Ensure RFC-6960 compliance

.gitignore

@@ -1,3 +1,4 @@
+*.der
 /.idea/
 /cacertocsp
 /config.yaml

internal/ocspsource/ocspsource.go

@@ -23,6 +23,7 @@ import (
 	"crypto/x509"
 	"crypto/x509/pkix"
 	"encoding/asn1"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"math/big"
@@ -34,6 +35,8 @@ import (
 	"golang.org/x/crypto/ocsp"
 )
 
+var unknownIssuer = errors.New("issuer key hash does not match any of the known issuers")
+
 // CertificateIssuer is an abstraction for the OCSP responder that knows certificates of a specific CA
 type CertificateIssuer struct {
 	responderCertificate *x509.Certificate
@@ -156,7 +159,16 @@ func WithIssuer(issuer *CertificateIssuer) Option {
 func (o *OcspSource) Response(r *ocsp.Request) ([]byte, http.Header, error) {
 	issuer, err := o.getIssuer(r.IssuerKeyHash, r.HashAlgorithm)
 	if err != nil {
-		return nil, nil, fmt.Errorf("cannot answer request: %v", err)
+		if errors.Is(err, unknownIssuer) {
+			logrus.Infof("received request for unknown issuer key hash %s", hex.EncodeToString(r.IssuerKeyHash))
+			issuer = o.getDefaultIssuer()
+			response, err := issuer.buildUnknownResponse(r.SerialNumber)
+			if err != nil {
+				return nil, nil, fmt.Errorf("could not build OCSP response: %v", err)
+			}
+			return response, nil, nil
+		}
+		return nil, nil, fmt.Errorf("cannot answer request: %w", err)
 	}
 
 	response, err := issuer.LookupResponse(r.SerialNumber)
@@ -177,7 +189,7 @@ func (o *OcspSource) getIssuer(keyHash []byte, algorithm crypto.Hash) (*Certific
 			return issuer, nil
 		}
 	}
-	return nil, errors.New("issuer key hash does not match any of the known issuers")
+	return nil, unknownIssuer
 }
 
 // BackgroundTasks delegates background task handling to the CertificateIssuer instances of the OcspSource
@@ -195,3 +207,7 @@ func (o *OcspSource) BackgroundTasks(ctx context.Context) {
 	wg.Wait()
 	logrus.Info("stopped OCSP source background tasks")
 }
+
+func (o *OcspSource) getDefaultIssuer() *CertificateIssuer {
+	return o.issuers[0]
+}

internal/ocspsource/opensslcertdb.go

@@ -102,7 +102,17 @@ func (o *OpenSSLCertDB) LookupResponseTemplate(number *big.Int) *ocsp.Response {
 	if response, ok := o.content[serial]; ok {
 		return response
 	}
-	return nil
+	// RFC https://datatracker.ietf.org/doc/html/rfc6960#section-2.2 states that certificates we cannot answer for
+	// should be marked as good. This would allow a CRL as sole certificate source.
+	//
+	// We might change this to an unknown response when we have a more accurate certificate source like an API or an
+	// up-to-date openssl index.txt.
+	logrus.Debugf("received request for certificate %s that is not in our data source", serial)
+	response := &ocsp.Response{
+		Status:       ocsp.Good,
+		SerialNumber: number,
+	}
+	return response
 }
 
 // WatchChanges creates file system monitoring for the index.txt file of the OpenSSLCertDB
@@ -172,7 +182,6 @@ func parseLine(line string) (string, *ocsp.Response) {
 	}
 
 	response := ocsp.Response{
-		Status:       0,
 		SerialNumber: serialNumber,
 	}