diff --git a/external-sop/src/main/java/sop/external/operation/ChangeKeyPasswordExternal.java b/external-sop/src/main/java/sop/external/operation/ChangeKeyPasswordExternal.java
deleted file mode 100644
index d53d6a5..0000000
--- a/external-sop/src/main/java/sop/external/operation/ChangeKeyPasswordExternal.java
+++ /dev/null
@@ -1,62 +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.ChangeKeyPassword;
-
-import javax.annotation.Nonnull;
-import java.io.InputStream;
-import java.util.ArrayList;
-import java.util.List;
-import java.util.Properties;
-
-public class ChangeKeyPasswordExternal implements ChangeKeyPassword {
-    private final List<String> commandList = new ArrayList<>();
-    private final List<String> envList;
-
-    private int keyPasswordCounter = 0;
-
-    public ChangeKeyPasswordExternal(String binary, Properties environment) {
-        this.commandList.add(binary);
-        this.commandList.add("decrypt");
-        this.envList = ExternalSOP.propertiesToEnv(environment);
-    }
-
-    @Override
-    @Nonnull
-    public ChangeKeyPassword noArmor() {
-        this.commandList.add("--no-armor");
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public ChangeKeyPassword oldKeyPassphrase(@Nonnull String oldPassphrase) {
-        this.commandList.add("--old-key-password=@ENV:KEY_PASSWORD_" + keyPasswordCounter);
-        this.envList.add("KEY_PASSWORD_" + keyPasswordCounter + "=" + oldPassphrase);
-        keyPasswordCounter++;
-
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public ChangeKeyPassword newKeyPassphrase(@Nonnull String newPassphrase) {
-        this.commandList.add("--new-key-password=@ENV:KEY_PASSWORD_" + keyPasswordCounter);
-        this.envList.add("KEY_PASSWORD_" + keyPasswordCounter + "=" + newPassphrase);
-        keyPasswordCounter++;
-
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public Ready keys(@Nonnull InputStream inputStream) throws SOPGPException.KeyIsProtected, SOPGPException.BadData {
-        return ExternalSOP.executeTransformingOperation(Runtime.getRuntime(), commandList, envList, inputStream);
-    }
-}
diff --git a/external-sop/src/main/kotlin/sop/external/operation/ChangeKeyPasswordExternal.kt b/external-sop/src/main/kotlin/sop/external/operation/ChangeKeyPasswordExternal.kt
new file mode 100644
index 0000000..a45e59f
--- /dev/null
+++ b/external-sop/src/main/kotlin/sop/external/operation/ChangeKeyPasswordExternal.kt
@@ -0,0 +1,37 @@
+// 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.ChangeKeyPassword
+
+/** Implementation of the [ChangeKeyPassword] operation using an external SOP binary. */
+class ChangeKeyPasswordExternal(binary: String, environment: Properties) : ChangeKeyPassword {
+
+    private val commandList: MutableList<String> = mutableListOf(binary, "change-key-password")
+    private val envList = ExternalSOP.propertiesToEnv(environment).toMutableList()
+
+    private var keyPasswordCounter = 0
+
+    override fun noArmor(): ChangeKeyPassword = apply { commandList.add("--no-armor") }
+
+    override fun oldKeyPassphrase(oldPassphrase: String): ChangeKeyPassword = apply {
+        commandList.add("--old-key-password=@ENV:KEY_PASSWORD_$keyPasswordCounter")
+        envList.add("KEY_PASSWORD_$keyPasswordCounter=$oldPassphrase")
+        keyPasswordCounter += 1
+    }
+
+    override fun newKeyPassphrase(newPassphrase: String): ChangeKeyPassword = apply {
+        commandList.add("--new-key-password=@ENV:KEY_PASSWORD_$keyPasswordCounter")
+        envList.add("KEY_PASSWORD_$keyPasswordCounter=$newPassphrase")
+        keyPasswordCounter += 1
+    }
+
+    override fun keys(keys: InputStream): Ready =
+        ExternalSOP.executeTransformingOperation(Runtime.getRuntime(), commandList, envList, keys)
+}