refactor! remove pkg directory (#145)

* refactor!: remove pkg directory

* chore: include breaking changes in draft

policy/README.md

@@ -0,0 +1,2 @@
+## policy
+This package is for attestation policy mapping and evaluation.

policy/evaluator.go

@@ -0,0 +1,11 @@
+package policy
+
+import (
+	"context"
+
+	"github.com/docker/attest/attestation"
+)
+
+type Evaluator interface {
+	Evaluate(ctx context.Context, resolver attestation.Resolver, pctx *Policy, input *Input) (*Result, error)
+}

policy/match.go

@@ -0,0 +1,65 @@
+package policy
+
+import (
+	"fmt"
+
+	"github.com/docker/attest/config"
+)
+
+type matchType string
+
+const (
+	matchTypePolicy        matchType = "policy"
+	matchTypeMatchNoPolicy matchType = "match_no_policy"
+	matchTypeNoMatch       matchType = "no_match"
+)
+
+type policyMatch struct {
+	matchType   matchType
+	policy      *config.PolicyMapping
+	rule        *config.PolicyRule
+	matchedName string
+}
+
+func findPolicyMatch(imageName string, mappings *config.PolicyMappings) (*policyMatch, error) {
+	if mappings == nil {
+		return &policyMatch{matchType: matchTypeNoMatch, matchedName: imageName}, nil
+	}
+	return findPolicyMatchImpl(imageName, mappings, make(map[*config.PolicyRule]bool))
+}
+
+func findPolicyMatchImpl(imageName string, mappings *config.PolicyMappings, matched map[*config.PolicyRule]bool) (*policyMatch, error) {
+	for _, rule := range mappings.Rules {
+		if rule.Pattern.MatchString(imageName) {
+			switch {
+			case rule.PolicyID == "" && rule.Replacement == "":
+				return nil, fmt.Errorf("rule %s has neither policy-id nor rewrite", rule.Pattern)
+			case rule.PolicyID != "" && rule.Replacement != "":
+				return nil, fmt.Errorf("rule %s has both policy-id and rewrite", rule.Pattern)
+			case rule.PolicyID != "":
+				policy := mappings.Policies[rule.PolicyID]
+				if policy != nil {
+					return &policyMatch{
+						matchType:   matchTypePolicy,
+						policy:      policy,
+						rule:        rule,
+						matchedName: imageName,
+					}, nil
+				}
+				return &policyMatch{
+					matchType:   matchTypeMatchNoPolicy,
+					rule:        rule,
+					matchedName: imageName,
+				}, nil
+			case rule.Replacement != "":
+				if matched[rule] {
+					return nil, fmt.Errorf("rewrite loop detected")
+				}
+				matched[rule] = true
+				imageName = rule.Pattern.ReplaceAllString(imageName, rule.Replacement)
+				return findPolicyMatchImpl(imageName, mappings, matched)
+			}
+		}
+	}
+	return &policyMatch{matchType: matchTypeNoMatch, matchedName: imageName}, nil
+}

policy/match_test.go

@@ -0,0 +1,121 @@
+package policy
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/docker/attest/config"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestFindPolicyMatch(t *testing.T) {
+	testCases := []struct {
+		name       string
+		imageName  string
+		mappingDir string
+
+		expectError       bool
+		expectedMatchType matchType
+		expectedPolicyID  string
+		expectedImageName string
+	}{
+		{
+			name:       "alpine",
+			mappingDir: "doi",
+			imageName:  "docker.io/library/alpine",
+
+			expectedMatchType: matchTypePolicy,
+			expectedPolicyID:  "docker-official-images",
+			expectedImageName: "docker.io/library/alpine",
+		},
+		{
+			name:       "no match",
+			mappingDir: "doi",
+			imageName:  "docker.io/something/else",
+
+			expectedMatchType: matchTypeNoMatch,
+			expectedImageName: "docker.io/something/else",
+		},
+		{
+			name:       "match, no policy",
+			mappingDir: "local",
+			imageName:  "docker.io/library/alpine",
+
+			expectedMatchType: matchTypeMatchNoPolicy,
+			expectedImageName: "docker.io/library/alpine",
+		},
+		{
+			name:       "simple rewrite",
+			mappingDir: "simple-rewrite",
+			imageName:  "mycoolmirror.org/library/alpine",
+
+			expectedMatchType: matchTypePolicy,
+			expectedPolicyID:  "docker-official-images",
+			expectedImageName: "docker.io/library/alpine",
+		},
+		{
+			name:       "rewrite no match",
+			mappingDir: "rewrite-to-no-match",
+			imageName:  "mycoolmirror.org/library/alpine",
+
+			expectedMatchType: matchTypeNoMatch,
+			expectedImageName: "badredirect.org/alpine",
+		},
+		{
+			name:       "rewrite to match, no policy",
+			mappingDir: "rewrite-to-local",
+			imageName:  "mycoolmirror.org/library/alpine",
+
+			expectedMatchType: matchTypeMatchNoPolicy,
+			expectedImageName: "docker.io/library/alpine",
+		},
+		{
+			name:       "multiple rewrites",
+			mappingDir: "rewrite-multiple",
+			imageName:  "myevencoolermirror.org/library/alpine",
+
+			expectedMatchType: matchTypePolicy,
+			expectedPolicyID:  "docker-official-images",
+			expectedImageName: "docker.io/library/alpine",
+		},
+		{
+			name:       "invalid rewrites",
+			mappingDir: "rewrite-invalid",
+			imageName:  "mycoolmirror.org/library/alpine",
+
+			expectError:       true,
+			expectedMatchType: matchTypePolicy,
+			expectedPolicyID:  "docker-official-images",
+			expectedImageName: "docker.io/library/alpine",
+		},
+		{
+			name:       "rewrite loop",
+			mappingDir: "rewrite-loop",
+			imageName:  "yin/alpine",
+
+			expectError: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			mappings, err := config.LoadLocalMappings(filepath.Join("testdata", "mappings", tc.mappingDir))
+			require.NoError(t, err)
+			match, err := findPolicyMatch(tc.imageName, mappings)
+			if tc.expectError {
+				require.Error(t, err)
+				// TODO: check error matches expected error message
+				return
+			}
+			require.NoError(t, err)
+			assert.Equal(t, tc.expectedMatchType, match.matchType)
+			if match.matchType == matchTypePolicy {
+				if assert.NotNil(t, match.policy) {
+					assert.Equal(t, tc.expectedPolicyID, match.policy.ID)
+				}
+			}
+			assert.Equal(t, tc.expectedImageName, match.matchedName)
+		})
+	}
+}

