Verify embedded SCTs

cmd/cosign/cli/dockerfile.go

@@ -92,6 +92,7 @@ Shell-like variables in the Dockerfile's FROM lines will be substituted with val
 					CertEmail:       o.CertVerify.CertEmail,
 					CertOidcIssuer:  o.CertVerify.CertOidcIssuer,
 					CertChain:       o.CertVerify.CertChain,
+					EnforceSCT:      o.CertVerify.EnforceSCT,
 					Sk:              o.SecurityKey.Use,
 					Slot:            o.SecurityKey.Slot,
 					Output:          o.Output,

cmd/cosign/cli/fulcio/depcheck_test.go

@@ -25,7 +25,6 @@ func TestNoDeps(t *testing.T) {
 	depcheck.AssertNoDependency(t, map[string][]string{
 		"github.com/sigstore/cosign/cmd/cosign/cli/fulcio": {
 			// Avoid pulling in a variety of things that are massive dependencies.
-			"github.com/google/certificate-transparency-go",
 			"github.com/google/trillian",
 			"github.com/envoyproxy/go-control-plane",
 			"github.com/gogo/protobuf/protoc-gen-gogo",

cmd/cosign/cli/fulcio/fulcioverifier/ctl/verify.go

@@ -0,0 +1,229 @@
+// Copyright 2022 The Sigstore Authors.
+//
+// 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 ctl
+
+import (
+	"context"
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/sha256"
+	"crypto/x509"
+	"encoding/json"
+	"encoding/pem"
+	"fmt"
+	"os"
+
+	ct "github.com/google/certificate-transparency-go"
+	ctx509 "github.com/google/certificate-transparency-go/x509"
+	"github.com/google/certificate-transparency-go/x509util"
+	"github.com/pkg/errors"
+	"github.com/sigstore/cosign/cmd/cosign/cli/fulcio/fulcioverifier/ctutil"
+
+	"github.com/sigstore/cosign/pkg/cosign/tuf"
+	"github.com/sigstore/sigstore/pkg/cryptoutils"
+)
+
+// This is the CT log public key target name
+var ctPublicKeyStr = `ctfe.pub`
+
+// Setting this env variable will over ride what is used to validate
+// the SCT coming back from Fulcio.
+const altCTLogPublicKeyLocation = "SIGSTORE_CT_LOG_PUBLIC_KEY_FILE"
+
+// logIDMetadata holds information for mapping a key ID hash (log ID) to associated data.
+type logIDMetadata struct {
+	pubKey crypto.PublicKey
+	status tuf.StatusKind
+}
+
+// ContainsSCT checks if the certificate contains embedded SCTs. cert can either be
+// DER or PEM encoded.
+func ContainsSCT(cert []byte) (bool, error) {
+	embeddedSCTs, err := x509util.ParseSCTsFromCertificate(cert)
+	if err != nil {
+		return false, err
+	}
+	if len(embeddedSCTs) != 0 {
+		return true, nil
+	}
+	return false, nil
+}
+
+// VerifySCT verifies SCTs against the Fulcio CT log public key.
+//
+// The SCT is a `Signed Certificate Timestamp`, which promises that
+// the certificate issued by Fulcio was also added to the public CT log within
+// some defined time period.
+//
+// VerifySCT can verify an SCT list embedded in the certificate, or a detached
+// SCT provided by Fulcio.
+//
+// By default the public keys comes from TUF, but you can override this for test
+// purposes by using an env variable `SIGSTORE_CT_LOG_PUBLIC_KEY_FILE`. If using
+// an alternate, the file can be PEM, or DER format.
+func VerifySCT(ctx context.Context, certPEM, chainPEM, rawSCT []byte) error {
+	// fetch SCT verification key
+	pubKeys := make(map[[sha256.Size]byte]logIDMetadata)
+	rootEnv := os.Getenv(altCTLogPublicKeyLocation)
+	if rootEnv == "" {
+		tufClient, err := tuf.NewFromEnv(ctx)
+		if err != nil {
+			return err
+		}
+		defer tufClient.Close()
+
+		targets, err := tufClient.GetTargetsByMeta(tuf.CTFE, []string{ctPublicKeyStr})
+		if err != nil {
+			return err
+		}
+		for _, t := range targets {
+			pub, err := cryptoutils.UnmarshalPEMToPublicKey(t.Target)
+			if err != nil {
+				return err
+			}
+			ctPub, ok := pub.(*ecdsa.PublicKey)
+			if !ok {
+				return fmt.Errorf("invalid public key: was %T, require *ecdsa.PublicKey", pub)
+			}
+			keyID, err := ctutil.GetCTLogID(ctPub)
+			if err != nil {
+				return errors.Wrap(err, "error getting CTFE public key hash")
+			}
+			pubKeys[keyID] = logIDMetadata{ctPub, t.Status}
+		}
+	} else {
