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diff --git a/content/post/hold-on-to-your-hat-and-learn-system-transparency-in-five-minutes.md b/content/post/hold-on-to-your-hat-and-learn-system-transparency-in-five-minutes.md
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+---
+title: "Hold on to your hat and learn System Transparency in five minutes!"
+date: 2020-10-12
+---
+What do we really know about the systems that run our critical applications?
+Not enough is probably a fair summary: much can go wrong between device reset
+and execution of a user-land application. System Transparency helps you verify
+that what you think is running remotely actually runs, and not, say, a modified
+operating system that contains a secret backdoor. I will break it down
+top-to-bottom after first motivating the rationale and objective briefly.
+
+## Rationale and objective
+Anyone in a position of power should probably be subject to a proportional
+amount of transparency. It is an important safeguard that deters malicious
+activities, while at the same time making it possible to fix honest mistakes.
+Such a principle can of course be applied in real life, but I mainly refer to
+the different components that compose a digital system: hardware, firmware,
+operating systems, applications, and so forth. Generally I would say that _power
+is decreased by transparency because most abuse can be detected_. For example,
+it would be proportional by Intel to open up their proprietary management engine
+because it is powerful enough to
+	[hijack your system](https://www.wired.com/2017/05/hack-brief-intel-fixes-critical-bug-lingered-7-dang-years/).
+
+
+The scenario to keep in mind is accordingly as follows. A remote server is
+running a service somewhere that processes your data based on a policy. You
+might have reason to believe that said policy is followed now, but will it be
+in the future when
+	[intruders](https://www.eff.org/deeplinks/2020/07/after-weeks-hack-it-past-time-twitter-end-end-encrypt-direct-messages)
+and
+	[law enforcement](https://www.eff.org/cases/apple-challenges-fbi-all-writs-act-order)
+knock down the door? I, for one, would prefer if we could _verify_ that the
+system in question works as intended (and not just trust that to be the case
+blindly). The other benefit of such remote system verification is more subtle:
+the service provider could use it to determine if intention matches deployment.
+Of course there might be unknown bugs, but by making every part of the system
+as transparent as possible it will be easier to find vulnerabilities and assess
+trustworthiness.
+
+## Breaking it down, top-to-bottom
+The idea is to first make transparent what is allowed running on a given system.
+You can view this as the top-most layer that represents an operating system
+package with installed programs, configurations, and so forth. Thereafter we
+need to enforce that nothing else than the transparent operating system package
+is allowed running with a bottom layer. Such enforcement is based on hardware
+features that should be transparent as well.
+
+### Reproducible and publicly auditable operating system packages
+Suppose that we have an operating system package that we would like to deploy.
+As a first step we need to
+	[build it reproducibly](https://reproducible-builds.org/), such that anyone can inspect
+the source code and determine if the resulting package lives up to the claimed
+promises. A possible issue that one might find, for example, is that there is
+interactive system access installed: pretty much anything could run after a
+reconfiguration. Therefore, a transparent system should restrict arbitrary
+access and provision updates as new operating system packages that, again,
+build reproducibly. For those that are familiar with functional programming, it
+is essentially an
+	[immutable infrastructure](https://web.archive.org/web/20200518230417/http://chadfowler.com/2013/06/23/immutable-deployments.html).
+An independent benefit of such maintenance is that
+	[malware persistence](https://github.com/Karneades/malware-persistence#overview-of-often-and-less-often-used-persistence-mechanisms)
+becomes trickier.
+
+A reproducible operating system package serves a limited purpose unless it is
+publicly available. Therefore, we should insert it into a
+	[transparency log](https://transparency.dev/).
+This means that anyone can verify whether a package builds reproducibly, and if
+it contains, say, a secret backdoor that would be detected by source inspection.
+
+### Measured and remotely attested boot
+Now we need to enforce that the publicly disclosed operating system packages
+run on our servers and nothing else. At a first glance it might sound daunting,
+but today’s hardware platforms ship some pretty useful security features. For
+example, there is usually a separate hardware domain for key management,
+cryptographic hashing, Platform Configuration Registers (PCRs), and digital
+signatures. It is possible to measure code, data structures, and configurations
+into a PCR before execution to form a hash chain, such that all initial system
+states can be aggregated into a single value. The system’s boot process can be
+aborted if a measurement diverges from the expected value, e.g., because the
+boot loader did not enforce transparency logging as required by the top layer.
