wip: implement verification attestor that returns a slsa VSA

Co-authored-by: Kris Coleman <[email protected]>

attestation/policyverify/policyverify.go

@@ -0,0 +1,213 @@
+package policyverify
+
+import (
+	"crypto"
+	"crypto/x509"
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"github.com/testifysec/go-witness/attestation"
+	"github.com/testifysec/go-witness/cryptoutil"
+	"github.com/testifysec/go-witness/dsse"
+	"github.com/testifysec/go-witness/log"
+	"github.com/testifysec/go-witness/policy"
+	"github.com/testifysec/go-witness/slsa"
+	"github.com/testifysec/go-witness/source"
+	"github.com/testifysec/go-witness/timestamp"
+)
+
+const (
+	Name = "policyverify"
+	Type = slsa.VerificationSummaryPredicate
+)
+
+var (
+	_ attestation.Subjecter = &Attestor{}
+	_ attestation.Attestor  = &Attestor{}
+)
+
+type Attestor struct {
+	slsa.VerificationSummary
+
+	policyEnvelope   dsse.Envelope
+	policyVerifiers  []cryptoutil.Verifier
+	collectionSource source.Sourcer
+	subjectDigests   []string
+}
+
+type Option func(*Attestor)
+
+func VerifyWithPolicyEnvelope(policyEnvelope dsse.Envelope) Option {
+	return func(a *Attestor) {
+		a.policyEnvelope = policyEnvelope
+	}
+}
+
+func VerifyWithPolicyVerifiers(policyVerifiers []cryptoutil.Verifier) Option {
+	return func(a *Attestor) {
+		a.policyVerifiers = append(a.policyVerifiers, policyVerifiers...)
+	}
+}
+
+func VerifyWithSubjectDigests(subjectDigests []cryptoutil.DigestSet) Option {
+	return func(a *Attestor) {
+		for _, set := range subjectDigests {
+			for _, digest := range set {
+				a.subjectDigests = append(a.subjectDigests, digest)
+			}
+		}
+	}
+}
+
+func VerifyWithCollectionSource(source source.Sourcer) Option {
+	return func(a *Attestor) {
+		a.collectionSource = source
+	}
+}
+
+func New(opts ...Option) *Attestor {
+	a := &Attestor{}
+	for _, opt := range opts {
+		opt(a)
+	}
+
+	return a
+}
+
+func (a *Attestor) Name() string {
+	return Name
+}
+
+func (a *Attestor) Type() string {
+	return Type
+}
+
+func (a *Attestor) RunType() attestation.RunType {
+	return attestation.ExecuteRunType
+}
+
+func (a *Attestor) Subjects() map[string]cryptoutil.DigestSet {
+	subjects := map[string]cryptoutil.DigestSet{}
+	for _, digest := range a.subjectDigests {
+		subjects[fmt.Sprintf("artifact:%v", digest)] = cryptoutil.DigestSet{
+			cryptoutil.DigestValue{Hash: crypto.SHA256, GitOID: false}: digest,
+		}
+	}
+
+	subjects[fmt.Sprintf("policy:%v", a.VerificationSummary.Policy.URI)] = a.VerificationSummary.Policy.Digest
+	return subjects
+}
+
+func (a *Attestor) Attest(ctx *attestation.AttestationContext) error {
+	if _, err := a.policyEnvelope.Verify(dsse.VerifyWithVerifiers(a.policyVerifiers...)); err != nil {
+		return fmt.Errorf("could not verify policy: %w", err)
+	}
+
+	pol := policy.Policy{}
+	if err := json.Unmarshal(a.policyEnvelope.Payload, &pol); err != nil {
+		return fmt.Errorf("failed to unmarshal policy from envelope: %w", err)
+	}
+
+	pubKeysById, err := pol.PublicKeyVerifiers()
+	if err != nil {
+		return fmt.Errorf("failed to get public keys from policy: %w", err)
+	}
+
+	pubkeys := make([]cryptoutil.Verifier, 0)
+	for _, pubkey := range pubKeysById {
+		pubkeys = append(pubkeys, pubkey)
+	}
+
+	trustBundlesById, err := pol.TrustBundles()