policy/mock.go

@@ -0,0 +1,32 @@
+package policy
+
+import (
+	"context"
+
+	"github.com/docker/attest/attestation"
+)
+
+type MockPolicyEvaluator struct {
+	EvaluateFunc func(ctx context.Context, resolver attestation.Resolver, pctx *Policy, input *Input) (*Result, error)
+}
+
+func (pe *MockPolicyEvaluator) Evaluate(ctx context.Context, resolver attestation.Resolver, pctx *Policy, input *Input) (*Result, error) {
+	if pe.EvaluateFunc != nil {
+		return pe.EvaluateFunc(ctx, resolver, pctx, input)
+	}
+	return AllowedResult(), nil
+}
+
+func GetMockPolicy() Evaluator {
+	return &MockPolicyEvaluator{
+		EvaluateFunc: func(_ context.Context, _ attestation.Resolver, _ *Policy, _ *Input) (*Result, error) {
+			return AllowedResult(), nil
+		},
+	}
+}
+
+func AllowedResult() *Result {
+	return &Result{
+		Success: true,
+	}
+}

policy/policy.go

@@ -0,0 +1,38 @@
+package policy
+
+import (
+	"fmt"
+
+	"github.com/docker/attest/attestation"
+	"github.com/docker/attest/config"
+	"github.com/docker/attest/oci"
+)
+
+func CreateImageDetailsResolver(imageSource *oci.ImageSpec) (oci.ImageDetailsResolver, error) {
+	switch imageSource.Type {
+	case oci.OCI:
+		return attestation.NewOCILayoutResolver(imageSource)
+	case oci.Docker:
+		return oci.NewRegistryImageDetailsResolver(imageSource)
+	}
+	return nil, fmt.Errorf("unsupported image source type: %s", imageSource.Type)
+}
+
+func CreateAttestationResolver(resolver oci.ImageDetailsResolver, mapping *config.PolicyMapping) (attestation.Resolver, error) {
+	if mapping.Attestations != nil {
+		if mapping.Attestations.Style == config.AttestationStyleAttached {
+			switch resolver := resolver.(type) {
+			case *oci.RegistryImageDetailsResolver:
+				return attestation.NewRegistryResolver(resolver)
+			case *attestation.LayoutResolver:
+				return resolver, nil
+			default:
+				return nil, fmt.Errorf("unsupported image details resolver type: %T", resolver)
+			}
+		}
+		if mapping.Attestations.Repo != "" {
+			return attestation.NewReferrersResolver(resolver, attestation.WithReferrersRepo(mapping.Attestations.Repo))
+		}
+	}
+	return attestation.NewReferrersResolver(resolver)
+}