+		fmt.Fprintf(os.Stderr, "**Warning** Using a non-standard public key for verifying SCT: %s\n", rootEnv)
+		raw, err := os.ReadFile(rootEnv)
+		if err != nil {
+			return errors.Wrap(err, "error reading alternate public key file")
+		}
+		pubKey, err := getAlternatePublicKey(raw)
+		if err != nil {
+			return errors.Wrap(err, "error parsing alternate public key from the file")
+		}
+		keyID, err := ctutil.GetCTLogID(pubKey)
+		if err != nil {
+			return errors.Wrap(err, "error getting CTFE public key hash")
+		}
+		pubKeys[keyID] = logIDMetadata{pubKey, tuf.Active}
+	}
+	if len(pubKeys) == 0 {
+		return errors.New("none of the CTFE keys have been found")
+	}
+
+	// parse certificate and chain
+	cert, err := x509util.CertificateFromPEM(certPEM)
+	if err != nil {
+		return err
+	}
+	certChain, err := x509util.CertificatesFromPEM(chainPEM)
+	if err != nil {
+		return err
+	}
+	if len(certChain) == 0 {
+		return errors.New("no certificate chain found")
+	}
+
+	// fetch embedded SCT if present
+	embeddedSCTs, err := x509util.ParseSCTsFromCertificate(certPEM)
+	if err != nil {
+		return err
+	}
+	// SCT must be either embedded or in header
+	if len(embeddedSCTs) == 0 && len(rawSCT) == 0 {
+		return errors.New("no SCT found")
+	}
+
+	// check SCT embedded in certificate
+	if len(embeddedSCTs) != 0 {
+		for _, sct := range embeddedSCTs {
+			pubKeyMetadata, ok := pubKeys[sct.LogID.KeyID]
+			if !ok {
+				return errors.New("ctfe public key not found for embedded SCT")
+			}
+			err := ctutil.VerifySCT(pubKeyMetadata.pubKey, []*ctx509.Certificate{cert, certChain[0]}, sct, true)
+			if err != nil {
+				return errors.Wrap(err, "error verifying embedded SCT")
+			}
+			if pubKeyMetadata.status != tuf.Active {
+				fmt.Fprintf(os.Stderr, "**Info** Successfully verified embedded SCT using an expired verification key\n")
+			}
+		}
+		return nil
+	}
+
+	// check SCT in response header
+	var addChainResp ct.AddChainResponse
+	if err := json.Unmarshal(rawSCT, &addChainResp); err != nil {
+		return errors.Wrap(err, "unmarshal")
+	}
+	sct, err := addChainResp.ToSignedCertificateTimestamp()
+	if err != nil {
+		return err
+	}
+	pubKeyMetadata, ok := pubKeys[sct.LogID.KeyID]
+	if !ok {
+		return errors.New("ctfe public key not found")
+	}
+	err = ctutil.VerifySCT(pubKeyMetadata.pubKey, []*ctx509.Certificate{cert}, sct, false)
+	if err != nil {
+		return errors.Wrap(err, "error verifying SCT")
+	}
+	if pubKeyMetadata.status != tuf.Active {
+		fmt.Fprintf(os.Stderr, "**Info** Successfully verified SCT using an expired verification key\n")
+	}
+	return nil
+}
+
+// VerifyEmbeddedSCT verifies an embedded SCT in a certificate.
+func VerifyEmbeddedSCT(ctx context.Context, chain []*x509.Certificate) error {
+	if len(chain) < 2 {
+		return errors.New("certificate chain must contain at least a certificate and its issuer")
+	}
+	certPEM, err := cryptoutils.MarshalCertificateToPEM(chain[0])
+	if err != nil {
+		return err
+	}
+	chainPEM, err := cryptoutils.MarshalCertificatesToPEM(chain[1:])
+	if err != nil {
+		return err
+	}
+	return VerifySCT(ctx, certPEM, chainPEM, []byte{})
+}
+
+// Given a byte array, try to construct a public key from it.
+// Will try first to see if it's PEM formatted, if not, then it will
+// try to parse it as der publics, and failing that
+func getAlternatePublicKey(in []byte) (crypto.PublicKey, error) {
+	var pubKey crypto.PublicKey
+	var err error
+	var derBytes []byte
+	pemBlock, _ := pem.Decode(in)
+	if pemBlock == nil {
+		fmt.Fprintf(os.Stderr, "Failed to decode non-standard public key for verifying SCT using PEM decode, trying as DER")
+		derBytes = in
+	} else {
+		derBytes = pemBlock.Bytes
+	}
+	pubKey, err = x509.ParsePKIXPublicKey(derBytes)
+	if err != nil {
+		// Try using the PKCS1 before giving up.
+		pubKey, err = x509.ParsePKCS1PublicKey(derBytes)
+		if err != nil {
+			return nil, errors.Wrap(err, "failed to parse alternate public key")
+		}
+	}
+	return pubKey, nil
+}