+It is also possible to sign PCR values and attest them remotely. In other
+words, if these features work we can prove to a third party how the system
+booted.
+
+### Open source firmware and LinuxBoot
+An immediate concern is that much trust is placed in the underlying hardware
+platform. Naturally, it begs the question if such trust is misplaced. A
+	[talk by Ron Minnich](https://osseu17.sched.com/event/ByYt/replace-your-exploit-ridden-firmware-with-linux-ronald-minnich-google)
+brings you up to speed on why the answer is probably "yes". Let us focus on
+solutions instead: open hardware, firmware, and boot loaders. It is paramount
+that these components are vetted thoroughly in the open because they may
+compromise the system
+	[while running or before it is even started](https://securelist.com/mosaicregressor/98849/).
+
+So, System Transparency implements a flavor of
+	[LinuxBoot](https://www.linuxboot.org/)
+called
+	[stboot](https://github.com/system-transparency/system-transparency/blob/master/README.md#bootloader-stboot).
+It can replace much of the later-stage UEFI components with a Linux kernel and a
+user-land environment in Go, such that a subset of proprietary firmware is
+removed in favor of an open source option that is safer and customizable. For
+example, one possible customization is to enforce transparency logging as a
+criteria to boot into the host operating system. It is possible to eliminate
+UEFI all together by re-flashing the firmware with
+	[coreboot](https://doc.coreboot.org/)
+and specifying stboot as a payload. The TL;DR is that coreboot is (mostly) open
+source firmware that does the bare minimum hardware initialization. It was
+recently
+	[ported to a modern server platform](https://mullvad.net/en/blog/2019/8/7/open-source-firmware-future/).
+
+### Set-up ceremony and tamper-evident hardware
+Assuming an open platform that enforces transparency logging as described
+above, you can be somewhat sure that said operating system packages run. The
+problem is that you cannot easily know if that assumption is true. I am not
+claiming that there is a slam-dunk solution here, but measures can be taken to
+reduce the risk of a broken setup. For example, assemble and install the
+platform while witnessed live by several independent parties that write down
+and publish a log book of events that occurred:
+	"[neutralized the management engine](https://github.com/corna/me_cleaner)",
+"added open firmware with checksum XYZ", etc. We can also define some physical
+security boundaries that, if breached, automatically activate defensive
+mechanisms that preserve the system’s overall integrity after setup.
+
+## Concluding remarks
+The described System Transparency design shows how a service provider can
+facilitate trust by engineering a system that is more trustworthy. I would like
+to emphasize _more trustworthy:_ all of the applied techniques have merit on
+their own, and if one part does not fit the use-case or current practice it
+might be reasonable to cut it. For example, if you lease cloud servers that
+only allow starting stboot from UEFI: so be it. Simply assume that there will
+be no firmware and physical attacks for the time being. It is still a
+significant improvement when compared to obscure operating system packages
+since the attack surface and overall trust domain is reduced.
+[The growing problem of malicious Tor relays in the cloud](https://medium.com/@nusenu/how-malicious-tor-relays-are-exploiting-users-in-2020-part-i-1097575c0cac)
+could benefit from such a solution because a class of real-world attackers would
+not see any traffic (if enforced by Tor). As another example: suppose your
+interest is mainly to harden your own internal infrastructure, and not so much
+about making it transparent for everyone. It is not a strict requirement to put
+the operating system package in the public, i.e., a hash is enough to convince
+yourself that nothing else was allowed running.
+
+## Acknowledgments
+Fredrik Strömberg provided valuable feedback on this story, which is sponsored
+by my
+	[System Transparency](https://system-transparency.org/)
+employment at Mullvad VPN.
diff --git a/content/post/observations-from-a-trillian-play-date.md b/content/post/observations-from-a-trillian-play-date.md
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@@ -0,0 +1,130 @@
+---
+title: "Observations from a Trillian play-date"
+date: 2020-11-23
+---
+Have you ever heard about
+	[Trillian](https://transparency.dev/)
+in the context of transparency logging?  Perhaps you view it as an integral part
+of
+	[Certificate Transparency](https://certificate.transparency.dev/),
+a solution for arbitrary transparency applications, or both. Even if you know
+Certificate Transparency quite well the Trillian details might be a bit blurry
+until you sit down and get some hands-on experience: at least that was the case
+for me.  Therefore, Trillian and I had a little play-date. I thought I would
+share a few observations that in hindsight are obvious but at the same time
+helpful.