policy/policy_test.go

@@ -0,0 +1,175 @@
+package policy_test
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/docker/attest/attestation"
+	"github.com/docker/attest/config"
+	"github.com/docker/attest/internal/test"
+	"github.com/docker/attest/oci"
+	"github.com/docker/attest/policy"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func loadAttestation(t *testing.T, path string) *attestation.Envelope {
+	ex, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	env := new(attestation.Envelope)
+	err = json.Unmarshal(ex, env)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return env
+}
+
+func TestRegoEvaluator_Evaluate(t *testing.T) {
+	ctx, _ := test.Setup(t)
+	resolveErrorStr := "failed to resolve policy by id: policy with id non-existent-policy-id not found"
+	TestDataPath := filepath.Join("..", "test", "testdata")
+	ExampleAttestation := filepath.Join(TestDataPath, "example_attestation.json")
+
+	re := policy.NewRegoEvaluator(true)
+
+	defaultResolver := attestation.MockResolver{
+		Envs: []*attestation.Envelope{loadAttestation(t, ExampleAttestation)},
+	}
+
+	testCases := []struct {
+		policyPath      string
+		expectSuccess   bool
+		isCanonical     bool
+		resolver        attestation.Resolver
+		opts            *policy.Options
+		policyID        string
+		resolveErrorStr string
+	}{
+		{policyPath: "testdata/policies/allow", expectSuccess: true, resolver: defaultResolver},
+		{policyPath: "testdata/policies/allow", expectSuccess: true, resolver: defaultResolver, policyID: "docker-official-images"},
+		{policyPath: "testdata/policies/allow", resolver: defaultResolver, policyID: "non-existent-policy-id", resolveErrorStr: resolveErrorStr},
+		{policyPath: "testdata/policies/deny", resolver: defaultResolver},
+		{policyPath: "testdata/policies/verify-sig", expectSuccess: true, resolver: defaultResolver},
+		{policyPath: "testdata/policies/wrong-key", resolver: defaultResolver},
+		{policyPath: "testdata/policies/allow-canonical", expectSuccess: true, isCanonical: true, resolver: defaultResolver},
+		{policyPath: "testdata/policies/allow-canonical", resolver: defaultResolver},
+		{policyPath: "testdata/policies/no-rego", resolver: defaultResolver, resolveErrorStr: "no policy file found in policy mapping"},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.policyPath, func(t *testing.T) {
+			input := &policy.Input{
+				Digest: "sha256:test-digest",
+				PURL:   "test-purl",
+			}
+			if !tc.isCanonical {
+				input.Tag = "test"
+			}
+
+			if tc.opts == nil {
+				tc.opts = &policy.Options{
+					LocalTargetsDir: test.CreateTempDir(t, "", "tuf-targets"),
+					PolicyID:        tc.policyID,
+					LocalPolicyDir:  tc.policyPath,
+					DisableTUF:      true,
+				}
+			}
+			imageName, err := tc.resolver.ImageName(ctx)
+			require.NoError(t, err)
+			resolver := policy.NewResolver(nil, tc.opts)
+			policy, err := resolver.ResolvePolicy(ctx, imageName)
+			if tc.resolveErrorStr != "" {
+				require.Error(t, err)
+				assert.Contains(t, err.Error(), tc.resolveErrorStr)
+				return
+			}
+			require.NoErrorf(t, err, "failed to resolve policy")
+			require.NotNil(t, policy, "policy should not be nil")
+			result, err := re.Evaluate(ctx, tc.resolver, policy, input)
+			require.NoErrorf(t, err, "Evaluate failed")
+
+			if tc.expectSuccess {
+				assert.True(t, result.Success, "Evaluate should have succeeded")
+			} else {
+				assert.False(t, result.Success, "Evaluate should have failed")
+			}
+		})
+	}
+}
+
+func TestLoadingMappings(t *testing.T) {
+	policyMappings, err := config.LoadLocalMappings(filepath.Join("testdata", "policies", "allow"))
+	require.NoError(t, err)
+	assert.Equal(t, len(policyMappings.Rules), 3)
+	for _, mirror := range policyMappings.Rules {
+		if mirror.PolicyID != "" {
+			assert.Equal(t, "docker-official-images", mirror.PolicyID)
+		}
+	}
+}
+
+func TestCreateAttestationResolver(t *testing.T) {
+	mockResolver := attestation.MockResolver{
+		Envs: []*attestation.Envelope{},
+	}
+	layoutResolver := &attestation.LayoutResolver{}
+	registryResolver := &oci.RegistryImageDetailsResolver{}
+
+	nilRepoReferrers := &config.PolicyMapping{
+		Attestations: &config.AttestationConfig{
+			Style: config.AttestationStyleReferrers,
+		},
+	}
+	referrers := &config.PolicyMapping{
+		Attestations: &config.AttestationConfig{
+			Repo:  "localhost:5000/repo",
+			Style: config.AttestationStyleReferrers,
+		},
+	}
+	attached := &config.PolicyMapping{
+		Attestations: &config.AttestationConfig{
+			Style: config.AttestationStyleAttached,
+		},
+	}
+
+	testCases := []struct {
+		name     string
+		resolver oci.ImageDetailsResolver
+		mapping  *config.PolicyMapping
+		errorStr string
+	}{
+		{name: "referrers", resolver: layoutResolver, mapping: referrers},
+		{name: "referrers (no mapped repo)", resolver: layoutResolver, mapping: nilRepoReferrers},
+		{name: "referrers (no mapping)", resolver: layoutResolver, mapping: &config.PolicyMapping{Attestations: nil}},
+		{name: "attached (registry)", resolver: registryResolver, mapping: attached},
+		{name: "attached (layout)", resolver: layoutResolver, mapping: attached},
+		{name: "attached (unsupported)", resolver: mockResolver, mapping: attached, errorStr: "unsupported image details resolver type"},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			resolver, err := policy.CreateAttestationResolver(tc.resolver, tc.mapping)
+			if tc.errorStr == "" {
+				require.NoError(t, err)
+			} else {
+				assert.Contains(t, err.Error(), tc.errorStr)
+			}
+			if tc.mapping.Attestations == nil {
+				return
+			}
+			switch resolver.(type) {
+			case *attestation.ReferrersResolver:
+				assert.Equal(t, tc.mapping.Attestations.Style, config.AttestationStyleReferrers)
+			case *attestation.RegistryResolver:
+				assert.Equal(t, tc.mapping.Attestations.Style, config.AttestationStyleAttached)
+			case *attestation.LayoutResolver:
+				assert.Equal(t, tc.mapping.Attestations.Style, config.AttestationStyleAttached)
+			}
+		})
+	}
+}

