diff --git a/external-sop/src/main/java/sop/external/operation/ExtractCertExternal.java b/external-sop/src/main/java/sop/external/operation/ExtractCertExternal.java
deleted file mode 100644
index 538359a..0000000
--- a/external-sop/src/main/java/sop/external/operation/ExtractCertExternal.java
+++ /dev/null
@@ -1,44 +0,0 @@
-// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package sop.external.operation;
-
-import sop.Ready;
-import sop.exception.SOPGPException;
-import sop.external.ExternalSOP;
-import sop.operation.ExtractCert;
-
-import javax.annotation.Nonnull;
-import java.io.InputStream;
-import java.util.ArrayList;
-import java.util.List;
-import java.util.Properties;
-
-/**
- * Implementation of the {@link ExtractCert} operation using an external SOP binary.
- */
-public class ExtractCertExternal implements ExtractCert {
-
-    private final List<String> commandList = new ArrayList<>();
-    private final List<String> envList;
-
-    public ExtractCertExternal(String binary, Properties properties) {
-        this.commandList.add(binary);
-        this.commandList.add("extract-cert");
-        this.envList = ExternalSOP.propertiesToEnv(properties);
-    }
-
-    @Override
-    @Nonnull
-    public ExtractCert noArmor() {
-        this.commandList.add("--no-armor");
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public Ready key(@Nonnull InputStream keyInputStream) throws SOPGPException.BadData {
-        return ExternalSOP.executeTransformingOperation(Runtime.getRuntime(), commandList, envList, keyInputStream);
-    }
-}
diff --git a/external-sop/src/main/kotlin/sop/external/operation/ExtractCertExternal.kt b/external-sop/src/main/kotlin/sop/external/operation/ExtractCertExternal.kt
new file mode 100644
index 0000000..9b86733
--- /dev/null
+++ b/external-sop/src/main/kotlin/sop/external/operation/ExtractCertExternal.kt
@@ -0,0 +1,24 @@
+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package sop.external.operation
+
+import java.io.InputStream
+import java.util.Properties
+import sop.Ready
+import sop.external.ExternalSOP
+import sop.operation.ExtractCert
+
+/** Implementation of the [ExtractCert] operation using an external SOP binary. */
+class ExtractCertExternal(binary: String, environment: Properties) : ExtractCert {
+
+    private val commandList = mutableListOf(binary, "extract-cert")
+    private val envList = ExternalSOP.propertiesToEnv(environment)
+
+    override fun noArmor(): ExtractCert = apply { commandList.add("--no-armor") }
+
+    override fun key(keyInputStream: InputStream): Ready =
+        ExternalSOP.executeTransformingOperation(
+            Runtime.getRuntime(), commandList, envList, keyInputStream)
+}