+
+## Problem statement and overview
+I agree with Daz Wilkin that
+	[it is somewhat daunting to get started with Trillian](https://medium.com/google-cloud/google-trillian-for-noobs-9b81547e9c4a).
+Putting it all together involves many different components and configurations,
+especially if you need the high reliability and scale that Trillian supports. It
+does not have to be that complicated though. Trillian is pretty much a database
+which includes an append-only Merkle tree:
+
+1. **Trillian log server:** exposes a gRPC API that is used by an
+application-dependent front-end or so-called _Trillian personality_. Requests and
+responses trigger operations on the underlying database, such as queuing new
+data requests and assembling cryptographic Merkle tree proofs.
+2. **Trillian log signer:** checks the database periodically and sequences it
+into a Merkle tree. The term _log signer_ was confusing for me initially because
+it is usually the front-end personality that adds externally visible signatures.
+Therefore, I found it helpful to think of this component as a _log sequencer_.
+
+I will not talk much about the front-end personality. It is the part of Trillian
+that you or your ecosystem has to implement. It will include definitions of
+public endpoints, the data to be logged, who is allowed logging it, etc.
+
+## Trillian as a database abstraction
+The simplest description of Trillian is probably as a regular database. You can
+insert any item of your choice after serializing it as zeroes and ones, and come
+back later on and retrieve it. In reality it is more accurate to say that
+Trillian is hooked-up to a database, such as MariaDB using the schema over
+	[here](https://github.com/google/trillian/blob/master/storage/mysql/schema/storage.sql).
+This means that before getting started a database must be configured such that
+there is a record in the Trees table that identifies a particular Trillian
+instance.
+
+```
+CREATE TABLE IF NOT EXISTS Trees(
+    TreeId BIGINT NOT NULL,
+    HashAlgorithm ENUM(‘SHA256’) NOT NULL,
+    SignatureAlgorithm ENUM(‘ECDSA’, ‘RSA’, ‘ED25519’) NOT NULL,
+    PrivateKey MEDIUMBLOB NOT NULL,
+    PublicKey MEDIUMBLOB NOT NULL,
+    ...
+);
+```
+
+Initially I was confused by the public-key cryptography that is part of the
+database schema: is it not the front-end personality that attaches signatures,
+for, say, Signed Certificate Timestamps (SCTs) in Certificate Transparency?
+Well, yes. But the scenario in mind here is that there might be a front-end
+personality that runs in a different trust domain, such that the Trillian
+back-end needs to sign some data to prove its origin. The front-end determines
+what becomes externally visible regardless of if these signatures are used.
+
+New add-data requests are queued by the Trillian log server in an unordered
+table of pending leaves. Each such leaf also has an optional appendix, which
+allows extra data to be stored but without merging it into the Merkle tree.  For
+example, it might be reasonable to hold on to an associated signature if the
+front-end personality requires that the data is signed as an admission criteria.
+
+## Trillian as a Merkle tree abstraction
+The log signer sequences the pending leaf data periodically. By sequencing I
+mean taking the unordered leaves that one or more log servers queued, and then
+appending them to the current Merkle tree on specific indices. In other words,
+not even the log servers know the index of an added leaf until it is merged. It
+is important to keep in mind because several proposals in the past assumed that
+Trillian logs are timestamp ordered, but strictly speaking there is no such
+guarantee unless the front-end takes responsibility of sequencing (in which case
+there is a specific pre-ordered Trillian API that can be used).
+
+The Merkle tree itself is viewed as many smaller sub-trees in the database,
+where only the bottom layer of each sub-tree is stored physically. Any interior
+node can be computed on the fly, which apparently
+	[saves up to 50% of space](https://github.com/google/trillian/blob/master/docs/storage/storage.md).
+The log server accesses the database to interact with the sequenced Merkle tree,
+e.g., to retrieve tree heads and build audit paths (hashes in the tree). As
+such, there is no explicit communication between the log server and signer.