policy/rego.go

@@ -0,0 +1,277 @@
+package policy
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/docker/attest/attestation"
+	intoto "github.com/in-toto/in-toto-golang/in_toto"
+	"github.com/open-policy-agent/opa/ast"
+	"github.com/open-policy-agent/opa/rego"
+	"github.com/open-policy-agent/opa/storage"
+	"github.com/open-policy-agent/opa/storage/inmem"
+	"github.com/open-policy-agent/opa/tester"
+	"github.com/open-policy-agent/opa/topdown"
+	"github.com/open-policy-agent/opa/types"
+	opa "github.com/open-policy-agent/opa/util"
+	"sigs.k8s.io/yaml"
+)
+
+type regoEvaluator struct {
+	debug bool
+}
+
+const (
+	DefaultQuery  = "result := data.attest.result"
+	resultBinding = "result"
+)
+
+func NewRegoEvaluator(debug bool) Evaluator {
+	return &regoEvaluator{
+		debug: debug,
+	}
+}
+
+func (re *regoEvaluator) Evaluate(ctx context.Context, resolver attestation.Resolver, pctx *Policy, input *Input) (*Result, error) {
+	var regoOpts []func(*rego.Rego)
+
+	// Create a new in-memory store
+	store := inmem.New()
+	params := storage.TransactionParams{}
+	params.Write = true
+	txn, err := store.NewTransaction(ctx, params)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, target := range pctx.InputFiles {
+		// load yaml as data (no rego opt for this!?)
+		if filepath.Ext(target.Path) == ".yaml" {
+			yamlData, err := loadYAML(target.Path, target.Content)
+			if err != nil {
+				return nil, err
+			}
+			err = store.Write(ctx, txn, storage.AddOp, storage.Path{}, yamlData)
+			if err != nil {
+				return nil, err
+			}
+		} else {
+			regoOpts = append(regoOpts, rego.Module(target.Path, string(target.Content)))
+		}
+	}
+
+	err = store.Commit(ctx, txn)
+	if err != nil {
+		store.Abort(ctx, txn)
+		return nil, err
+	}
+
+	if re.debug {
+		regoOpts = append(regoOpts,
+			rego.EnablePrintStatements(true),
+			rego.PrintHook(topdown.NewPrintHook(os.Stderr)),
+			rego.Dump(os.Stderr),
+		)
+	}
+	query := DefaultQuery
+	if pctx.Query != "" {
+		query = pctx.Query
+	}
+	regoOpts = append(regoOpts,
+		rego.Query(query),
+		rego.Input(input),
+		rego.Store(store),
+		rego.GenerateJSON(jsonGenerator[Result]()),
+	)
+	for _, custom := range RegoFunctions(resolver) {
+		regoOpts = append(regoOpts, custom.Func)
+	}
+
+	r := rego.New(regoOpts...)
+	rs, err := r.Eval(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(rs) == 0 {
+		return nil, fmt.Errorf("no policy evaluation result")
+	}
+	binding, ok := rs[0].Bindings[resultBinding]
+	if !ok {
+		return nil, fmt.Errorf("failed to extract verification result")
+	}
+	result, ok := binding.(Result)
+	if !ok {
+		return nil, fmt.Errorf("failed to extract verification result")
+	}
+
+	return &result, nil
+}
+
+func jsonGenerator[T any]() func(t *ast.Term, ec *rego.EvalContext) (any, error) {
+	return func(t *ast.Term, _ *rego.EvalContext) (any, error) {
+		// TODO: this is horrible - we're converting the AST to JSON and then back to AST, then using ast.As to convert it to a struct
+		// We can't use ast.As directly because it fails if the AST contains a set
+		json, err := ast.JSON(t.Value)
+		if err != nil {
+			return nil, err
+		}
+		v, err := ast.InterfaceToValue(json)
+		if err != nil {
+			return nil, err
+		}
+		var result T
+		err = ast.As(v, &result)
+		if err != nil {
+			return nil, err
+		}
+		return result, nil
+	}
+}
+
+var dynamicObj = types.NewObject(nil, types.NewDynamicProperty(types.A, types.A))
+
+var verifyDecl = &ast.Builtin{
+	Name:             "attest.verify",
+	Decl:             types.NewFunction(types.Args(dynamicObj, dynamicObj), dynamicObj),
+	Nondeterministic: true,
+}
+
+var attestDecl = &ast.Builtin{
+	Name:             "attest.fetch",
+	Decl:             types.NewFunction(types.Args(types.S), dynamicObj),
+	Nondeterministic: true,
+}
+
