content: draft: define how downstream users can verify the SLSA sourc…

…e track level of revisions (#1094)

fixes #1071
fixes #1042
refs #241

This PR modifies _draft_ content of the SLSA spec.

## Context

See [discussions
here](https://docs.google.com/document/d/13Xt8mA_2b00McGX2vkyhu4GQdFAqtXPu7YXE8ZA6ISE/edit?resourcekey=0-EqfHF79tUWAKp4PzsE3z1A&tab=t.0#heading=h.fhg4lsemfpz2)
[and
here](https://docs.google.com/document/d/1PwhekVB1iDpcgCQRNVN_aesoVdOiTruoebCs896aGxw/edit#bookmark=id.oqoqjt4urxm).

Google document requires
[slsa-discussion@googlegroups.com](mailto:slsa-discussion@googlegroups.com)
membership.

## VSA for source

Define how downstream users can verify the SLSA source track level of
revisions by using a [VSAs](http://slsa.dev/verification_summary)
produced by the Source Control Platform (SCP).

To use these VSAs users do not need to know the specifics of how any
given SCP or Version Control System (VCS) meets the SLSA source
requirements (which may vary greatly from implementation to
implementation). Instead it is left to the SCP or another trusted
'authority' to make that determination for downstream users.

The question of _how_ the authority ensures those claims to be true is
left undefined in this change.

Future updates can include guidance for how to verify source level when
combined with [build provenance](https://slsa.dev/provenance).

## Example scenario

1. A user wants to verify
9a04d1e
is SLSA source level 3.
2. The user 'trusts' GitHub as the authority for source revisions
managed by GitHub.
3. The user requests a VSA for
9a04d1e
from a TBD API
4. The user verifies the VSA following [the standard
instructions](https://slsa.dev/spec/draft/verification_summary#how-to-verify)
or using [standard
tooling](https://github.com/slsa-framework/slsa-verifier?tab=readme-ov-file#verification-summary-attestations-vsa)
and looking for `SLSA_SOURCE_LEVEL_2` in the `verifiedLevels` field.

---------

Signed-off-by: Tom Hennen <tomhennen@google.com>
Signed-off-by: Tom Hennen <TomHennen@users.noreply.github.com>
Co-authored-by: Zachariah Cox <zachariahcox@github.com>
Co-authored-by: Aditya Sirish <8928778+adityasaky@users.noreply.github.com>
Co-authored-by: Marcela Melara <marcela.melara@intel.com>

docs/spec/draft/source-requirements.md

@@ -8,6 +8,8 @@ Open issues are tracked with the [source-track](https://github.com/slsa-framewor
 -   [] [How to communicate SLSA source track metadata?](https://github.com/slsa-framework/slsa/issues/1071)
 -   [] [Create guidance for consumers on how to evaluate the source platform](https://github.com/slsa-framework/slsa/issues/1078)
 -   [] [Clarify what must be retained during source migrations](https://github.com/slsa-framework/slsa/issues/1079)
+-   [] [Define source control 'system'](https://github.com/slsa-framework/slsa/issues/1128)
+-   [] [Define where source attestations are stored](https://github.com/slsa-framework/slsa/issues/1129)
 
 ## Objective
 
@@ -237,3 +239,120 @@ Branches may have differing security postures, and a change can be approved for
 
