WIP: Replace nesting with independent instancing

pgpainless-core/src/main/java/org/pgpainless/decryption_verification/MessageDecryptionStream.java

@@ -12,41 +12,50 @@ import org.bouncycastle.bcpg.Packet;
 import org.bouncycastle.bcpg.PacketTags;
 import org.bouncycastle.bcpg.PublicKeyEncSessionPacket;
 import org.bouncycastle.bcpg.SignaturePacket;
-import org.bouncycastle.bcpg.SymmetricEncDataPacket;
-import org.bouncycastle.bcpg.SymmetricEncIntegrityPacket;
 import org.bouncycastle.bcpg.SymmetricKeyEncSessionPacket;
-import org.bouncycastle.bcpg.TrustPacket;
-import org.bouncycastle.bcpg.UserAttributePacket;
-import org.bouncycastle.bcpg.UserIDPacket;
 import org.bouncycastle.openpgp.PGPCompressedData;
+import org.bouncycastle.openpgp.PGPEncryptedData;
 import org.bouncycastle.openpgp.PGPEncryptedDataList;
 import org.bouncycastle.openpgp.PGPException;
 import org.bouncycastle.openpgp.PGPLiteralData;
 import org.bouncycastle.openpgp.PGPOnePassSignatureList;
+import org.bouncycastle.openpgp.PGPPBEEncryptedData;
 import org.bouncycastle.openpgp.PGPSignatureList;
+import org.bouncycastle.openpgp.operator.PBEDataDecryptorFactory;
 import org.pgpainless.algorithm.OpenPgpPacket;
+import org.pgpainless.decryption_verification.automaton.InputAlphabet;
+import org.pgpainless.decryption_verification.automaton.NestingPDA;
 import org.pgpainless.exception.MalformedOpenPgpMessageException;
+import org.pgpainless.exception.MessageNotIntegrityProtectedException;
+import org.pgpainless.exception.MissingDecryptionMethodException;
 import org.pgpainless.implementation.ImplementationFactory;
+import org.pgpainless.util.Passphrase;
 
+import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
+import java.io.SequenceInputStream;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.NoSuchElementException;
 import java.util.Stack;
 
-public class PGPDecryptionStream extends InputStream {
+public class MessageDecryptionStream extends InputStream {
 
-    PushdownAutomaton automaton = new PushdownAutomaton();
+    private final ConsumerOptions options;
+
+    NestingPDA automaton = new NestingPDA();
     // nested streams, outermost at the bottom of the stack
     Stack<Layer> packetLayers = new Stack<>();
+    List<PublicKeyEncSessionPacket> pkeskList = new ArrayList<>();
+    List<SymmetricKeyEncSessionPacket> skeskList = new ArrayList<>();
 
-    public PGPDecryptionStream(InputStream inputStream) throws IOException, PGPException {
+    public MessageDecryptionStream(InputStream inputStream, ConsumerOptions options)
-        try {
+            throws IOException, PGPException {
-            packetLayers.push(Layer.initial(inputStream));
+        this.options = options;
-            walkLayer();
+        packetLayers.push(Layer.initial(inputStream));
-        } catch (MalformedOpenPgpMessageException e) {
+        walkLayer();
-            throw e.toRuntimeException();
-        }
     }
 
     private void walkLayer() throws PGPException, IOException {
@@ -54,6 +63,7 @@ public class PGPDecryptionStream extends InputStream {
             return;
         }
 
+        // We are currently in the deepest layer
         Layer layer = packetLayers.peek();
         BCPGInputStream inputStream = (BCPGInputStream) layer.inputStream;
 
@@ -65,33 +75,23 @@ public class PGPDecryptionStream extends InputStream {
             OpenPgpPacket tag = nextTagOrThrow(inputStream);
             switch (tag) {
 
-                case PKESK:
+                case LIT:
-                    PublicKeyEncSessionPacket pkeskPacket = (PublicKeyEncSessionPacket) inputStream.readPacket();
+                    automaton.next(InputAlphabet.LiteralData);
-                    PGPEncryptedDataList encList = null;
+                    PGPLiteralData literalData = new PGPLiteralData(inputStream);
-                    break;
+                    packetLayers.push(Layer.literalMessage(literalData.getDataStream()));
-                case SIG:
+                    break loop;
-                    automaton.next(PushdownAutomaton.InputAlphabet.Signatures);
 
