diff --git a/external-sop/src/main/java/sop/external/operation/InlineSignExternal.java b/external-sop/src/main/java/sop/external/operation/InlineSignExternal.java
deleted file mode 100644
index 1a86002..0000000
--- a/external-sop/src/main/java/sop/external/operation/InlineSignExternal.java
+++ /dev/null
@@ -1,74 +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.enums.InlineSignAs;
-import sop.exception.SOPGPException;
-import sop.external.ExternalSOP;
-import sop.operation.InlineSign;
-
-import javax.annotation.Nonnull;
-import java.io.IOException;
-import java.io.InputStream;
-import java.util.ArrayList;
-import java.util.List;
-import java.util.Properties;
-
-/**
- * Implementation of the {@link InlineSign} operation using an external SOP binary.
- */
-public class InlineSignExternal implements InlineSign {
-
-    private final List<String> commandList = new ArrayList<>();
-    private final List<String> envList;
-
-    private int keyCounter = 0;
-    private int withKeyPasswordCounter = 0;
-
-    public InlineSignExternal(String binary, Properties environment) {
-        commandList.add(binary);
-        commandList.add("inline-sign");
-        envList = ExternalSOP.propertiesToEnv(environment);
-    }
-
-    @Override
-    @Nonnull
-    public InlineSign noArmor() {
-        commandList.add("--no-armor");
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public InlineSign key(@Nonnull InputStream key) throws SOPGPException.KeyCannotSign, SOPGPException.BadData, SOPGPException.UnsupportedAsymmetricAlgo, IOException {
-        String envVar = "KEY_" + keyCounter++;
-        commandList.add("@ENV:" + envVar);
-        envList.add(envVar + "=" + ExternalSOP.readString(key));
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public InlineSign withKeyPassword(@Nonnull byte[] password) throws SOPGPException.UnsupportedOption, SOPGPException.PasswordNotHumanReadable {
-        String envVar = "WITH_KEY_PASSWORD_" + withKeyPasswordCounter++;
-        commandList.add("--with-key-password=@ENV:" + envVar);
-        envList.add(envVar + "=" + new String(password));
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public InlineSign mode(@Nonnull InlineSignAs mode) throws SOPGPException.UnsupportedOption {
-        commandList.add("--as=" + mode);
-        return this;
-    }
-
-    @Override
-    @Nonnull
-    public Ready data(@Nonnull InputStream data) throws SOPGPException.KeyIsProtected, SOPGPException.ExpectedText {
-        return ExternalSOP.executeTransformingOperation(Runtime.getRuntime(), commandList, envList, data);
-    }
-}
diff --git a/external-sop/src/main/kotlin/sop/external/operation/InlineSignExternal.kt b/external-sop/src/main/kotlin/sop/external/operation/InlineSignExternal.kt
new file mode 100644
index 0000000..a304e85
--- /dev/null
+++ b/external-sop/src/main/kotlin/sop/external/operation/InlineSignExternal.kt
@@ -0,0 +1,40 @@
+// 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.enums.InlineSignAs
+import sop.external.ExternalSOP
+import sop.operation.InlineSign
+
+/** Implementation of the [InlineSign] operation using an external SOP binary. */
+class InlineSignExternal(binary: String, environment: Properties) : InlineSign {
+
+    private val commandList = mutableListOf(binary, "inline-sign")
+    private val envList = ExternalSOP.propertiesToEnv(environment).toMutableList()
+
+    private var argCounter = 0
+
+    override fun mode(mode: InlineSignAs): InlineSign = apply { commandList.add("--as=$mode") }
+
+    override fun data(data: InputStream): Ready =
+        ExternalSOP.executeTransformingOperation(Runtime.getRuntime(), commandList, envList, data)
+
+    override fun noArmor(): InlineSign = apply { commandList.add("--no-armor") }
+
+    override fun key(key: InputStream): InlineSign = apply {
+        commandList.add("@ENV:KEY_$argCounter")
+        envList.add("KEY_$argCounter=${ExternalSOP.readString(key)}")
+        argCounter += 1
+    }
+
+    override fun withKeyPassword(password: ByteArray): InlineSign = apply {
+        commandList.add("--with-key-password=@ENV:WITH_KEY_PASSWORD_$argCounter")
+        envList.add("WITH_KEY_PASSWORD_$argCounter=${String(password)}")
+        argCounter += 1
+    }
+}