+func wrapFunctionResult(value *ast.Term, err error) (*ast.Term, error) {
+	var terms [][2]*ast.Term
+	if err != nil {
+		terms = append(terms, [2]*ast.Term{ast.StringTerm("error"), ast.StringTerm(err.Error())})
+	}
+	if value != nil {
+		terms = append(terms, [2]*ast.Term{ast.StringTerm("value"), value})
+	}
+	return ast.ObjectTerm(terms...), nil
+}
+
+func handleErrors1(f func(rCtx rego.BuiltinContext, a *ast.Term) (*ast.Term, error)) rego.Builtin1 {
+	return func(rCtx rego.BuiltinContext, a *ast.Term) (*ast.Term, error) {
+		return wrapFunctionResult(f(rCtx, a))
+	}
+}
+
+func handleErrors2(f func(rCtx *rego.BuiltinContext, a, b *ast.Term) (*ast.Term, error)) rego.Builtin2 {
+	return func(rCtx rego.BuiltinContext, a, b *ast.Term) (*ast.Term, error) {
+		return wrapFunctionResult(f(&rCtx, a, b))
+	}
+}
+
+func RegoFunctions(resolver attestation.Resolver) []*tester.Builtin {
+	return []*tester.Builtin{
+		{
+			Decl: verifyDecl,
+			Func: rego.Function2(
+				&rego.Function{
+					Name:             verifyDecl.Name,
+					Decl:             verifyDecl.Decl,
+					Memoize:          true,
+					Nondeterministic: verifyDecl.Nondeterministic,
+				},
+				handleErrors2(verifyInTotoEnvelope)),
+		},
+		{
+			Decl: attestDecl,
+			Func: rego.Function1(
+				&rego.Function{
+					Name:             attestDecl.Name,
+					Decl:             attestDecl.Decl,
+					Memoize:          true,
+					Nondeterministic: attestDecl.Nondeterministic,
+				},
+				handleErrors1(fetchInTotoAttestations(resolver))),
+		},
+	}
+}
+
+func fetchInTotoAttestations(resolver attestation.Resolver) rego.Builtin1 {
+	return func(rCtx rego.BuiltinContext, predicateTypeTerm *ast.Term) (*ast.Term, error) {
+		predicateTypeStr, ok := predicateTypeTerm.Value.(ast.String)
+		if !ok {
+			return nil, fmt.Errorf("predicateTypeTerm is not a string")
+		}
+		predicateType := string(predicateTypeStr)
+
+		envelopes, err := resolver.Attestations(rCtx.Context, predicateType)
+		if err != nil {
+			return nil, err
+		}
+
+		// Convert each envelope to an ast.Value.
+		values := make([]*ast.Term, len(envelopes))
+		for i, envelope := range envelopes {
+			value, err := ast.InterfaceToValue(envelope)
+			if err != nil {
+				return nil, err
+			}
+			values[i] = ast.NewTerm(value)
+		}
+
+		// Wrap the values in an ast.Set and convert it to an ast.Term.
+		set := ast.NewTerm(ast.NewSet(values...))
+
+		return set, nil
+	}
+}
+
+func verifyInTotoEnvelope(rCtx *rego.BuiltinContext, envTerm, optsTerm *ast.Term) (*ast.Term, error) {
+	env := new(attestation.Envelope)
+	opts := new(attestation.VerifyOptions)
+	err := ast.As(envTerm.Value, env)
+	if err != nil {
+		return nil, fmt.Errorf("failed to cast envelope: %w", err)
+	}
+	err = ast.As(optsTerm.Value, &opts)
+	if err != nil {
+		return nil, fmt.Errorf("failed to cast verifier options: %w", err)
+	}
+
+	payload, err := attestation.VerifyDSSE(rCtx.Context, env, opts)
+	if err != nil {
+		return nil, err
+	}
+
+	statement := new(intoto.Statement)
+
+	switch env.PayloadType {
+	case intoto.PayloadType:
+		err = json.Unmarshal(payload, statement)
+		if err != nil {
+			return nil, fmt.Errorf("failed to unmarshal statement: %w", err)
+		}
+		// TODO: implement other types of envelope
+	default:
+		return nil, fmt.Errorf("unsupported payload type: %s", env.PayloadType)
+	}
+
+	value, err := ast.InterfaceToValue(statement)
+	if err != nil {
+		return nil, err
+	}
+	return ast.NewTerm(value), nil
+}
+
+func loadYAML(path string, bs []byte) (interface{}, error) {
+	var x interface{}
+	bs, err := yaml.YAMLToJSON(bs)
+	if err != nil {
+		return nil, fmt.Errorf("%v: error converting YAML to JSON: %v", path, err)
+	}
+	err = opa.UnmarshalJSON(bs, &x)
+	if err != nil {
+		return nil, fmt.Errorf("%s: %w", path, err)
+	}
+	return x, nil
+}

