diff --git a/pgpainless-core/src/test/java/org/pgpainless/decryption_verification/syntax_check/PDATest.java b/pgpainless-core/src/test/java/org/pgpainless/decryption_verification/syntax_check/PDATest.java
index e4877d94..8fa107f6 100644
--- a/pgpainless-core/src/test/java/org/pgpainless/decryption_verification/syntax_check/PDATest.java
+++ b/pgpainless-core/src/test/java/org/pgpainless/decryption_verification/syntax_check/PDATest.java
@@ -73,6 +73,42 @@ public class PDATest {
         assertTrue(check.isValid());
     }
 
+    @Test
+    public void testOPSSignedEncryptedMessageIsValid() {
+        PDA check = new PDA();
+        check.next(InputAlphabet.OnePassSignature);
+        check.next(InputAlphabet.EncryptedData);
+        check.next(InputAlphabet.Signature);
+        check.next(InputAlphabet.EndOfSequence);
+        assertTrue(check.isValid());
+    }
+
+    @Test
+    public void anyInputAfterEOSIsNotValid() {
+        PDA check = new PDA();
+        check.next(InputAlphabet.LiteralData);
+        check.next(InputAlphabet.EndOfSequence);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> check.next(InputAlphabet.Signature));
+    }
+
+    @Test
+    public void testEncryptedMessageWithAppendedStandalongSigIsNotValid() {
+        PDA check = new PDA();
+        check.next(InputAlphabet.EncryptedData);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> check.next(InputAlphabet.Signature));
+    }
+
+    @Test
+    public void testOPSSignedEncryptedMessageWithMissingSigIsNotValid() {
+        PDA check = new PDA();
+        check.next(InputAlphabet.OnePassSignature);
+        check.next(InputAlphabet.EncryptedData);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> check.next(InputAlphabet.EndOfSequence));
+    }
+
     @Test
     public void testTwoLiteralDataIsNotValid() {
         PDA check = new PDA();
@@ -89,6 +125,48 @@ public class PDATest {
                 () -> check.next(InputAlphabet.Signature));
     }
 
+    @Test
+    public void testOPSAloneIsNotValid() {
+        PDA check = new PDA();
+        check.next(InputAlphabet.OnePassSignature);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> check.next(InputAlphabet.EndOfSequence));
+    }
+
+    @Test
+    public void testOPSLitWithMissingSigIsNotValid() {
+        PDA check = new PDA();
+        check.next(InputAlphabet.OnePassSignature);
+        check.next(InputAlphabet.LiteralData);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> check.next(InputAlphabet.EndOfSequence));
+    }
+
+    @Test
+    public void testCompressedMessageWithStandalongAppendedSigIsNotValid() {
+        PDA check = new PDA();
+        check.next(InputAlphabet.CompressedData);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> check.next(InputAlphabet.Signature));
+    }
+
+    @Test
+    public void testOPSCompressedDataWithMissingSigIsNotValid() {
+        PDA check = new PDA();
+        check.next(InputAlphabet.OnePassSignature);
+        check.next(InputAlphabet.CompressedData);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> check.next(InputAlphabet.EndOfSequence));
+    }
+
+    @Test
+    public void testCompressedMessageFollowedByTrailingLiteralDataIsNotValid() {
+        PDA check = new PDA();
+        check.next(InputAlphabet.CompressedData);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> check.next(InputAlphabet.LiteralData));
+    }
+
     @Test
     public void testOPSWithPrependedSigIsValid() {
         PDA check = new PDA();