As an adopter of SLSA, you have likely encountered the in-toto project. in-toto attestations are part of SLSA’s recommended suite for expressing software supply chain claims. As in-toto maintainers, we’ve interacted with a number of people who know of in-toto through SLSA but don’t fully understand the project. For example, some were surprised to learn that “in-toto isn’t just a format of attestations”, and that the framework also defines verification workflows that make use of attestations that are not SLSA Provenance. So, we decided to author this post as a quick primer on in-toto, how SLSA uses in-toto, and how other attestations can be used to complement SLSA.
in-toto Primer
The in-toto Attestation Framework defines a fixed, lightweight “Statement” that communicates some information about the execution of a software supply chain, such as if the source was reviewed, or if it went through a SLSA conformant build process. The information is communicated as a “Predicate” using a context-specific schema identified by a “Predicate Type”, and applies to one or more software supply chain “Subjects”. SLSA Provenance is one such predicate type.
in-toto models the software supply chain as a series of “steps”. Supply chain owners, i.e. project owners or maintainers, define the set of actors authorized to perform each step and rules that govern the artifacts used and produced by the step. They can also include other arbitrary checks called “inspections” to be performed during the verification workflow. These policies are recorded in a piece of metadata called a supply chain “Layout”, which is cryptographically signed by the supply chain owners. As each step is performed, one or more attestations are generated recording various aspects of the process, and during verification, in-toto applies the policies in the layout against the set of attestations produced during the execution of the supply chain.
What SLSA gets from in-toto?
SLSA v1 is focused on the build aspects of the software supply chain. It defines the Provenance predicate to record the build characteristics of the produced artifacts. The specification describes how to verify Provenance as well as lays out other requirements that are to be verified separately. The SLSA community has also indicated a future source track focused on the security posture of how source is stored and developed prior to being fed into a build process.
This is where other in-toto predicate types can complement SLSA. in-toto allows for defining use case specific predicates that contain contextual information. Apart from SLSA Provenance, the framework currently lists definitions for in-toto Links, results of runtime traces, Software Bill of Materials (SBOM) specifications such as SPDX and CycloneDX, the Software Supply Chain Attribute Integrity (SCAI) specification, and SLSA Verification Summary Attestations. in-toto also has a process for members of the community to propose new predicate types. Proposals are reviewed by the maintainers of the project who are supply chain security stakeholders at Google, Verizon, Intel, Kusari, and TestifySec. This ensures that predicate types are of high quality, useful to different organizations and their specific use cases, and, most importantly, consistent so that attestations are usable beyond organization boundaries.
With the upcoming enhancements to in-toto layouts, supply chain owners can define granular policies for all of the different predicates they generate during the execution of their software supply chain. For example, they can embed SLSA Provenance specific checks alongside others that verify best practices such as two party code reviews, successful test runs, and so on were adhered to. Taken together, a bundle of in-toto attestations and a layout can be used to comprehensively verify a software supply chain’s security posture.
End-to-end attestations for your software supply chain
Let’s take a look at how other in-toto attestations can complement SLSA Provenance. Here are a series of attestations (truncated where necessary) for the execution of a supply chain. For the sake of this example, the attestations are presented without their outer signing layer; in the real world, they’d all be signed by the actors performing the corresponding operations.
Check out the source code
First, the source repository is checked out and its Git commit is recorded using an in-toto link.
{
"_type": "https://in-toto.io/Statement/v1",
"subject": [{
"name": "foo",
"digest": {
"gitCommit": "abcdef123456"
}
}],
"predicateType": "https://in-toto.io/attestation/link/v0.3",
"predicate": {
"name": "checkout",
"command": "git clone https://git.example.com/foo.git",
"materials": {},
"byproducts": {},
"environment": {}
}
}
Is the code reviewed?
The attestation bundle includes a code review for the commit that was checked out.
{
"_type": "https://in-toto.io/Statement/v1",
"subject": [{
"name": "foo",
"digest": {
"gitCommit": "abcdef123456"
}
}],
"predicateType": "https://in-toto.io/attestation/human-review/v0.1",
"predicate": {
"result": "PASSED",
"reviewLink": "https://cr.example.com/foo/abcdef123456",
"timestamp": "2023-04-02T12:34:23Z"
}
}
Run tests
Prior to actually executing the build, the source’s tests are run. This test run and its results are recorded as a standalone attestation.