+
+## Trillian as an API
+The final part of the puzzle is the interface that the front-end personality can
+use while talking to Trillian. Fortunately, it is relatively straight forward.
+You will only send requests and receives responses from the log server that
+exposes a gRPC API. Possible requests and responses are documented
+	[here](https://github.com/google/trillian/blob/master/docs/api.md).
+This is really the place to look if you want to know what will "just work".
+
+For example, you will notice that there is a `QueueLeafRequest` that takes as
+input some data that goes into the Merkle tree and the leaf’s Appendix, as well
+as an identity hash that tells Trillian what should be counted as a duplicate.
+You may also take advantage of the built-in Trillian rate limiting by specifying
+a custom `charge_to` string. You can think of this as saying "Dear Trillian,
+this IP address requested to add a leaf and it is signed using a certificate
+that ends in the following trust anchor". In response a resource exhaustion
+error might be returned if too many requests were observed for a given quota
+string.
+
+Other requests I would suggest you look into include retrieving a leaf, a signed
+tree head, an inclusion proof, and a consistency proof. It goes a pretty long
+way if you want to get what details are (not) in the front-end personality.
+
+## Concluding remarks
+The view that Trillian is a database with an append-only Merkle tree is by no
+means wrong, but it is also not a complete description. For example, there is
+also a map mode that associates keys with values without being append-only. If
+you look further into Trillian you will also realize that there are many details
+that matter for deployment but not so much if we just want to get the hang of
+things. For example, there is built-in functionality for running several log
+server and signing instances, coordinating them, exporting health metrics,
+choosing database back-ends, configuring rate limiting, and more. If that sounds
+interesting you can get an enhanced intuition by reading the
+	[manual deployment scenario](https://github.com/google/certificate-transparency-go/blob/master/trillian/docs/ManualDeployment.md)
+documentation for Certificate Transparency.
+
+## Acknowledgments
+Fredrik Strömberg provided valuable feedback on this story, which is sponsored
+by my
+	[System Transparency](https://system-transparency.org/)
+employment at Mullvad VPN.
diff --git a/content/post/trillian-log-sequencing-demystified.md b/content/post/trillian-log-sequencing-demystified.md
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+---
+title: "Trillian log sequencing: demystified?"
+date: 2021-02-09
+---
+One way to view
+	[Trillian](https://transparency.dev/)
+is as
+	[a database with an append-only Merkle tree](https://www.rgdd.se/post/observations-from-a-trillian-play-date/).
+That Merkle tree is managed by a separate component called a log signer. It runs
+in a dedicated process that basically merges pending leaves that have yet to be
+incorporated into the Merkle tree. This part of the log signer is structured as
+a sequencing job that runs periodically. I spent a few hours learning more about
+the details and thought shared knowledge is better.
+
+## Problem definition and overview
+Trillian’s
+	[log signer](https://github.com/google/trillian/blob/3ae67195ffd778d37275c6972445f4e7f9e21410/cmd/trillian_log_signer/main.go)
+comes with a whole bunch of configuration options that are spread across several
+different files. Some of these options are more difficult to grasp than others,
+such as `num_sequencers`, `sequencer_interval`, and `batch_size`. I don't mean
+difficult as in understanding that there may be several sequencers that run
+periodically, but rather what that actually means in terms of concurrency and
+how much can be sequenced per time unit.
+
+The short answer is as follows:
+1. Regardless of how many sequencers you configure there will be no concurrent
+sequencing of a particular Merkle tree. The number of sequencers is only
+relevant if there are multiple Merkle trees.
+2. The sequencer interval tells you how often the log signer wakes up to do a
+sequencing job. It sequences no more than the configured batch size before going
+back to sleep. If the interval elapsed already there will be no sleep.
+
+In other words, to avoid building up a large backlog you need to consider how
+often the sequencer runs and how much work it is willing to do every time. The
+longer answer is detailed below, and it includes a little bit of additional
+context regarding what you may (not) do with these three parameters. For
+example, the sequencer job is most reliable if it runs often over small batches.
+
+## Log signer
+The most central part of the log signer is probably its forever loop. For
+reference it is implemented by Trillian’s
+	[operation manager](https://github.com/google/trillian/blob/3ae67195ffd778d37275c6972445f4e7f9e21410/log/operation_manager.go#L110),
+see the `OperationLoop` function. In a nutshell, the log signer wakes up,
+performs some jobs, and goes back to sleep. Pretty much a busy working day!
