Improve PGP fingerprint handling

Warn if no fingerprint is specified and give more details, if
fingerprint comparison fails.

Closes #555

cmd/csaf_checker/processor.go

@@ -1449,7 +1449,7 @@ func (p *processor) checkWellknownSecurityDNS(domain string) error {
 }
 
 // checkPGPKeys checks if the OpenPGP keys are available and valid, fetches
-// the the remotely keys and compares the fingerprints.
+// the remotely keys and compares the fingerprints.
 // As a result of these a respective error messages are passed to badPGP method
 // in case of errors. It returns nil if all checks are passed.
 func (p *processor) checkPGPKeys(_ string) error {
@@ -1518,8 +1518,13 @@ func (p *processor) checkPGPKeys(_ string) error {
 			continue
 		}
 
+		if key.Fingerprint == "" {
+			p.badPGPs.warn("No fingerprint for public OpenPGP key found.")
+			continue
+		}
+
 		if !strings.EqualFold(ckey.GetFingerprint(), string(key.Fingerprint)) {
-			p.badPGPs.error("Fingerprint of public OpenPGP key %s does not match remotely loaded.", u)
+			p.badPGPs.error("Given Fingerprint (%q) of public OpenPGP key %q does not match remotely loaded (%q).", string(key.Fingerprint), u, ckey.GetFingerprint())
 			continue
 		}
 		if p.keys == nil {

cmd/csaf_downloader/downloader.go

@@ -366,10 +366,15 @@ func (d *downloader) loadOpenPGPKeys(
 			continue
 		}
 
+		if key.Fingerprint == "" {
+			slog.Warn("No fingerprint for public OpenPGP key found.")
+			continue
+		}
+
 		if !strings.EqualFold(ckey.GetFingerprint(), string(key.Fingerprint)) {
 			slog.Warn(
 				"Fingerprint of public OpenPGP key does not match remotely loaded",
-				"url", u)
+				"url", u, "fingerprint", key.Fingerprint, "remote-fingerprint", ckey.GetFingerprint())
 			continue
 		}
 		if d.keys == nil {