policy/resolver.go

@@ -0,0 +1,194 @@
+package policy
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path"
+	"path/filepath"
+
+	"github.com/distribution/reference"
+	"github.com/docker/attest/config"
+	"github.com/docker/attest/internal/util"
+	"github.com/docker/attest/tuf"
+)
+
+type Resolver struct {
+	tufClient tuf.Downloader
+	opts      *Options
+}
+
+func NewResolver(tufClient tuf.Downloader, opts *Options) *Resolver {
+	return &Resolver{
+		tufClient: tufClient,
+		opts:      opts,
+	}
+}
+
+func (r *Resolver) ResolvePolicy(_ context.Context, imageName string) (*Policy, error) {
+	p, err := r.resolvePolicyByID()
+	if err != nil {
+		return nil, fmt.Errorf("failed to resolve policy by id: %w", err)
+	}
+	if p != nil {
+		return p, nil
+	}
+	imageName, err = normalizeImageName(imageName)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse image name: %w", err)
+	}
+	localMappings, err := config.LoadLocalMappings(r.opts.LocalPolicyDir)
+	if err != nil {
+		return nil, fmt.Errorf("failed to load local policy mappings: %w", err)
+	}
+	match, err := findPolicyMatch(imageName, localMappings)
+	if err != nil {
+		return nil, err
+	}
+	if match.matchType == matchTypePolicy {
+		return r.resolveLocalPolicy(match.policy, imageName, match.matchedName)
+	}
+	if !r.opts.DisableTUF {
+		tufMappings, err := config.LoadTUFMappings(r.tufClient, r.opts.LocalTargetsDir)
+		if err != nil {
+			return nil, fmt.Errorf("failed to load tuf policy mappings as fallback: %w", err)
+		}
+
+		// it's a mirror of a tuf policy
+		if match.matchType == matchTypeMatchNoPolicy {
+			for _, mapping := range tufMappings.Policies {
+				if mapping.ID == match.rule.PolicyID {
+					return r.resolveTUFPolicy(mapping, imageName, match.matchedName)
+				}
+			}
+		}
+
+		// try to resolve a tuf policy directly
+		match, err = findPolicyMatch(imageName, tufMappings)
+		if err != nil {
+			return nil, err
+		}
+		if match.matchType == matchTypePolicy {
+			return r.resolveTUFPolicy(match.policy, imageName, match.matchedName)
+		}
+	}
+	return nil, nil
+}
+
+func (r *Resolver) resolveLocalPolicy(mapping *config.PolicyMapping, imageName string, matchedName string) (*Policy, error) {
+	if r.opts.LocalPolicyDir == "" {
+		return nil, fmt.Errorf("local policy dir not set")
+	}
+	var URI string
+	var digest map[string]string
+	files := make([]*File, 0, len(mapping.Files))
+	for _, f := range mapping.Files {
+		filename := f.Path
+		filePath := path.Join(r.opts.LocalPolicyDir, filename)
+		fileContents, err := os.ReadFile(filePath)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read policy file %s: %w", filename, err)
+		}
+		files = append(files, &File{
+			Path:    filename,
+			Content: fileContents,
+		})
+		// if the file is a policy file, store the URI and digest
+		if filepath.Ext(filename) == ".rego" {
+			// TODO: support multiple rego files, need some way to identify the main policy file
+			if URI != "" {
+				return nil, fmt.Errorf("multiple policy files found in policy mapping")
+			}
+			URI = filePath
+			digest = map[string]string{"sha256": util.SHA256Hex(fileContents)}
+		}
+	}
+	if URI == "" {
+		return nil, fmt.Errorf("no policy file found in policy mapping")
+	}
+	policy := &Policy{
+		InputFiles: files,
+		Mapping:    mapping,
+		URI:        URI,
+		Digest:     digest,
+	}
+	if imageName != matchedName {
+		policy.ResolvedName = matchedName
+	}
+	return policy, nil
+}
+
+func (r *Resolver) resolveTUFPolicy(mapping *config.PolicyMapping, imageName string, matchedName string) (*Policy, error) {
+	var URI string
+	var digest map[string]string
+	files := make([]*File, 0, len(mapping.Files))
+	for _, f := range mapping.Files {
+		filename := f.Path
+		file, err := r.tufClient.DownloadTarget(filename, filepath.Join(r.opts.LocalTargetsDir, filename))
+		if err != nil {
+			return nil, fmt.Errorf("failed to download policy file %s: %w", filename, err)
+		}
+		files = append(files, &File{
+			Path:    filename,
+			Content: file.Data,
+		})
+		// if the file is a policy file, store the URI and digest
+		if filepath.Ext(filename) == ".rego" {
+			// TODO: support multiple rego files, need some way to identify the main policy file
+			if URI != "" {
+				return nil, fmt.Errorf("multiple policy files found in policy mapping")
+			}
+			URI = file.TargetURI
+			digest = map[string]string{"sha256": file.Digest}
+		}
+	}
+	if URI == "" {
+		return nil, fmt.Errorf("no policy file found in policy mapping")
+	}
+	policy := &Policy{
+		InputFiles: files,
+		Mapping:    mapping,
+		URI:        URI,
+		Digest:     digest,
+	}
+	if imageName != matchedName {
+		policy.ResolvedName = matchedName
+	}
+	return policy, nil
+}
+
+func (r *Resolver) resolvePolicyByID() (*Policy, error) {
+	if r.opts.PolicyID != "" {
+		localMappings, err := config.LoadLocalMappings(r.opts.LocalPolicyDir)
+		if err != nil {
+			return nil, fmt.Errorf("failed to load local policy mappings: %w", err)
+		}
+		if localMappings != nil {
+			policy := localMappings.Policies[r.opts.PolicyID]
+			if policy != nil {
+				return r.resolveLocalPolicy(policy, "", "")
+			}
+		}
+
+		if !r.opts.DisableTUF {
+			tufMappings, err := config.LoadTUFMappings(r.tufClient, r.opts.LocalTargetsDir)
+			if err != nil {
+				return nil, fmt.Errorf("failed to load tuf policy mappings by id: %w", err)
+			}
+			policy := tufMappings.Policies[r.opts.PolicyID]
+			if policy != nil {
+				return r.resolveTUFPolicy(policy, "", "")
+			}
+		}
+		return nil, fmt.Errorf("policy with id %s not found", r.opts.PolicyID)
+	}
+	return nil, nil
+}
+
+func normalizeImageName(imageName string) (string, error) {
+	named, err := reference.ParseNormalizedNamed(imageName)
+	if err != nil {
+		return "", fmt.Errorf("failed to parse image name: %w", err)
+	}
+	return named.Name(), nil
+}