+
+### Coordination
+Before proceeding you need to know that a log signer may manage multiple
+different Merkle trees. It might sound odd at first, but hopefully less so if
+you think about the existence of transparency applications that use
+	[temporal sharding](https://www.digicert.com/dc/blog/scaling-certificate-transparency-logs-temporal-sharding/):
+the split of one transparency log into multiple smaller ones so that the leaves
+are grouped by, say, yearly relevance like 2020 and 2021. This allows parts of
+the log to be retired as soon as they are no longer needed. You can also run
+multiple different log signers to avoid single points of failure. At most one
+log signer is responsible for a particular Merkle tree at a time though. This
+requires coordination, which is one part of the forever loop.
+
+### Sequencing
+The other part is log sequencing. After waking up once per interval as defined
+by the configured `sequencer_interval`, the log signer takes a pass over all
+Merkle trees that it manages. A sequencing job is scheduled for each Merkle tree
+that the log signer is currently the master for. It is determined by an election
+protocol which is used for coordination in the case of multiple log signers,
+otherwise master is assumed as there is nothing to coordinate. Next, these jobs
+are distributed to a number of concurrent sequencers that work on _different
+Merkle trees_. This means that there is no concurrency whatsoever when _a
+particular Merkle tree_ is being sequenced. Moreover,
+	[what is selected for sequencing](https://github.com/google/trillian/blob/3ae67195ffd778d37275c6972445f4e7f9e21410/storage/mysql/queue.go#L33)
+is deterministic based on Trillian’s internal timestamp order and the number of
+leaves that the sequencer is willing to process per job.
+
+In other words, there is no point setting `num_sequencers` higher than the
+number of Merkle trees that you manage. You may set it lower, in which case at
+least one sequencer will move on to a different sequencing job (and thus a
+different Merkle tree) once it is done. Now it is also evident that the
+configured `sequencer_interval` and `batch_size` determines an upper bound for
+the number of writes per time unit. It is an upper bound because your hardware
+might not be capable of handling, say, 10k writes per second.
+
+## Concluding remarks
+When I noticed the `sequencer_interval` parameter I wondered if it could be used
+to configure a tree head frequency, such that the Trillian front-end personality
+would only see a fixed number of updates per time interval. For example, you
+might want that because the
+	[second version of Certificate Transparency requires control over it](https://datatracker.ietf.org/doc/html/draft-ietf-trans-rfc6962-bis-34#section-4.1)
+and some gossip-audit models
+	[assume it](https://tools.ietf.org/html/draft-ietf-trans-gossip-05).
+If not supported by the Trillian back-end, the front-end personality has to take
+on the role. While it is trivial in the case of a single personality instance
+(e.g., pick a tree head every hour), it might require some additional
+coordination if there are multiple concurrent instances running. So, it would be
+convenient if the _already coordinated_ log signer could enforce a tree head
+frequency.
+
+In theory it is of course possible to set the sequencer interval and batch size
+to accommodate both a frequency and an upper bound for writes per second. In
+practice it is apparently recommended to use short intervals and batch sizes of
+up to 1000 leaves. This recommendation involves quite a bit of nuance, and
+relates to things like
+	[which back-end is used for the underlying database](https://github.com/google/trillian/issues/1845).
+If tree head frequencies are generally useful it might land as a back-end
+feature at some point. For now, it is better enforced by the front-end
+personality.
+
+## Acknowledgments
+This story is sponsored by my
+	[System Transparency](https://system-transparency.org/)
+employment at Mullvad VPN.  Martin Hutchinson and Pavel Kalinnikov provided
+valuable insights as part of a conversation in the
+	[Trillian slack](https://gtrillian.slack.com).
+Mistakes, if any, are my own.
diff --git a/content/post/what-happened-at-ct-days-2020.md b/content/post/what-happened-at-ct-days-2020.md
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@@ -0,0 +1,160 @@
+---
+title: "What happened at CT days 2020?"
+date: 2020-09-14
+---
+This year’s
+	[CT days](https://groups.google.com/a/chromium.org/forum/#!topic/ct-policy/JWVVhZTL5RM)
+were hosted remotely on September 8–9. The agenda covered a wide range of
+topics, such as making CT more newcomer friendly, updating user-agent policies,
+and what it takes to operate a log at scale. I do not intend to write about all
+of it, and especially not every little detail. You will be brought up to speed
+on some highlights and get further reading. All credit obviously goes to the
+people who presented sessions on this material.
