diff --git a/external-sop/src/main/java/sop/external/operation/GenerateKeyExternal.java b/external-sop/src/main/java/sop/external/operation/GenerateKeyExternal.java
deleted file mode 100644
index a8244cb..0000000
--- a/external-sop/src/main/java/sop/external/operation/GenerateKeyExternal.java
+++ /dev/null
@@ -1,78 +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.GenerateKey;
-
-import javax.annotation.Nonnull;
-import java.util.ArrayList;
-import java.util.List;
-import java.util.Properties;
-
-/**
- * Implementation of the {@link GenerateKey} operation using an external SOP binary.
- */
-public class GenerateKeyExternal implements GenerateKey {
-
-    private final List<String> commandList = new ArrayList<>();
-    private final List<String> envList;
-
-    private int keyPasswordCounter = 0;
-
-    public GenerateKeyExternal(String binary, Properties environment) {
-        this.commandList.add(binary);
-        this.commandList.add("generate-key");
-        this.envList = ExternalSOP.propertiesToEnv(environment);
-    }
-
-    @Override
-    @Nonnull
-    public GenerateKey noArmor() {
-        this.commandList.add("--no-armor");
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public GenerateKey userId(@Nonnull String userId) {
-        this.commandList.add(userId);
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public GenerateKey withKeyPassword(@Nonnull String password)
-            throws SOPGPException.PasswordNotHumanReadable, SOPGPException.UnsupportedOption {
-        this.commandList.add("--with-key-password=@ENV:KEY_PASSWORD_" + keyPasswordCounter);
-        this.envList.add("KEY_PASSWORD_" + keyPasswordCounter + "=" + password);
-        keyPasswordCounter++;
-
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public GenerateKey profile(@Nonnull String profile) {
-        commandList.add("--profile=" + profile);
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public GenerateKey signingOnly() {
-        commandList.add("--signing-only");
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public Ready generate()
-            throws SOPGPException.MissingArg, SOPGPException.UnsupportedAsymmetricAlgo {
-        return ExternalSOP.executeProducingOperation(Runtime.getRuntime(), commandList, envList);
-    }
-}
diff --git a/external-sop/src/main/kotlin/sop/external/operation/GenerateKeyExternal.kt b/external-sop/src/main/kotlin/sop/external/operation/GenerateKeyExternal.kt
new file mode 100644
index 0000000..37116a4
--- /dev/null
+++ b/external-sop/src/main/kotlin/sop/external/operation/GenerateKeyExternal.kt
@@ -0,0 +1,38 @@
+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package sop.external.operation
+
+import java.util.Properties
+import sop.Ready
+import sop.external.ExternalSOP
+import sop.operation.GenerateKey
+
+/** Implementation of the [GenerateKey] operation using an external SOP binary. */
+class GenerateKeyExternal(binary: String, environment: Properties) : GenerateKey {
+
+    private val commandList = mutableListOf(binary, "generate-key")
+    private val envList = ExternalSOP.propertiesToEnv(environment).toMutableList()
+
+    private var argCounter = 0
+
+    override fun noArmor(): GenerateKey = apply { commandList.add("--no-armor") }
+
+    override fun userId(userId: String): GenerateKey = apply { commandList.add(userId) }
+
+    override fun withKeyPassword(password: String): GenerateKey = apply {
+        commandList.add("--with-key-password=@ENV:KEY_PASSWORD_$argCounter")
+        envList.add("KEY_PASSWORD_$argCounter=$password")
+        argCounter += 1
+    }
+
+    override fun profile(profile: String): GenerateKey = apply {
+        commandList.add("--profile=$profile")
+    }
+
+    override fun signingOnly(): GenerateKey = apply { commandList.add("--signing-only") }
+
+    override fun generate(): Ready =
+        ExternalSOP.executeProducingOperation(Runtime.getRuntime(), commandList, envList)
+}