Although it’s tempting to attempt to set up a single commonplace for how one can use SLSA, it’s not doable: SLSA shouldn’t be a line dance the place everybody does the identical strikes, on the similar time, to the identical music. It’s a various system with totally different types, strikes, and prospers. The open supply group, organizations, and shoppers might all implement SLSA in another way, however they will nonetheless work with one another.
On this three-part sequence, we’ll discover how three fictional organizations would apply SLSA to satisfy their totally different wants. In doing so, we’ll reply a few of the fundamental questions that newcomers to SLSA have:
Half 1: The fundamentals
- How and when do you confirm a bundle with SLSA?
- How you can deal with artifacts with out provenance?
Half 2: The small print
- The place is the provenance saved?
- The place is the suitable coverage saved and who ought to confirm it?
- What ought to the insurance policies verify?
- How do you determine belief & distribute keys?
Half 3: Placing all of it collectively
- What does a safe, heterogeneous provide chain seem like?
Our fictional instance includes three organizations that need to use SLSA:
Squirrel: a bundle supervisor with a lot of builders and customers
Oppy: an open supply working system with an enterprise distribution
Acme: a mid sized enterprise.
Squirrel needs to make SLSA as simple for his or her customers as doable, even when meaning abstracting some particulars away. In the meantime, Oppy doesn’t need to summary something away from their customers beneath the philosophy that they need to explicitly perceive precisely what they’re consuming.
Acme is making an attempt to provide a container picture that accommodates three artifacts:
- The Squirrel bundle ‘foo’
- The Oppy bundle ‘baz’
- A customized executable, ‘bar’, written by Acme staff
This sequence demonstrates one strategy to utilizing SLSA that lets Acme confirm the Squirrel and Oppy packages ‘foo’ and ‘baz’ and its clients confirm the container picture. Although not each recommended answer is ideal, the options described is usually a start line for dialogue and a basis for brand new options.
Squirrel, Oppy, and Acme will comply with an analogous qualification course of for the supply management techniques they plan to help.
Sooner or later, a number of of the organizations might want to do full verification of every artifact to find out whether it is acceptable for a given use case. That is completed by checking if the artifact meets the suitable coverage.
Usually, full verification would happen with SLSA provenance, supply attestations, and maybe different specialised attestations (like vulnerability scan outcomes). Whereas having to coordinate this information for all of its dependencies looks like a number of work to Acme, they’re ready to do full verification if Squirrel and Oppy are unable to.
When Acme isn’t utilizing full verification, they will as an alternative use delegated verification the place they verify if an artifact is suitable for a use case by checking if another trusted occasion who carried out a full verification (resembling Squirrel or Oppy) believes the artifact is suitable.
Delegated verification is simpler to carry out rapidly with restricted information and community connectivity. It could even be simpler for some customers who worth if somebody they belief verified the artifact is nice.
Squirrel likes how simple delegated verification would make issues for his or her customers and plans to help it by making a Verification Abstract Attestation (VSA) after they carry out full verification.
Verification (full or delegated) might occur at quite a few totally different occasions.
Squirrel plans to carry out full verification when an artifact is printed to their repo. It will make sure that packages within the repo have met their corresponding coverage. It’s additionally useful as a result of all of the required information will be gathered when latency isn’t crucial.
If this had been the one time verification is carried out, it could put the repository’s storage within the trusted computing base (TCB) of its customers. Squirrel’s plans to make use of delegated verification (and challenge VSAs) can stop this. The signature on the VSA will stop the artifacts from being tampered with whereas sitting in storage, even when they’re simply SLSA 0. Downstream customers will simply must confirm the VSA.
Acme additionally needs to do some kind of verification on the import to their inside repo because it simplifies their safety story. They’re not fairly certain what this may seem like but.
Acme additionally needs to do verification when an artifact is definitely put in since it may possibly take away quite a few intermediaries from their TCB (their repo, the community, upstream storage techniques).
In the event that they carry out full verification at set up then they have to collect all of the required data. That might be a number of information, however it is likely to be simplified by gathering the information from exterior sources and caching it of their inside repo. A bigger drawback is that it requires Acme to have established belief in all events that produced that data (e.g. each builder of each bundle). For a posh provide chain which may be troublesome.
If Acme performs delegated verification, they solely want the VSA for the packages being put in and to explicitly belief a handful of events. This permits the advanced full verification to be carried out as soon as whereas permitting all customers of that bundle to carry out a a lot easier operation.
Given these tradeoffs Acme prefers delegated verification at set up time. Squirrel additionally actually likes the concept and plans to construct set up time verification immediately into the Squirrel instrument.
On use
Time of use verification permits probably the most context with which choices will be made (“is that this job allowed to run this code and is it free from vulns proper now?”). It additionally permits coverage modifications to have an effect on already constructed & put in software program (which can or is probably not fascinating).
Acme needs their customers to have the ability to confirm on use with out too many dependencies so that they plan to offer VSAs customers can use to carry out delegated verification after they begin the container (maybe utilizing one thing like Kyverno).
Such an artifact would possible not be accepted at increased SLSA ranges, however the provenance can be utilized to: 1) stop tampering with the artifact after it’s been imported and a couple of) be a knowledge level for future evaluation (e.g. ought to we prioritize asking for foo.tgz to be distributed with native SLSA provenance?).
Acme is likely to be inquisitive about taking this strategy in some unspecified time in the future, however they don’t want it in the mean time.
In our subsequent publish we’ll cowl particular approaches that can be utilized to reply questions like “the place ought to attestations and insurance policies be saved?” and “how do I belief the attestations that I obtain?”