+
+## New community website
+You might be familiar with the
+	[current CT website](https://web.archive.org/web/20200829193007/https://www.certificate-transparency.org/).
+It is a little bit like an explosion of technical details and links from several
+years back that, if you digest it all, tell you how Google’s CT project works.
+This is not particularly welcoming for newcomers that need to grasp what CT is
+today and how it fits into the broader picture of the web’s public-key
+infrastructure. For example, CT is no longer Google’s own logging project, but
+rather an ecosystem of different people and organizations that come together
+with one mission: to detect maliciously or mistakenly issued certificates. This
+happens to be the first thing you will find when browsing the
+	[new community website](https://certificate.transparency.dev/).
+It is nifty looking, and I encourage you to browse it yourself. You will notice
+that the origin story of CT and its broader context is described, which helps
+the reader pick up the fundamentals from a combination of text and
+visualizations. 
+
+![Working together to detect maliciously issued certificates](/img/ctdev.png)
+**source**: https://certificate.transparency.dev/
+
+[Ryan Hurst](https://unmitigatedrisk.com/) explained that we can think of the
+new community website as a place where newcomers can be directed to understand
+the value of CT and how it works.  Moreover, it is our collective responsibility
+to add anything that is missing and keep it up-to-date. Anyone can
+	[submit pull requests](https://github.com/google/certificate-transparency-community-site)
+on GitHub.
+
+## Policy updates
+I can second that it is not always easy to understand every nuance of CT
+enforcement by different user agents. For example, I remember
+	[filing a bug](https://bugs.chromium.org/p/chromium/issues/detail?id=1053971#c1)
+not too long ago when noticing that Chromium (not to be confused with Google
+Chrome) disabled CT by default. In the future there might be a separate and
+lighter-weight Chromium CT policy that embedders could use as a starting point,
+but for now Google’s policy will be shaped solely for Chrome. This is reflected
+by the new CT policy website that is being drafted: the so-called
+	[Chrome Certificate Transparency Policy](https://googlechrome.github.io/CertificateTransparency/ct_policy.html).
+[Devon O’Brien](https://twitter.com/modyoloN) appropriately described it as a
+complete overhaul of what the current policy states and how these requirements
+are framed specifically for enforcement in Chrome.
+
+The CT community website will link the updated CT policy once it goes live in
+the near future. In the mean time, you can enjoy the draft and provide feedback
+to the
+	[Chrome CT team](mailto:chrome-certificate-transparency@google.com)
+or the
+	[CT policy group](https://groups.google.com/a/chromium.org/g/ct-policy).
+The recommended starting point is the different states that a CT log can be in.
+I am not going to detail it, but it was mentioned as a pro-tip in multiple
+sessions.  You will also notice that informative reference material was added,
+with more to come as the broader community identifies components that are
+unclear or missing.
+
+There are several policy changes in the midst of the above updates. While not
+particularly huge, they are there and worth pointing out. All new CT logs must
+be temporally sharded. A log that is temporally sharded accepts logging of
+certificates that fall into a range of expiry dates. For example, a 2020-shard
+would not accept certificates that expire in 2021. The status of sharding moves
+from allowed to required, solving the issue of logs that grow forever and ever.
+A log must additionally not return multiple different SCTs for a single log
+entry, which solves undefined behavior in
+	[RFC 6962](https://tools.ietf.org/html/rfc6962)
+that could lead to unverifiable SCTs in quirky corner cases. Note that a
+certificate that passes the Google Chrome CT policy is now said to be CT
+compliant. It is analogous to the current wording of a certificate being CT
+qualified, and avoids confusion with CT logs that can also be qualified in the
+browser.
+
+[Clint Wilson](https://twitter.com/clintw_) announced
+	[Apple’s intended policy updates](https://groups.google.com/a/chromium.org/forum/?oldui=1#!topic/ct-policy/JWVVhZTL5RM).