policy/resolver_test.go

@@ -0,0 +1,65 @@
+package policy_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/docker/attest/internal/test"
+	"github.com/docker/attest/policy"
+	"github.com/docker/attest/tuf"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestResolvePolicy(t *testing.T) {
+	localPolicyPath := "testdata/policies/allow"
+	tufPolicyPath := "testdata/policies/allow-canonical"
+	noLocalPolicyPath := "testdata/policies/no-policy"
+	testPolicyID := "docker-official-images"
+	testImageName := "localhost:5001/test/repo:tag"
+
+	testCases := []struct {
+		name              string
+		policyPath        string
+		policyID          string
+		localOverridesTUF bool // if a policy is provided locally, it should override TUF
+		DisableTUF        bool
+	}{
+		{name: "resolve by id (TUF only)", policyID: testPolicyID, DisableTUF: false},
+		{name: "resolve by id (local mapping, TUF policy)", policyPath: noLocalPolicyPath, policyID: testPolicyID, DisableTUF: false},
+		{name: "resolve by id (local mapping, local policy, no TUF)", policyPath: localPolicyPath, policyID: testPolicyID, DisableTUF: true},
+		{name: "resolve by id (local mapping, local policy)", policyPath: localPolicyPath, policyID: testPolicyID, DisableTUF: false, localOverridesTUF: true},
+		{name: "resolve by match (TUF only)", DisableTUF: false},
+		{name: "resolve by match (local mapping, TUF policy)", policyPath: noLocalPolicyPath, DisableTUF: false},
+		{name: "resolve by match (local mapping, local policy, no TUF)", policyPath: localPolicyPath, DisableTUF: true},
+		{name: "resolve by match (local mapping, local policy)", policyPath: localPolicyPath, DisableTUF: false, localOverridesTUF: true},
+	}
+
+	var tufClient tuf.Downloader
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			opts := &policy.Options{}
+			tempDir := test.CreateTempDir(t, "", "tuf-dest")
+			if !tc.DisableTUF {
+				tufClient = tuf.NewMockTufClient(tufPolicyPath)
+			}
+			if tc.policyID != "" {
+				opts.PolicyID = tc.policyID
+			}
+			if tc.policyPath != "" {
+				opts.LocalPolicyDir = tc.policyPath
+			}
+			opts.DisableTUF = tc.DisableTUF
+			opts.LocalTargetsDir = tempDir
+			resolver := policy.NewResolver(tufClient, opts)
+			policy, err := resolver.ResolvePolicy(context.Background(), testImageName)
+			require.NoError(t, err)
+			assert.NotNil(t, policy)
+			if tc.DisableTUF || tc.localOverridesTUF {
+				assert.Contains(t, policy.URI, localPolicyPath)
+			} else {
+				assert.Contains(t, policy.URI, tufPolicyPath)
+			}
+		})
+	}
+}

policy/testdata/mappings/doi/mapping.yaml

@@ -0,0 +1,10 @@
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images

policy/testdata/mappings/local/mapping.yaml

@@ -0,0 +1,10 @@
+version: v1
+kind: policy-mapping
+policies:
+  - id: local-policy
+    description: Local Policy
+    files:
+      - path: local-policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images # note this policy does not exist in this file

policy/testdata/mappings/rewrite-invalid/mapping.yaml

@@ -0,0 +1,13 @@
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images
+  - pattern: "^mycoolmirror[.]org/library/(.*)$"
+    rewrite: "docker.io/library/$1"
+    policy-id: docker-official-images # invalid to specify both rewrite and policy-id

policy/testdata/mappings/rewrite-loop/mapping.yaml

@@ -0,0 +1,14 @@
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images
+  - pattern: "^yin/(.*)$"
+    rewrite: "yang/$1"
+  - pattern: "^yang/(.*)$"
+    rewrite: "yin/$1"

policy/testdata/mappings/rewrite-multiple/mapping.yaml

@@ -0,0 +1,14 @@
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images
+  - pattern: "^mycoolmirror[.]org/library/(.*)$"
+    rewrite: "docker.io/library/$1"
+  - pattern: "^myevencoolermirror[.]org/library/(.*)$"
+    rewrite: "mycoolmirror.org/library/$1"

policy/testdata/mappings/rewrite-to-local/mapping.yaml

@@ -0,0 +1,12 @@
+version: v1
+kind: policy-mapping
+policies:
+  - id: local-policy
+    description: Local Policy
+    files:
+      - path: local-policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images # note this policy does not exist in this file
+  - pattern: "^mycoolmirror[.]org/library/(.*)$"
+    rewrite: "docker.io/library/$1"

policy/testdata/mappings/rewrite-to-no-match/mapping.yaml

@@ -0,0 +1,12 @@
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images
+  - pattern: "^mycoolmirror[.]org/library/(.*)$"
+    rewrite: "badredirect.org/$1" # no matching rule for this rewrite

policy/testdata/mappings/simple-rewrite/mapping.yaml

@@ -0,0 +1,12 @@
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images
+  - pattern: "^mycoolmirror[.]org/library/(.*)$"
+    rewrite: "docker.io/library/$1"

policy/testdata/policies/allow-canonical/doi/policy.rego

@@ -0,0 +1,11 @@
+package attest
+
+import rego.v1
+
+default canonical = false
+
+canonical if {
+  not input.tag
+}
+
+result := {"success": canonical}

policy/testdata/policies/allow-canonical/mapping.yaml

@@ -0,0 +1,17 @@
+# map repos to policies
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    attestations:
+      repo: "localhost:5001/library-refs"
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images
+  - pattern: ^localhost:5001/(.*)$
+    rewrite: docker.io/library/$1
+  - pattern: ^registry[.]local:5000/(.*)$
+    rewrite: docker.io/library/$1

policy/testdata/policies/allow/doi/policy.rego

@@ -0,0 +1,7 @@
+package attest
+
+import rego.v1
+
+result := {
+  "success": true,
+}

