diff --git a/pgpainless-core/src/test/java/org/pgpainless/key/protection/PassphraseTest.java b/pgpainless-core/src/test/java/org/pgpainless/key/protection/PassphraseTest.java
index 1aa1b80c..1b021d01 100644
--- a/pgpainless-core/src/test/java/org/pgpainless/key/protection/PassphraseTest.java
+++ b/pgpainless-core/src/test/java/org/pgpainless/key/protection/PassphraseTest.java
@@ -16,7 +16,9 @@
 package org.pgpainless.key.protection;
 
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
@@ -68,4 +70,20 @@ public class PassphraseTest {
         assertNull(trimmedEmpty.getChars());
         assertTrue(trimmedEmpty.isEmpty());
     }
+
+    @Test
+    public void equalsTest() {
+        assertNotEquals(Passphrase.fromPassword("passphrase"), Passphrase.fromPassword("Password"));
+        assertNotEquals(Passphrase.fromPassword("password"), null);
+        assertNotEquals(Passphrase.fromPassword("password"), "password");
+        Passphrase passphrase = Passphrase.fromPassword("passphrase");
+        assertEquals(passphrase, passphrase);
+    }
+
+    @Test
+    public void hashCodeTest() {
+        assertNotEquals(0, Passphrase.fromPassword("passphrase").hashCode());
+        assertNotEquals(Passphrase.fromPassword("passphrase").hashCode(), Passphrase.fromPassword("password").hashCode());
+        assertEquals(0, Passphrase.emptyPassphrase().hashCode());
+    }
 }
diff --git a/pgpainless-core/src/test/java/org/pgpainless/util/ArmorUtilsTest.java b/pgpainless-core/src/test/java/org/pgpainless/util/ArmorUtilsTest.java
index e254dea7..019fed58 100644
--- a/pgpainless-core/src/test/java/org/pgpainless/util/ArmorUtilsTest.java
+++ b/pgpainless-core/src/test/java/org/pgpainless/util/ArmorUtilsTest.java
@@ -17,6 +17,7 @@ package org.pgpainless.util;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
@@ -26,8 +27,11 @@ import java.util.List;
 
 import org.bouncycastle.bcpg.ArmoredInputStream;
 import org.bouncycastle.bcpg.ArmoredOutputStream;
+import org.bouncycastle.openpgp.PGPException;
+import org.bouncycastle.openpgp.PGPSecretKeyRing;
 import org.junit.jupiter.api.Test;
 import org.pgpainless.algorithm.HashAlgorithm;
+import org.pgpainless.key.TestKeys;
 
 public class ArmorUtilsTest {
 
@@ -89,4 +93,31 @@ public class ArmorUtilsTest {
         assertThrows(IllegalArgumentException.class, () -> ArmorUtils.addMessageIdHeader(armor, "contains spaces 7890123456789012"));
         assertThrows(IllegalArgumentException.class, () -> ArmorUtils.addMessageIdHeader(armor, "contains\nnewlines\n12345678901234"));
     }
+
+    @Test
+    public void testAddCommentAndHashHeaders() throws PGPException, IOException {
+        PGPSecretKeyRing secretKeys = TestKeys.getEmilSecretKeyRing();
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        ArmoredOutputStream armor = new ArmoredOutputStream(out);
+        ArmorUtils.addCommentHeader(armor, "This is a comment.");
+        ArmorUtils.addHashAlgorithmHeader(armor, HashAlgorithm.SHA224);
+
+        secretKeys.encode(armor);
+        armor.close();
+
+        String armored = out.toString();
+        assertTrue(armored.contains("Hash: SHA224"));
+        assertTrue(armored.contains("Comment: This is a comment."));
+    }
+
+    @Test
+    public void toAsciiArmoredString() throws PGPException, IOException {
+        PGPSecretKeyRing secretKeys = TestKeys.getEmilSecretKeyRing();
+        ByteArrayOutputStream bytes = new ByteArrayOutputStream();
+        secretKeys.encode(bytes);
+
+        ByteArrayInputStream in = new ByteArrayInputStream(bytes.toByteArray());
+        String ascii = ArmorUtils.toAsciiArmoredString(in);
+        assertTrue(ascii.startsWith("-----BEGIN PGP PRIVATE KEY BLOCK-----\n"));
+    }
 }
diff --git a/pgpainless-core/src/test/java/org/pgpainless/util/MultiMapTest.java b/pgpainless-core/src/test/java/org/pgpainless/util/MultiMapTest.java
index fa21efdb..6699ee5f 100644
--- a/pgpainless-core/src/test/java/org/pgpainless/util/MultiMapTest.java
+++ b/pgpainless-core/src/test/java/org/pgpainless/util/MultiMapTest.java
@@ -128,5 +128,6 @@ public class MultiMapTest {
         assertEquals(fromMap.get("key"), Collections.singleton("value"));
 
         assertNotEquals(fromMap, map);
+        assertNotEquals(fromMap, null);
     }
 }
diff --git a/pgpainless-core/src/test/java/org/pgpainless/util/NotationRegistryTest.java b/pgpainless-core/src/test/java/org/pgpainless/util/NotationRegistryTest.java
index 002b24b8..e646a2b5 100644
--- a/pgpainless-core/src/test/java/org/pgpainless/util/NotationRegistryTest.java
+++ b/pgpainless-core/src/test/java/org/pgpainless/util/NotationRegistryTest.java
@@ -16,6 +16,7 @@
 package org.pgpainless.util;
 
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import org.junit.jupiter.api.Test;
@@ -37,4 +38,10 @@ public class NotationRegistryTest {
         assertFalse(registry.isKnownNotation("proof@metacode.biz"), "Notation is no longer known after registry is cleared.");
         assertFalse(registry.isKnownNotation("unknown@notation.data"));
     }
+
+    @Test
+    public void addKnownNotation_nullThrows() {
+        NotationRegistry registry = new NotationRegistry();
+        assertThrows(NullPointerException.class, () -> registry.addKnownNotation(null));
+    }
 }