diff --git a/pgpainless-core/src/main/java/org/pgpainless/decryption_verification/MissingPublicKeyCallback.java b/pgpainless-core/src/main/kotlin/org/pgpainless/decryption_verification/MissingPublicKeyCallback.kt
similarity index 67%
rename from pgpainless-core/src/main/java/org/pgpainless/decryption_verification/MissingPublicKeyCallback.java
rename to pgpainless-core/src/main/kotlin/org/pgpainless/decryption_verification/MissingPublicKeyCallback.kt
index 9b6f4a1e..723530b7 100644
--- a/pgpainless-core/src/main/java/org/pgpainless/decryption_verification/MissingPublicKeyCallback.java
+++ b/pgpainless-core/src/main/kotlin/org/pgpainless/decryption_verification/MissingPublicKeyCallback.kt
@@ -1,33 +1,28 @@
-// SPDX-FileCopyrightText: 2018 Paul Schaub <vanitasvitae@fsfe.org>
+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
 //
 // SPDX-License-Identifier: Apache-2.0
 
-package org.pgpainless.decryption_verification;
+package org.pgpainless.decryption_verification
 
-import javax.annotation.Nonnull;
-import javax.annotation.Nullable;
+import org.bouncycastle.openpgp.PGPPublicKeyRing
 
-import org.bouncycastle.openpgp.PGPPublicKeyRing;
-
-public interface MissingPublicKeyCallback {
+fun interface MissingPublicKeyCallback {
 
     /**
      * This method gets called if we encounter a signature made by a key which was not provided for signature verification.
      * If you cannot provide the requested key, it is safe to return null here.
      * PGPainless will then continue verification with the next signature.
      *
-     * Note: The key-id might belong to a subkey, so be aware that when looking up the {@link PGPPublicKeyRing},
+     * Note: The key-id might belong to a subkey, so be aware that when looking up the [PGPPublicKeyRing],
      * you may not only search for the key-id on the key rings primary key!
      *
      * It would be super cool to provide the OpenPgp fingerprint here, but unfortunately one-pass-signatures
      * only contain the key id.
      *
      * @param keyId ID of the missing signing (sub)key
-     *
      * @return keyring containing the key or null
      *
      * @see <a href="https://datatracker.ietf.org/doc/html/rfc4880#section-5.4">RFC</a>
      */
-    @Nullable PGPPublicKeyRing onMissingPublicKeyEncountered(@Nonnull Long keyId);
-
-}
+    fun onMissingPublicKeyEncountered(keyId: Long): PGPPublicKeyRing?
+}
\ No newline at end of file
diff --git a/pgpainless-core/src/test/java/org/pgpainless/decryption_verification/VerifyWithMissingPublicKeyCallbackTest.java b/pgpainless-core/src/test/java/org/pgpainless/decryption_verification/VerifyWithMissingPublicKeyCallbackTest.java
index 52877626..0d58e7dd 100644
--- a/pgpainless-core/src/test/java/org/pgpainless/decryption_verification/VerifyWithMissingPublicKeyCallbackTest.java
+++ b/pgpainless-core/src/test/java/org/pgpainless/decryption_verification/VerifyWithMissingPublicKeyCallbackTest.java
@@ -14,7 +14,6 @@ import java.io.IOException;
 import java.nio.charset.StandardCharsets;
 import java.security.InvalidAlgorithmParameterException;
 import java.security.NoSuchAlgorithmException;
-import javax.annotation.Nonnull;
 
 import org.bouncycastle.openpgp.PGPException;
 import org.bouncycastle.openpgp.PGPPublicKey;
@@ -63,8 +62,8 @@ public class VerifyWithMissingPublicKeyCallbackTest {
                         .addVerificationCert(unrelatedKeys)
                         .setMissingCertificateCallback(new MissingPublicKeyCallback() {
                             @Override
-                            public PGPPublicKeyRing onMissingPublicKeyEncountered(@Nonnull Long keyId) {
-                                assertEquals(signingKey.getKeyID(), (long) keyId, "Signing key-ID mismatch.");
+                            public PGPPublicKeyRing onMissingPublicKeyEncountered(long keyId) {
+                                assertEquals(signingKey.getKeyID(), keyId, "Signing key-ID mismatch.");
                                 return signingPubKeys;
                             }
                         }));