Added der support and a sample der cert in the certs folder.

main.go

@@ -45,6 +45,8 @@ var fileExtToFormat = map[string]string{
 	".p12":   "PKCS12",
 	".pfx":   "PKCS12",
 	".jceks": "JCEKS",
+	".jks":   "JCEKS",
+	".der":   "DER",
 }
 
 type certWithAlias struct {
@@ -105,6 +107,12 @@ func getCerts(file, format string) ([]certWithAlias, error) {
 			certs = append(certs, certWithAlias{cert: cert})
 			block, data = pem.Decode(data)
 		}
+	case "DER":
+		cert, err := x509.ParseCertificate(data)
+		if err != nil {
+			return nil, err
+		}
+		certs = append(certs, certWithAlias{cert: cert})
 	case "PKCS12":
 		scanner := bufio.NewReader(os.Stdin)
 		fmt.Print("Enter password: ")