fix: verification of attestations from rekor

Few different issues here

* Intermediates from rekor need to be base64 decoded before using them.
* Trusted time stamps were only being considered while parsing entries
  from rekor but not while verifying envelopes in witness.Verify. To get
  around this I added an option to attach a trusted time to a specific
  DSSE signature which makes sure this gets used when verifying the
  envelopes.

Signed-off-by: Mikhail Swift <mikhail@testifysec.com>

Author: Mikhail Swift

pkg/cryptoutil/x509.go

@@ -68,6 +68,7 @@ func (v *X509Verifier) BelongsToRoot(root *x509.Certificate) error {
 	_, err := v.cert.Verify(x509.VerifyOptions{
 		Roots:         rootPool,
 		Intermediates: intermediatePool,
+		CurrentTime:   v.trustedTime,
 	})
 
 	return err

pkg/dsse/dsse.go

@@ -50,6 +50,8 @@ type Signature struct {
 	Signature     []byte   `json:"sig"`
 	Certificate   []byte   `json:"certificate,omitempty"`
 	Intermediates [][]byte `json:"intermediates,omitempty"`
+
+	trustedTime time.Time
 }
 
 // preauthEncode wraps the data to be signed or verified and it's type in the DSSE protocol's
@@ -112,7 +114,6 @@ type verificationOptions struct {
 	roots         []*x509.Certificate
 	intermediates []*x509.Certificate
 	verifiers     []cryptoutil.Verifier
-	trustedTime   time.Time
 }
 
 func WithRoots(roots []*x509.Certificate) VerificationOption {
@@ -133,12 +134,6 @@ func WithVerifiers(verifiers []cryptoutil.Verifier) VerificationOption {
 	}
 }
 
-func WithTrustedTime(time time.Time) VerificationOption {
-	return func(vo *verificationOptions) {
-		vo.trustedTime = time
-	}
-}
-
 func (e Envelope) Verify(opts ...VerificationOption) ([]cryptoutil.Verifier, error) {
 	options := &verificationOptions{}
 	for _, opt := range opts {
@@ -170,7 +165,7 @@ func (e Envelope) Verify(opts ...VerificationOption) ([]cryptoutil.Verifier, err
 			}
 
 			sigIntermediates = append(sigIntermediates, options.intermediates...)
-			verifier, err := cryptoutil.NewX509Verifier(cert, sigIntermediates, options.roots, options.trustedTime)
+			verifier, err := cryptoutil.NewX509Verifier(cert, sigIntermediates, options.roots, sig.trustedTime)
 			if err != nil {
 				return nil, err
 			}
@@ -213,3 +208,42 @@ func TryParseCertificate(data []byte) (*x509.Certificate, error) {
 
 	return cert, nil
 }
+
+type SignatureOption func(so *signatureOptions)
+
+type signatureOptions struct {
+	cert          []byte
+	intermediates [][]byte
+	trustedTime   time.Time
+}
+
+func SignatureWithCertificate(certBytes []byte) SignatureOption {
+	return func(so *signatureOptions) {
+		so.cert = certBytes
+	}
+}
+
+func SignatureWithIntermediates(intermediates [][]byte) SignatureOption {
+	return func(so *signatureOptions) {
+		so.intermediates = intermediates
+	}
+}
+func SignatureWithTrustedTime(trustedTime time.Time) SignatureOption {
+	return func(so *signatureOptions) {
+		so.trustedTime = trustedTime
+	}
+}
+func NewSignature(keyID string, sig []byte, opts ...SignatureOption) Signature {
+	so := signatureOptions{}
+	for _, opt := range opts {
+		opt(&so)
+	}
+
+	return Signature{
+		KeyID:         keyID,
+		Signature:     sig,
+		Certificate:   so.cert,
+		Intermediates: so.intermediates,
+		trustedTime:   so.trustedTime,
+	}
+}

pkg/rekor/rekor.go

@@ -161,21 +161,23 @@ func ParseEnvelopeFromEntry(entry *models.LogEntryAnon) (dsse.Envelope, error) {
 			return env, fmt.Errorf("failed to decode signature: %w", err)
 		}
 
-		verifier, err := cryptoutil.NewVerifierFromReader(bytes.NewReader(sig.PublicKey), cryptoutil.VerifyWithTrustedTime(time.Unix(*entry.IntegratedTime, 0)))
+		trustedTime := time.Unix(*entry.IntegratedTime, 0)
+		verifier, err := cryptoutil.NewVerifierFromReader(bytes.NewReader(sig.PublicKey), cryptoutil.VerifyWithTrustedTime(trustedTime))
 		if err != nil {
 			return env, fmt.Errorf("failed to create verifier from public key on rekor entry: %w", err)
 		}
 
-		envSig := dsse.Signature{
-			Signature: decodedSig,
-			KeyID:     sig.Keyid,
-		}
-
+		envSig := dsse.NewSignature(sig.Keyid, decodedSig, dsse.SignatureWithTrustedTime(trustedTime))
 		_, ok := verifier.(*cryptoutil.X509Verifier)
 		if ok {
 			envSig.Certificate = sig.PublicKey
 			for _, intermediate := range sig.Intermediates {
-				envSig.Intermediates = append(envSig.Intermediates, intermediate)
+				decoded, err := base64.StdEncoding.DecodeString(string(intermediate))
+				if err != nil {
+					continue
+				}
+
+				envSig.Intermediates = append(envSig.Intermediates, decoded)
 			}
 		}