document the new feature

Signed-off-by: Ramon Petgrave <[email protected]>

docs/API-Library.md

@@ -0,0 +1,68 @@
+# Api Library
+
+## Verifers
+
+We have exported functions for using slsa-verifier within your own golang packages
+
+- slsa-verifier/verifiers/verifier.go
+
+### Npmjs
+
+With `VerifyNpmPackageWithSigstoreTufClient`, you can pass in your own Tuf client with custom options.
+For example, use the embedded Tuf root with `sigstoreTuf.DefaultOptions().WithForceCache()`.
+
+Example:
+
+```go
+package main
+
+import (
+	"context"
+	"fmt"
+
+	sigstoreTuf "github.com/sigstore/sigstore-go/pkg/tuf"
+	options "github.com/slsa-framework/slsa-verifier/v2/options"
+	apiVerify "github.com/slsa-framework/slsa-verifier/v2/verifiers"
+	apiUtils "github.com/slsa-framework/slsa-verifier/v2/verifiers/utils"
+)
+
+func main() {
+	doVerify()
+}
+
+func doVerify() (*apiUtils.TrustedBuilderID, error) {
+	packageVersion := "0.1.127"
+	packageName := "@ianlewis/actions-test"
+	builderId := "https://github.com/slsa-framework/slsa-github-generator/.github/workflows/builder_nodejs_slsa3.yml"
+	attestations := []byte(`{"attestations":[{"predicateType":"https://giEntries":[{"logIndex":"2035" ... `)
+	tarballHash := "ab786dbef723164a605e55ff0ebe83f8e879159bd411980d4423c9b1646b858a537b4bc4d494fc8f71195db715e5c5e9ab4b8809f8b1b399cd30ac053d180ba7"
+	provenanceOpts := &options.ProvenanceOpts{
+		ExpectedSourceURI:      "github.com/ianlewis/actions-test",
+		ExpectedDigest:         "ab786dbef723164a605e55ff0ebe83f8e879159bd411980d4423c9b1646b858a537b4bc4d494fc8f71195db715e5c5e9ab4b8809f8b1b399cd30ac053d180ba7",
+		ExpectedPackageName:    &packageName,
+		ExpectedPackageVersion: &packageVersion,
+	}
+	builderOpts := &options.BuilderOpts{
+		ExpectedID: &builderId,
+	}
+	// example: force using the embedded root, without going online for a refresh
+	// opts := sigstoreTuf.DefaultOptions().WithForceCache()
+	// example: supply your own root
+	// opts := sigstoreTuf.DefaultOptions().WithRoot([]byte(`{"signed":{"_type":"root","spec_version":"1.0","version":9,"expires":"2024-09-12T06:53:10Z","keys":{"1e1d65ce98b10 ...`)).WithForceCache()
+	opts := sigstoreTuf.DefaultOptions()
+	client, err := sigstoreTuf.New(opts)
+	if err != nil {
+		fmt.Printf("creating SigstoreTuf client: %v", err)
+		return nil, fmt.Errorf("creating SigstoreTuf client: %w", err)
+	}
+	_, outBuilderID, err := apiVerify.VerifyNpmPackageWithSigstoreTufClient(context.Background(), attestations, tarballHash, provenanceOpts, builderOpts, client)
+	if err != nil {
+		fmt.Printf("Verifying npm package: FAILED: %v", err)
+		return nil, err
+	}
+	fmt.Printf("builderId: %s\n", outBuilderID.Name())
+	fmt.Println("Verifying npm package: PASSED")
+	return outBuilderID, nil
+}
+```
+