 The change management tool MUST record timestamps for all contributions and review-related activities.
 User-provided timestamps MUST NOT be used.
+
+## Communicating source levels
+
+SLSA source level details are communicated using attestations.
+These attestations either refer to a source revision itself or provide context needed to evaluate an attestation that _does_ refer to a revision.
+
+There are two broad categories of source attestations within the source track:
+
+1.  Summary attestations: Used to communicate to downstream users what high level security properties a given source revision meets.
+2.  Provenance attestations: Provide trustworthy, tamper-proof, metadata with the necessary information to determine what high level security properties a given source revision has.
+
+To provide interoperability and ensure ease of use, it's essential that the summary attestations are applicable across all Source Control Platforms.
+Due to the significant differences in how SCPs operate and how they may chose to meet the Source Track requirements it is preferable to
+allow for flexibility with the full attestations.  To that end SLSA leaves provenance attestations undefined and up to the SCPs to determine
+what works best in their environment.
+
+### Summary attestation
+
+Summary attestations are issued by some authority that has sufficient evidence to make the determination of a given
+revision's source level.  Summary attestations convey properties about the revision as a whole and summarize properties computed over all
+the changes that contributed to that revision over its history.
+
+The source track issues summary attestations using [Verification Summary Attestations (VSAs)](./verification_summary.md) as follows:
+
+1.  `subject.uri` SHOULD be set to a human readable URI of the revision.
+2.  `subject.digest` MUST include the revision identifier (e.g. `gitCommit`) and MAY include other digests over the contents of the revision (e.g. `gitTree`, `dirHash`, etc...).
+SCPs that do not use cryptographic digests MUST define a canonical type that is used to identify immutable revisions (e.g. `svn_revision_id`)[^1].
+3.  `subject.annotations.source_branches` SHOULD be set to a list of branches that pointed to this revision at any point in their history.
+    -   git branches MUST be fully qualified (e.g. `refs/head/main`) to reduce the likelyhood of confusing downstream tooling.
+4.  `resourceUri` MUST be set to the URI of the repository, preferably using [SPDX Download Location](https://spdx.github.io/spdx-spec/v2.3/package-information/#77-package-download-location-field).
+E.g. `git+https://github.com/foo/hello-world`.
+5.  `verifiedLevels` MUST include the SLSA source track level the verifier asserts the revision meets. One of `SLSA_SOURCE_LEVEL_0`, `SLSA_SOURCE_LEVEL_1`, `SLSA_SOURCE_LEVEL_2`, `SLSA_SOURCE_LEVEL_3`.
+MAY include additional properties as asserted by the verifier.  The verifier MUST include _only_ the highest SLSA source level met by the revision.
+6.  `dependencyLevels` MAY be empty as source revisions are typically terminal nodes in a supply chain.
+
+Verifiers MAY issue these attestations based on their understanding of the underlying system (e.g. based on design docs, security reviews, etc...),
+but at SLSA Source Level 3 MUST use tamper-proof [provenance attestations](#provenance-attestations) appropriate to their SCP when making the assessment.
+
+The SLSA source track MAY create additional tags to include in `verifiedLevels` which attest
+to other properties of a revision (e.g. if it was code reviewed).  All SLSA source tags will start with `SLSA_SOURCE_`.
+
+#### Populating source_branches
+
+The summary attestation issuer may choose to populate `source_branches` in any way they wish.
+Downstream users are expected to be familiar with the method used by the issuer.
+
+Example implementations:
+
+-   Issue a new VSA for each merged Pull Request and add the destination branch to `source_branches`.
+-   Issue a new VSA each time a 'consumable branch' is updated to point to a new revision.
+
+#### Example
+
+```json
+"_type": "https://in-toto.io/Statement/v1",
+"subject": [{
+  "uri": "https://github.com/foo/hello-world/commit/9a04d1ee393b5be2773b1ce204f61fe0fd02366a",
+  "digest": {"gitCommit": "9a04d1ee393b5be2773b1ce204f61fe0fd02366a"},
+  "annotations": {"source_branches": ["refs/heads/main", "refs/heads/release_1.0"]}
+}],
+
+"predicateType": "https://slsa.dev/verification_summary/v1",
+"predicate": {
+  "verifier": {
+    "id": "https://example.com/source_verifier",
+  },
+  "timeVerified": "1985-04-12T23:20:50.52Z",
+  "resourceUri": "git+https://github.com/foo/hello-world",
+  "policy": {
+    "uri": "https://example.com/slsa_source.policy",
+  },
+  "verificationResult": "PASSED",
+  "verifiedLevels": ["SLSA_SOURCE_LEVEL_3"],
+}
+```
+
+#### How to verify
+
+-   VSAs for source revisions MUST follow [the standard method of VSA verification](./verification_summary.md#how-to-verify).
+-   Users SHOULD check that an allowed branch is listed in `subject.annotations.source_branches` to ensure the revision is from an appropriate context within the repository.
+-   Users SHOULD check that the expected `SLSA_SOURCE_LEVEL_` is listed within `verifiedLevels`.
+-   Users MUST ignore any unrecognized values in `verifiedLevels`.
+
+### Provenance attestations
+
+Source provenance attestations provide tamper-proof evidence (ideally signed [in-toto attestations](https://github.com/in-toto/attestation/blob/main/README.md))
+that can be used to determine what SLSA Source Level or other high level properties a given revision meets.
+This evidence can be used by an authority as the basis for issuing a [Summary Attestation](#summary-attestation).
+
+SCPs and VCSes may have different methods of operating that necessitate different forms of evidence.
+E.g. GitHub-based workflows may need different evidence than Gerrit-based workflows, which would both likely be different from workflows that
+operate over Subversion repositories.
+
+These differences also mean that depending on the SCP and the repo's configuration the issuers of
+provenance attestations may vary from implementation to implementation, often because entities with
+the knowledge to issue them may vary.  The authority that issues
+[summary-attestations](#summary-attestation) MUST understand which entity should issue each provenance
+attestation type and ensure the full attestations come from the appropriate issuer.
+
+'Source provenance attestations' is a generic term used to refer to any type of attestation that provides
+evidence the process used to create a revision.
+
+Example source provenance attestations:
+
+-   A TBD attestation which describes the revision's parents and the actors involved in creating this revision.
+-   A "code review" attestation which describes the basics of any code review that took place.
+-   An "authentication" attestation which describes how the actors involved in any revision were authenticated.
+-   A [Vuln Scan attestation](https://github.com/in-toto/attestation/blob/main/spec/predicates/vuln.md)
+    which describes the results of a vulnerability scan over the contents of the revision.
+-   A [Test Results attestation](https://github.com/in-toto/attestation/blob/main/spec/predicates/test-result.md)
+ which describes the results of any tests run on the revision.
+-   An [SPDX attestation](https://github.com/in-toto/attestation/blob/main/spec/predicates/spdx.md)
+ which provides a software bill of materials for the revision.
+-   A [SCAI attestation](https://github.com/in-toto/attestation/blob/main/spec/predicates/scai.md) used to
+ describe which source quality tools were run on the revision.
+
+[^1]: in-toto attestations allow non-cryptographic digest types: https://github.com/in-toto/attestation/blob/main/spec/v1/digest_set.md#supported-algorithms.