+	if err != nil {
+		return fmt.Errorf("failed to load policy trust bundles: %w", err)
+	}
+
+	roots := make([]*x509.Certificate, 0)
+	intermediates := make([]*x509.Certificate, 0)
+	for _, trustBundle := range trustBundlesById {
+		roots = append(roots, trustBundle.Root)
+		intermediates = append(intermediates, trustBundle.Intermediates...)
+	}
+
+	timestampAuthoritiesById, err := pol.TimestampAuthorityTrustBundles()
+	if err != nil {
+		return fmt.Errorf("failed to load policy timestamp authorities: %w", err)
+	}
+
+	timestampVerifiers := make([]dsse.TimestampVerifier, 0)
+	for _, timestampAuthority := range timestampAuthoritiesById {
+		certs := []*x509.Certificate{timestampAuthority.Root}
+		certs = append(certs, timestampAuthority.Intermediates...)
+		timestampVerifiers = append(timestampVerifiers, timestamp.NewVerifier(timestamp.VerifyWithCerts(certs)))
+	}
+
+	verifiedSource := source.NewVerifiedSource(
+		a.collectionSource,
+		dsse.VerifyWithVerifiers(pubkeys...),
+		dsse.VerifyWithRoots(roots...),
+		dsse.VerifyWithIntermediates(intermediates...),
+		dsse.VerifyWithTimestampVerifiers(timestampVerifiers...),
+	)
+
+	accepted := true
+	policyResult, policyErr := pol.Verify(ctx.Context(), policy.WithSubjectDigests(a.subjectDigests), policy.WithVerifiedSource(verifiedSource))
+	if _, ok := policyErr.(policy.ErrPolicyDenied); ok {
+		accepted = false
+	} else if policyErr != nil {
+		return fmt.Errorf("failed to verify policy: %w", err)
+	}
+
+	a.VerificationSummary, err = verificationSummaryFromResults(a.policyEnvelope, policyResult, accepted)
+	if err != nil {
+		return fmt.Errorf("failed to generate verification summary: %w", err)
+	}
+
+	return policyErr
+}
+
+func calculateDigest(b []byte) (cryptoutil.DigestSet, error) {
+	return cryptoutil.CalculateDigestSetFromBytes(b, []crypto.Hash{crypto.SHA256})
+}
+
+func verificationSummaryFromResults(policyEnvelope dsse.Envelope, policyResult policy.PolicyResult, accepted bool) (slsa.VerificationSummary, error) {
+	inputAttestations := make([]slsa.ResourceDescriptor, 0, len(policyResult.EvidenceByStep))
+	for _, input := range policyResult.EvidenceByStep {
+		for _, attestation := range input {
+			digest, err := calculateDigest(attestation.Envelope.Payload)
+			if err != nil {
+				log.Debugf("failed to calculate evidence hash: %v", err)
+				continue
+			}
+
+			inputAttestations = append(inputAttestations, slsa.ResourceDescriptor{
+				URI:    attestation.Reference,
+				Digest: digest,
+			})
+		}
+	}
+
+	policyDigest, err := calculateDigest(policyEnvelope.Payload)
+	if err != nil {
+		return slsa.VerificationSummary{}, fmt.Errorf("failed to calculate policy digest: %w", err)
+	}
+
+	verificationResult := slsa.FailedVerificationResult
+	if accepted {
+		verificationResult = slsa.PassedVerificationResult
+	}
+
+	return slsa.VerificationSummary{
+		Verifier: slsa.Verifier{
+			ID: "witness",
+		},
+		TimeVerified: time.Now(),
+		Policy: slsa.ResourceDescriptor{
+			URI:    policyDigest[cryptoutil.DigestValue{Hash: crypto.SHA256, GitOID: false}], //TODO: find a better value for this...
+			Digest: policyDigest,
+		},
+		InputAttestations:  inputAttestations,
+		VerificationResult: verificationResult,
+	}, nil
+}