+They are also working towards definitions in undefined scenarios, such as
+expressing their policy in terms of days as opposed to months. The most notable
+policy update (in my opinion) is that the SCT diversity and quantity assumption
+for a CT compliant certificate is about to change. Currently, Apple considers a
+certificate CT compliant if accompanied by two SCTs from any pair of CT logs.
+Longer-lived certificates will require an additional SCT in the future, and log
+operator diversity will be added as well. This reduces the risk that a once CT
+compliant certificate will become non-compliant during its life time. The other
+benefit is that no single log operator can issue a certificate’s SCTs, which
+raises the bar towards unnoticed certificate mis-issuance. What it means that
+two log operators are diverse (also known as independent) is somewhat
+non-trivial: it can span many different dimensions, such as organization,
+country, and infrastructural providers. Google’s
+	[updated policy for log operators](https://googlechrome.github.io/CertificateTransparency/log_policy.html)
+will require a self-assertion that you are independent of all other log
+operators. My best guess is that Apple will rely on something similar to define
+log diversity.
+
+## Removing the one-Google log requirement?
+Google Chrome currently considers a certificate CT compliant if it is
+accompanied by two SCTs. One of these SCTs must additionally be issued by a log
+that Google runs. If Google’s CT logs are operated in good faith, we can be sure
+that there are no mis-issued certificates that go unnoticed.
+
+To state the obvious, the point of CT is not to trust that Google keeps the
+wider web safe from certificate mis-issuance. The current set-up is in fact
+quite error-prone: sub-optimal trust assumptions aside, a Google outage would
+essentially disable issuance of new CT compliant certificates on the web. Devon
+O’Brien expressed a desire to remove Google from the critical path of
+certificate issuance, and what considerations that need to go into such a
+decision. The major part is that Google must accept that they lose their
+privilege of observing all issued certificates up-front, which provides
+significant proactive security for Chrome users but might impend broader
+user-agent adoption. The result of such a change is that SCTs must be audited
+reactively in the background from many diverse vantage points, such that
+mis-issued certificates get noticed without the one-Google log policy.
+
+[Chris Thompson](https://notyetsecure.com/) presented Google’s plans on
+	[opt-in SCT auditing](https://docs.google.com/document/d/1G1Jy8LJgSqJ-B673GnTYIG4b7XRw2ZLtvvSlrqFcl4A/edit).
+The basic idea is to submit a random subset of SCTs to a Google-operated CT
+auditor. If an SCT is encountered that Google does not know about, that can be
+investigated further by challenging the issuing CT logs to prove certificate
+inclusion. The reason why this requires opt-in stems from the fact that the user
+is essentially sharing a random subset of its browsing history with Google. You
+might wonder who would opt-in for that, but it actually fits pretty well into
+the existing Safe Browsing Extended Reporting (SBER) programme. My largest
+concern is that opted-in users might be identifiable, and in that case the rest
+of us could still be attacked without high likelihood of detection.
+
+[Sarah Meiklejohn](https://smeiklej.com/) presented a follow-up session on
+privacy-preserving SCT auditing. It could be described as partial highlights of
+a systematic literature study that considered 15 different proposals against the
+following criteria:
+
+1. **Functionality**.  Does it work and in what threat model?
+2. **Privacy**.  What information do which parties learn?
+3. **Client-side performance**.  Bandwidth, computation, and storage?
+4. **Latency**.  How much is added, if any?
+5. **Server-side infrastructure costs**.  What needs to be changed or added?
+6. **Threat model**.  Mainly in terms of which parties need to trust each other.
+7. **Non-Google deployability**.  Can it be deployed without Google-scale?
+8. **Near-term deployability**.  Can we roll it out sooner rather than later?
+
+A subtle message is that a proposal without a third-party CT auditor is
+incomplete. I share this view because we cannot expect an end-user to take any
+reasonable action if log misbehavior is suspected. Therefore, it is not just
+proof fetching that needs to be private: also the process of reporting issues.
+
+## Acknowledgments
+Thanks to everyone that contributed to CT days 2020, both in terms of
+organization and putting in the actual work that the different sessions
+presented. A detailed summary and follow-up discussion might appear on the
+	[CT policy list](https://groups.google.com/a/chromium.org/forum/#!forum/ct-policy).
+Fredrik Strömberg provided valuable feedback on this story, which is sponsored
+by my
+	[System Transparency](https://www.system-transparency.org/)
+employment at Mullvad VPN.
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