policy/testdata/policies/allow/mapping.yaml

@@ -0,0 +1,17 @@
+# map repos to policies
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    attestations:
+      repo: "localhost:5001/library-refs"
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images
+  - pattern: ^localhost:5001/(.*)$
+    rewrite: docker.io/library/$1
+  - pattern: ^registry[.]local:5000/(.*)$
+    rewrite: docker.io/library/$1

policy/testdata/policies/deny/doi/policy.rego

@@ -0,0 +1,7 @@
+package attest
+
+import rego.v1
+
+result := {
+  "success": false,
+}

policy/testdata/policies/deny/mapping.yaml

@@ -0,0 +1,11 @@
+# map repos to policies
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images

policy/testdata/policies/no-policy/mapping.yaml

@@ -0,0 +1,10 @@
+# map repos to policies
+version: v1
+kind: policy-mapping
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images
+  - pattern: ^localhost:5001/(.*)$
+    rewrite: docker.io/library/$1
+  - pattern: ^registry[.]local:5000/(.*)$
+    rewrite: docker.io/library/$1

policy/testdata/policies/no-rego/doi/policy.yaml

@@ -0,0 +1 @@
+policy: "this is not rego"

policy/testdata/policies/no-rego/mapping.yaml

@@ -0,0 +1,11 @@
+# map repos to policies
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    files:
+      - path: doi/policy.yaml
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images

policy/testdata/policies/verify-sig/doi/policy.rego

@@ -0,0 +1,19 @@
+package attest
+
+import rego.v1
+
+keys := [{
+	"id": "a0c296026645799b2a297913878e81b0aefff2a0c301e97232f717e14402f3e4",
+	"key": "-----BEGIN PUBLIC KEY-----\nMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEgH23D1i2+ZIOtVjmfB7iFvX8AhVN\n9CPJ4ie9axw+WRHozGnRy99U2dRge3zueBBg2MweF0zrToXGig2v3YOrdw==\n-----END PUBLIC KEY-----",
+	"from": "2023-12-15T14:00:00Z",
+	"to": null,
+}]
+
+opts := {"keys": keys}
+
+success if {
+	some env in attest.fetch("foo")
+	statement := attest.verify(env, opts)
+}
+
+result := {"success": success}

policy/testdata/policies/verify-sig/mapping.yaml

@@ -0,0 +1,11 @@
+# map repos to policies
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images

policy/testdata/policies/wrong-key/doi/policy.rego

@@ -0,0 +1,30 @@
+package attest
+
+import rego.v1
+
+keys := [{
+	"id": "a0c296026645799b2a297913878e81b0aefff2a0c301e97232f717e14402f3e4",
+	"key": "-----BEGIN PUBLIC KEY-----\nMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEHyZpSgzvqFqNv7f3x7865OS38rAb\nQMcff55zM2UH/KR3Pr84a8QsGDNgaNGzJQJWjtMSgfV8WnNoffNK+svFNg==\n-----END PUBLIC KEY-----",
+	"from": "2023-12-15T14:00:00Z",
+	"to": null,
+}]
+
+default success := false
+
+provs(pred) := p if {
+	res := attest.fetch(pred)
+	not res.error
+	p := res.value
+}
+
+atts := union({provs("foo")})
+
+opts := {"keys": keys}
+
+success if {
+	some env in atts
+	res := attest.verify(env, opts)
+	not res.error
+}
+
+result := {"success": success}

policy/testdata/policies/wrong-key/mapping.yaml

@@ -0,0 +1,11 @@
+# map repos to policies
+version: v1
+kind: policy-mapping
+policies:
+  - id: docker-official-images
+    description: Docker Official Images
+    files:
+      - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images

policy/types.go

@@ -0,0 +1,62 @@
+package policy
+
+import (
+	"github.com/docker/attest/config"
+	"github.com/docker/attest/tuf"
+	intoto "github.com/in-toto/in-toto-golang/in_toto"
+)
+
+type Summary struct {
+	Subjects   []intoto.Subject `json:"subjects"`
+	SLSALevels []string         `json:"slsa_levels"`
+	Verifier   string           `json:"verifier"`
+	PolicyURI  string           `json:"policy_uri"`
+}
+
+type Violation struct {
+	Type        string            `json:"type"`
+	Description string            `json:"description"`
+	Attestation *intoto.Statement `json:"attestation"`
+	Details     map[string]any    `json:"details"`
+}
+
+type Result struct {
+	Success    bool        `json:"success"`
+	Violations []Violation `json:"violations"`
+	Summary    Summary     `json:"summary"`
+}
+
+type Options struct {
+	TUFClientOptions *tuf.ClientOptions
+	DisableTUF       bool
+	LocalTargetsDir  string
+	LocalPolicyDir   string
+	PolicyID         string
+	ReferrersRepo    string
+	AttestationStyle config.AttestationStyle
+	Debug            bool
+}
+
+type Policy struct {
+	InputFiles   []*File
+	Query        string
+	Mapping      *config.PolicyMapping
+	ResolvedName string
+	URI          string
+	Digest       map[string]string
+}
+
+type Input struct {
+	Digest         string `json:"digest"`
+	PURL           string `json:"purl"`
+	Tag            string `json:"tag,omitempty"`
+	Domain         string `json:"domain"`
+	NormalizedName string `json:"normalized_name"`
+	FamiliarName   string `json:"familiar_name"`
+	Platform       string `json:"platform"`
+}
+
+type File struct {
+	Path    string
+	Content []byte
+}