled in terms of actual security benefit delivered to the most people.

hedora2• 2 years ago

With SSH, if you care, you just need to check the fingerprints. With HTTPS, you have to implement certificate pinning (or set up a private CA and configure everything to ignore the system roots of trust), which is much more onerous.

Thanks for helping to improve https, since it helps in practice.

edit: Another way to put it is that if you want to reduce your attack surface to just the client and server machine, it's easy enough to scale SSH fingerprint verification to a few dozen machines. It's much, much harder to get SSL to provide that property for even two machines.

As for scaling to 100's of machines, it's not obvious to me that bootstrapping and rsyncing a known_hosts file and /etc/ssh_config that turns off trust on first use is harder than setting up a CA.

For millions of machines, SSH falls over (because known_hosts would be too big), but at that point, you're talking about what percentage of machines are compromised in steady state, not how to keep all attackers out. That's where HTTPS works well.

kfreds6• 2 years ago

While there's still work to do, the TLS ecosystem is unrivalled in terms of actual security benefit delivered to the most people.

Well said!

jancsika• 2 years ago

When at a previous company I set up an SSH CA, we found an SSH MITM within 3 days (though benign; an ISP-provided modem/router was resolving all hostnames to itself when the WAN was down), and the user was planning on just accepting the new SSH host key.

Wouldn't that be a problem with TOAU? :)

MattJ100• 2 years ago

It probably won't happen, but I'd love to see the concept of channel binding reach the web ecosystem. It allows you to securely verify the certificate, independent of any third party. However it only works when you and the server have a shared secret (which could be a password or something else). It therefore isn't really applicable to casual "anonymous" browsing. But for that kind of browsing, a third-party attestation (e.g. from a CA) is probably good enough anyway.

We use channel binding in the XMPP ecosystem and it's going well.

zozbot234• 2 years ago

If you're ok with TOFU ala ssh you can just use self-signed certificates. Skipping the browser warning is the "trust" equivalent.

harikb• 2 years ago

One of the folks behind this effort, Filippo (https://filippo.io/) is now, as he calls it, a full time professional open source maintainer https://words.filippo.io/full-time-maintainer/

That means I spend most of my time on maintenance, and I offer retainers to companies that benefit from my work and from access to my planning and my expertise.

He is trying a new way of life for highly-skilled OSS maintainers. Hopefully this works out well for him.

RIMR• 2 years ago

I knew I had heard of this guy before! I enjoyed this project of his: https://filippo.io/behindthesofa/

kfreds6• 2 years ago

Transparency logging is a fantastic complement to public-key cryptography.

A prioritised list of the most important cryptographic constructions may very well look like this: symmetric ciphers, hash functions, MACs, public-key cryptography, transparency logging.

If you're interested in security and/or cryptography and you don't know what transparency logging is, you're missing out.

gnfargbl2• 2 years ago

The "standard" list of all known CT logs is maintained by Google at https://www.gstatic.com/ct/log_list/v3/all_logs_list.json and follows the schema at https://www.gstatic.com/ct/log_list/v3/log_list_schema.json.

As far as I can tell, that schema currently only supports RFC6962 logs. Are there plans in train to support enumeration of Sunlight-format logs? Or am I just far too impatient?

mcpherrinm6• 2 years ago

Apple also has a list at https://valid.apple.com/ct/log_list/current_log_list.json

Getting them accepted as trusted lists is the goal, but how exactly that happens is not yet determined. Both the Apple and Chrome CT programs have responded to our announcement email at https://groups.google.com/a/chromium.org/g/ct-policy/c/v9Jzl...

As the specification, implementations, and ecosystem support evolve, we look forward to being able to include Sunlight logs alongside RFC6962 logs in Chrome. While we're not quite there yet, we encourage the CT community to experiment with the prototype logs and specification as much as possible.
That said, we’re not planning on accepting new Sunlight logs quite yet in the Apple CT Program (though we hope that is the end result). Before that jump, we’d especially like to hear additional input (even if it’s just “LGTM”) from CT log Monitors and Auditors, as well as CAs and other relying parties submitting certificates to CT logs. Along with that, we invite input from additional SMEs able to perform (informal) security and risk assessments of the proposal — especially if they’re accompanied by filed issues :)

The log list schema is a relatively small problem and should be easily enough to solve.

gnfargbl2• 2 years ago

Thanks for replying to what was a rather half-baked question, typed out in some excitement.

I guess a more thought-through version is as follows: Currently, the CT logs with state == usable are operated by only six entities (Cloudflare, DigiCert, Google, LE, Sectigo, TrustAsia). In the earlier days of CT logs, there were a larger number of log operators. I think it's a reasonable assumption that we've ended up in the current situation at least partly because of the cost of log operation?

Since Sunlight aims to drastically reduce the cost of log operation, might we expect the number of log operators to rise, and if so would there be likely changes to the ways that log operators are enumerated as "trusted", or would things likely be as they are now?

simonw2• 2 years ago

That's a fun JSON file. It's served with open CORS headers which means I can query it in Datasette Lite like this:

https://lite.datasette.io/?json=https://www.gstatic.com/ct/l...

poweredger640• 2 years ago

I was pretty sure CT was gonna die when Nimbus went down last year. Like, if Digicert or LE's logs went down while Nimbus was down, ha ha

mcpherrinm6• 2 years ago

There are six trusted operators, many with multiple logs. So there is some degree of redundancy, but this project is definitely an outcome of those same concerns.

savoyard• 2 years ago

Curiously, “Sunlight” was the codename for the original CT spec:

https://datatracker.ietf.org/doc/draft-laurie-pki-sunlight/0...

FiloSottile5• 2 years ago

Purely coincidental! ;-)

Sunlight, a Certificate Transparency log implementation

Related Stories