{
"_type": "https://in-toto.io/Statement/v1",
"subject": [{
"name": "foo",
"digest": {
"gitCommit": "abcdef123456"
}
}],
"predicateType": "https://in-toto.io/attestation/test-result/v0.1",
"predicate": {
"result": "PASSED",
"configuration": [{
"name": "foo/test.yml",
"digest": {
"gitBlob": "123456abcd"
}
}],
"url": "https://ci.example.com/23048",
"passedTests": ["(test.yml, ubuntu-latest)", "(test.yml, windows-latest)"],
"warnedTests": [],
"failedTests": []
}
}
Build the final product
The build service performs the actual build itself and records SLSA Provenance of the resulting artifact.
{
"_type": "https://in-toto.io/Statement/v1",
"subject": [{
"name": "bin/foo-v1.1",
"digest": {
"sha256": "ff332109abdd"
}
}],
"predicateType": "https://slsa.dev/provenance/v1",
"predicate": {
"buildDefinition": {
"buildType": "<buildType>",
"externalParameters": {
"repository": "https://git.example.com/foo.git",
"ref": "refs/heads/main"
},
"resolvedDependencies": [{
"name": "foo",
"uri": "git+https://git.example.com/foo@refs/heads/main",
"digest": {
"gitCommit": "abcdef123456"
}
}],
},
"runDetails": {
"builder": {
"id": "https://ci.example.com/",
},
}
}
}
Runtime trace results of the build
In addition to recording provenance, the bundle includes the results of a runtime trace of the build process.
{
"_type": "https://in-toto.io/Statement/v1",
"subject": [{
"name": "bin/foo-v1.1",
"digest": {
"sha256": "ff332109abdd"
}
}],
"predicateType": "https://in-toto.io/attestation/runtime-trace/v0.1",
"predicate": {
"monitor": {
"type": "https://github.com/cilium/tetragon/v0.8.4",
"tracePolicy": {
"policies": [{
"name": "connect",
"config": ""
}]
}
},
"monitoredProcess": {
"hostID": "https://ci.example.com",
"type": "custom",
"event": "https://ci.example.com/23055"
},
"monitorLog": {
"process": [],
"network": [],
"fileAccess": [{
"name": "foo/Makefile",
"digest": {
"sha256": "1209abcd56"
}
}, ...]
}
}
}
Build compiler configuration attributes
Finally, the bundle has a Software Supply Chain Attribute Integrity (SCAI) attestation that records some attributes of the build tool. Specifically, the attestation indicates the build step used GCC with stack protection enabled.
{
"_type": "https://in-toto.io/Statement/v1",
"subject": [{
"name": "bin/foo-v1.1",
"digest": { "sha256": "ff332109abdd" }
}],
"predicateType": "https://in-toto.io/attestation/scai/attribute-report/v0.2",
"predicate": {
"attributes": [{
"attribute": "WITH_STACK_PROTECTION",
"conditions": { "flags": "-fstack-protector*" },
}],
"producer": {
"uri": "file:///usr/bin/gcc",
"name": "gcc9.3.0",
"digest": {
"sha256": "78ab6a8..."
}
}
}
}
Verification
This set of metadata can be submitted to in-toto’s verification workflow using a layout conforming to the new, under-development schema. Successful verification would result in a Verification Summary Attestation.
{
"_type": "https://in-toto.io/Statement/v1",
"subject": [{
"name": "foo-v1.1",
"digest": {
"sha256": "ff332109abdd"
}
}],
"predicateType": "https://slsa.dev/verification_summary/v1",
"predicate": {
"verifier": {
"id": "https://example.com/publication_verifier"
},
"timeVerified": "2023-04-12T23:20:50.52Z",
"resourceUri": "https://download.example.com/foo-v1.1",
"policy": {
"uri": "https://example.com/foo.layout",
"digest": {"sha256": "dabc8907fa"}
},
"inputAttestations": [{
"uri": "https://example.com/foo-v1.1.intoto.jsonl",
"digest": {"sha256": "7843adcf34"}
}],
"verificationResult": "PASSED",
"verifiedLevels": ["SLSA_LEVEL_3"],
"slsaVersion": "1.0"
}
}
Get Involved
There are a number of exciting developments underway in in-toto such as the new layouts, several new predicate specifications, and improvements to our tooling. Come say hello at an in-toto community meeting and on #in-toto on the CNCF Slack workspace.