diff --git a/pgpainless-core/src/main/java/org/pgpainless/key/util/KeyIdUtil.java b/pgpainless-core/src/main/java/org/pgpainless/key/util/KeyIdUtil.java
deleted file mode 100644
index 78e03f71..00000000
--- a/pgpainless-core/src/main/java/org/pgpainless/key/util/KeyIdUtil.java
+++ /dev/null
@@ -1,37 +0,0 @@
-// SPDX-FileCopyrightText: 2021 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package org.pgpainless.key.util;
-
-import java.math.BigInteger;
-import java.util.regex.Pattern;
-
-public final class KeyIdUtil {
-
-    private KeyIdUtil() {
-
-    }
-
-    private static final Pattern LONG_KEY_ID = Pattern.compile("^[0-9A-Fa-f]{16}$");
-
-    /**
-     * Convert a long key-id into a key-id.
-     * A long key-id is a 16 digit hex string.
-     *
-     * @param longKeyId 16-digit hexadecimal string
-     * @return key-id converted to {@link Long}.
-     */
-    public static long fromLongKeyId(String longKeyId) {
-        if (!LONG_KEY_ID.matcher(longKeyId).matches()) {
-            throw new IllegalArgumentException("Provided long key-id does not match expected format. " +
-                    "A long key-id consists of 16 hexadecimal characters.");
-        }
-
-        return new BigInteger(longKeyId, 16).longValue();
-    }
-
-    public static String formatKeyId(long keyId) {
-        return String.format("%016X", keyId);
-    }
-}
diff --git a/pgpainless-core/src/main/kotlin/_kotlin/LongExtensions.kt b/pgpainless-core/src/main/kotlin/_kotlin/LongExtensions.kt
new file mode 100644
index 00000000..b2c64e2e
--- /dev/null
+++ b/pgpainless-core/src/main/kotlin/_kotlin/LongExtensions.kt
@@ -0,0 +1,25 @@
+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package _kotlin
+
+/**
+ * Format this Long as a 16 digit uppercase hex number.
+ */
+fun Long.hexKeyId(): String {
+    return String.format("%016X", this).uppercase()
+}
+
+/**
+ * Parse a 16 digit hex number into a Long.
+ */
+fun Long.Companion.fromHexKeyId(hexKeyId: String): Long {
+    require("^[0-9A-Fa-f]{16}$".toRegex().matches(hexKeyId)) {
+        "Provided long key-id does not match expected format. " +
+                "A long key-id consists of 16 hexadecimal characters."
+    }
+    // Calling toLong() only fails with a NumberFormatException.
+    //  Therefore, we call toULong(16).toLong(), which seems to work.
+    return hexKeyId.toULong(16).toLong()
+}
\ No newline at end of file
diff --git a/pgpainless-core/src/main/kotlin/org/pgpainless/key/util/KeyIdUtil.kt b/pgpainless-core/src/main/kotlin/org/pgpainless/key/util/KeyIdUtil.kt
new file mode 100644
index 00000000..b6ff1789
--- /dev/null
+++ b/pgpainless-core/src/main/kotlin/org/pgpainless/key/util/KeyIdUtil.kt
@@ -0,0 +1,36 @@
+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.key.util
+
+import _kotlin.fromHexKeyId
+import _kotlin.hexKeyId
+
+class KeyIdUtil {
+
+    companion object {
+
+        /**
+         * Convert a long key-id into a key-id.
+         * A long key-id is a 16 digit hex string.
+         *
+         * @param longKeyId 16-digit hexadecimal string
+         * @return key-id converted to {@link Long}.
+         */
+        @JvmStatic
+        @Deprecated("Superseded by Long extension method.",
+                ReplaceWith("Long.fromHexKeyId(longKeyId)"))
+        fun fromLongKeyId(longKeyId: String) = Long.fromHexKeyId(longKeyId)
+
+        /**
+         * Format a long key-ID as upper-case hex string.
+         * @param keyId keyId
+         * @return hex encoded key ID
+         */
+        @JvmStatic
+        @Deprecated("Superseded by Long extension method.",
+                ReplaceWith("keyId.hexKeyId()"))
+        fun formatKeyId(keyId: Long) = keyId.hexKeyId()
+    }
+}
\ No newline at end of file