attestation/policyverify/policyverify_test.go

@@ -0,0 +1,61 @@
+// Copyright 2023 The Witness Contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package policyverify
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/testifysec/go-witness/attestation"
+)
+
+func TestAttestorName(t *testing.T) {
+	a := New()
+	assert.Equal(t, a.Name(), "Expected Attestor Name Here")
+}
+
+func TestAttestorType(t *testing.T) {
+	a := New()
+	assert.Equal(t, a.Type(), "Expected Attestor Type Here")
+}
+
+func TestAttestorRunType(t *testing.T) {
+	a := New()
+	assert.Equal(t, a.RunType(), "Expected RunType Here")
+}
+
+func TestAttestorAttest(t *testing.T) {
+	// Arrange
+	a := New()
+	ctx := &attestation.AttestationContext{}
+
+	// Act
+	err := a.Attest(ctx)
+
+	// Assert
+	require.NoError(t, err)
+}
+
+func TestYourFunctionHere(t *testing.T) {
+	// Arrange
+	// Setup variables here
+
+	// Act
+	// Perform function to be tested here
+
+	// Assert
+	// Assert whether the expected result and actual result match or not.
+}

dsse/sign.go

@@ -68,6 +68,10 @@ func Sign(bodyType string, body io.Reader, opts ...SignOption) (Envelope, error)
 	env.Signatures = make([]Signature, 0)
 	pae := preauthEncode(bodyType, bodyBytes)
 	for _, signer := range so.signers {
+		if signer == nil {
+			continue
+		}
+
 		sig, err := signer.Sign(bytes.NewReader(pae))
 		if err != nil {
 			return env, err

policy/policy.go

@@ -168,7 +168,11 @@ func checkVerifyOpts(vo *verifyOptions) error {
 	return nil
 }
 
-func (p Policy) Verify(ctx context.Context, opts ...VerifyOption) (map[string][]source.VerifiedCollection, error) {
+type PolicyResult struct {
+	EvidenceByStep map[string][]source.VerifiedCollection
+}
+
+func (p Policy) Verify(ctx context.Context, opts ...VerifyOption) (PolicyResult, error) {
 	vo := &verifyOptions{
 		searchDepth: 3,
 	}
@@ -178,16 +182,16 @@ func (p Policy) Verify(ctx context.Context, opts ...VerifyOption) (map[string][]
 	}
 
 	if err := checkVerifyOpts(vo); err != nil {
-		return nil, err
+		return PolicyResult{}, err
 	}
 
 	if time.Now().After(p.Expires.Time) {
-		return nil, ErrPolicyExpired(p.Expires.Time)
+		return PolicyResult{}, ErrPolicyExpired(p.Expires.Time)
 	}
 
 	trustBundles, err := p.TrustBundles()
 	if err != nil {
-		return nil, err
+		return PolicyResult{}, err
 	}
 
 	attestationsByStep := make(map[string][]string)
@@ -202,7 +206,7 @@ func (p Policy) Verify(ctx context.Context, opts ...VerifyOption) (map[string][]
 		for stepName, step := range p.Steps {
 			statements, err := vo.verifiedSource.Search(ctx, stepName, vo.subjectDigests, attestationsByStep[stepName])
 			if err != nil {
-				return nil, err
+				return PolicyResult{}, err
 			}
 
 			approvedCollections := step.checkFunctionaries(statements, trustBundles)
@@ -218,11 +222,11 @@ func (p Policy) Verify(ctx context.Context, opts ...VerifyOption) (map[string][]
 		}
 
 		if accepted, err := p.verifyArtifacts(passedByStep); err == nil {
-			return accepted, nil
+			return PolicyResult{EvidenceByStep: accepted}, nil
 		}
 	}
 
-	return nil, ErrPolicyDenied{Reasons: []string{"failed to find set of attestations that satisfies the policy"}}
+	return PolicyResult{}, ErrPolicyDenied{Reasons: []string{"failed to find set of attestations that satisfies the policy"}}
 }
 
 // checkFunctionaries checks to make sure the signature on each statement corresponds to a trusted functionary for