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@@ -1,61 +1,91 @@ # System Transparency Front-End (STFE) -STFE is a Trillian personality that allows you to log signed artifact checksums. -A client consuming artifacts (such as a browser update, a Debian package, a document, -or more generally anything opaque) may enforce that such logging takes place by -mandating that there is a public trace of each artifact before even considering -to trust it. We refer to such a trace as a signed checksum entry: it is -composed of - an arbitrary identifier like `stfe_client v0.0.1`, - a checksum, - a signature, and - a namespace that is derived from the public verification key. -Tracking checksums as opposed to full-on artifacts makes it less costly to -operate, monitor, and audit the log. Because these checksum entries are signed -we can: +STFE is a [Trillian](https://transparency.dev/#trillian) +[personality](https://github.com/google/trillian/blob/master/docs/Personalities.md) +that allows you to log signed checksums. What a checksum covers is up to the +submitter. For example, it could be a Firefox update, a Debian package, or a +document. A log leaf contains: +- A _checksum_ that covers something opaque, e.g., an executable binary. +- An _identifier_ that is tied to what the checksum represents, e.g., name, +version, and platform. +- A _namespace_ that is tied to the submitters verification key, e.g., think of +it as a hashed public key. +- A _signature_ that covers `checksum` and `identifier` using the submitter's +secret signing key. + +The log verifies that the entry is signed for the specified namespace but +nothing more than that. A client that wishes to enforce transparency logging +could require that, say, a valid Debian package is only used if its checksum +appeared in the log with a correct identifier and namespace. Such a use-case +scenario enables us to: 1. **Facilitate detection of compromised signing keys**, e.g., a software -publisher can inspect the log to see if there are any unexpected artifact -checksums in their namespace. -2. **Ensure that everyone observes the same artifacts**, e.g., there should -never be two signed checksum entries with identical identifiers and namespaces -but different checksums. - -The scope of STFE should not be confused with properties such as _prevention_ or -even _recovery_ after detection. - -## Gossip-audit model -We glanced over the term _public trace_ a bit to quickly before. Simply adding -something into a transparency log serves a limited purpose unless (i) clients -_fail-close_ if an artifact does not appear in a log, and (ii) everyone observes -the same consistent transparency logs; meaning append-only, and that you and I -both get the same entries and cryptographic proofs when consuming the logs. The -first criteria requires several independent logs, such that the log ecosystem is -reliable enough. The second criteria is often overlooked and requires a -gossip-audit model. Therefore, we decided to build witness cosigning directly -into STFE. - -The idea is that many independent witnesses _cosign_ the log's signed tree head -(STH) if and only if they see a consistent append-only log. If enough reputable -parties signed-off the log's cryptographic state, you can be pretty sure that -you see the same log (and thus the same artifacts) as everyone else. Moreover, -if you already rely on witness cosigning for security, all you need from your -software publisher is an artifact, a public verification key, a cosigned -STH, and an inclusion proof that is based on it. That is excellent because -client-side verification becomes completely non-interactive! +publisher can inspect the log to see if there are any unexpected checksums in +their own signing namespace(s). +2. **Ensure that everyone observe the same checksums**, e.g., there should never +be two log entries with identical identifiers and namespaces but different +checksums. ## What has been done? -STFE is in a proof-of-concept stage. We have a +STFE is at the proof-of-concept stage. We have a [sketch](https://github.com/system-transparency/stfe/blob/main/doc/sketch.md) of the log's API, which basically defines data structures, data formats, and HTTP(S) endpoints. Be warned that it is a living design document that may be incomplete and subject to major revisions. For example, we are currently thinking about data formats and which parsers are reasonable to (not) force onto -the client-side tooling. +client-side tooling as well as server-side implementers and operators. + +There is a basic client (warning: _basic_) that can be used to interact with the +log, e.g., to add-entries and verify inclusion proofs against an STH. We have +yet to add client-side support for STFE's witness cosigning APIs. Witness +cosigning is part of the log's _gossip-audit model_, which must be well-defined +to keep the log honest.[^1] In the near future we will setup a public STFE prototype with zero promises of uptime, stability, etc. In the meantime you may get your hands dirty by running things locally. Rough documentation is available [here](https://github.com/system-transparency/stfe/blob/main/server/README.md). -There is a basic client (warning: _basic_) that can be used to interact with the -log, e.g., to add-entries and verify inclusion proofs against an STH. We have -yet to add client-side support for STFE's witness cosigning APIs. +## What should be considered? +Simply adding something into a transparency log is a great start that has merit +on its own. But, to make the most of a transparency log we should keep the +following factors in mind as the ecosystem bootstraps and develops: +1. Client-side tooling should ultimately fail-close if a signed checksum is not +transparency logged. This requires a reliable and available log ecosystem. +It is easier to achieve if there are multiple independent log operators. +2. Client-side tooling should verify that the signed checksums appeared in the +log. This requires inclusion proof verification. STFE forces it by never +issuing so-called _promises to log_ as in [Certificate +Transparency](https://tools.ietf.org/html/rfc6962).[^2] +3. Client-side tooling should verify that the log is append-only. This requires +consistency proof verification. +4. Client-side tooling should be convinced that they see the same append-only +log as everyone else. This requires a well-defined gossip-audit model. + +The final factor is often overlooked. While transparency logs are verifiable in +theory due to inclusion and consistency proofs, _it is paramount that the +different parties interacting with the log see the same entries and +cryptographic proofs_. Therefore, we built a proactive gossip-audit model +directly into STFE: _witness cosigning_.[^3] The idea is that many independent +witnesses _cosign_ the log's STH if and only if they see a consistent +append-only log. If enough reputable parties signed-off the log's cryptographic +state, you can be pretty sure that you see the same log (and thus the same +checksums) as everyone else. + +Moreover, if you already rely on witness cosigning for security, all you need +from, say, a software publisher, is an artifact, a public verification key, a +cosigned STH, and an inclusion proof that is based on it. That is excellent +because client-side verification becomes completely non-interactive! + +[^1]: The academic literature documents this challenge quite well with regards +to Certificate Transparency. See, for example, the work of +[Chuat _et al._](https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7346853), +[Nordberg _et al._](https://tools.ietf.org/html/draft-ietf-trans-gossip-05), and +[Dahlberg et al.](https://sciendo.com/article/10.2478/popets-2021-0024). +[^2]: So-called SCTs are signed promises that the log will merge a submitted +entry within a Maximum Merge Delay (MMD), e.g., 24 hours. It is a significant +system complexity because either the client needs to verify that these promises +were honored after the MMD passed, or the client needs to trust that the logs +are honest. +[^3]: Witness cosigning was initially proposed by [Syta _et al._](https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7546521). The +approach of [Meiklejohn _et al._](https://arxiv.org/pdf/2011.04551.pdf) is +closer to ours but the details differ. For example, witnesses poll STFE for +STHs rather than waiting for a single broadcast. |