+                case COMP:
+                    automaton.next(InputAlphabet.CompressedData);
+                    PGPCompressedData compressedData = new PGPCompressedData(inputStream);
+                    inputStream = new BCPGInputStream(compressedData.getDataStream());
+                    packetLayers.push(Layer.compressedData(inputStream));
+                    break;
+
+                case OPS:
+                    automaton.next(InputAlphabet.OnePassSignatures);
                     ByteArrayOutputStream buf = new ByteArrayOutputStream();
                     BCPGOutputStream bcpgOut = new BCPGOutputStream(buf);
-                    while (inputStream.nextPacketTag() == PacketTags.SIGNATURE || inputStream.nextPacketTag() == PacketTags.MARKER) {
-                        Packet packet = inputStream.readPacket();
-                        if (packet instanceof SignaturePacket) {
-                            SignaturePacket sig = (SignaturePacket) packet;
-                            sig.encode(bcpgOut);
-                        }
-                    }
-                    PGPSignatureList signatures = (PGPSignatureList) ImplementationFactory.getInstance()
-                            .getPGPObjectFactory(buf.toByteArray()).nextObject();
-                    break;
-                case SKESK:
-                    SymmetricKeyEncSessionPacket skeskPacket = (SymmetricKeyEncSessionPacket) inputStream.readPacket();
-
-                    break;
-                case OPS:
-                    automaton.next(PushdownAutomaton.InputAlphabet.OnePassSignatures);
-                    buf = new ByteArrayOutputStream();
-                    bcpgOut = new BCPGOutputStream(buf);
                     while (inputStream.nextPacketTag() == PacketTags.ONE_PASS_SIGNATURE || inputStream.nextPacketTag() == PacketTags.MARKER) {
                         Packet packet = inputStream.readPacket();
                         if (packet instanceof OnePassSignaturePacket) {
@@ -102,60 +102,103 @@ public class PGPDecryptionStream extends InputStream {
                     PGPOnePassSignatureList onePassSignatures = (PGPOnePassSignatureList) ImplementationFactory.getInstance()
                             .getPGPObjectFactory(buf.toByteArray()).nextObject();
                     break;
-                case SK:
+
+                case SIG:
+                    automaton.next(InputAlphabet.Signatures);
+
+                    buf = new ByteArrayOutputStream();
+                    bcpgOut = new BCPGOutputStream(buf);
+                    while (inputStream.nextPacketTag() == PacketTags.SIGNATURE || inputStream.nextPacketTag() == PacketTags.MARKER) {
+                        Packet packet = inputStream.readPacket();
+                        if (packet instanceof SignaturePacket) {
+                            SignaturePacket sig = (SignaturePacket) packet;
+                            sig.encode(bcpgOut);
+                        }
+                    }
+                    PGPSignatureList signatures = (PGPSignatureList) ImplementationFactory.getInstance()
+                            .getPGPObjectFactory(buf.toByteArray()).nextObject();
                     break;
-                case PK:
+
+                case PKESK:
+                    PublicKeyEncSessionPacket pkeskPacket = (PublicKeyEncSessionPacket) inputStream.readPacket();
+                    pkeskList.add(pkeskPacket);
                     break;
-                case SSK:
+
-                    break;
+                case SKESK:
-                case COMP:
+                    SymmetricKeyEncSessionPacket skeskPacket = (SymmetricKeyEncSessionPacket) inputStream.readPacket();
-                    automaton.next(PushdownAutomaton.InputAlphabet.CompressedData);
+                    skeskList.add(skeskPacket);
-                    PGPCompressedData compressedData = new PGPCompressedData(inputStream);
-                    inputStream = new BCPGInputStream(compressedData.getDataStream());
-                    packetLayers.push(Layer.CompressedData(inputStream));
                     break;
+
                 case SED:
-                    automaton.next(PushdownAutomaton.InputAlphabet.EncryptedData);
+                    if (!options.isIgnoreMDCErrors()) {
-                    SymmetricEncDataPacket symmetricEncDataPacket = (SymmetricEncDataPacket) inputStream.readPacket();
+                        throw new MessageNotIntegrityProtectedException();
-                    break;
+                    }
+                    // No break; we continue below!
+                case SEIPD:
+                    automaton.next(InputAlphabet.EncryptedData);
+                    PGPEncryptedDataList encryptedDataList = assembleEncryptedDataList(inputStream);
+
+                    for (PGPEncryptedData encData : encryptedDataList) {
+                        if (encData instanceof PGPPBEEncryptedData) {
+                            PGPPBEEncryptedData skenc = (PGPPBEEncryptedData) encData;
+                            for (Passphrase passphrase : options.getDecryptionPassphrases()) {
+                                PBEDataDecryptorFactory decryptorFactory = ImplementationFactory.getInstance()
+                                        .getPBEDataDecryptorFactory(passphrase);
+                                InputStream decryptedIn = skenc.getDataStream(decryptorFactory);
+                                packetLayers.push(Layer.encryptedData(new BCPGInputStream(decryptedIn)));
+                                walkLayer();
+                                break loop;
+                            }
+                        }
+                    }
+                    throw new MissingDecryptionMethodException("Cannot decrypt message.");
+
                 case MARKER:
                     inputStream.readPacket(); // discard
                     break;
-                case LIT:
+
-                    automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
+                case SK:
-                    PGPLiteralData literalData = new PGPLiteralData(inputStream);
+                case PK:
-                    packetLayers.push(Layer.LiteralMessage(literalData.getDataStream()));
+                case SSK:
-                    break loop;
-                case TRUST:
-                    TrustPacket trustPacket = (TrustPacket) inputStream.readPacket();
-                    break;
-                case UID:
-                    UserIDPacket userIDPacket = (UserIDPacket) inputStream.readPacket();
-                    break;
                 case PSK:
-                    break;
+                case TRUST:
+                case UID:
                 case UATTR:
-                    UserAttributePacket userAttributePacket = (UserAttributePacket) inputStream.readPacket();
+                    throw new MalformedOpenPgpMessageException("OpenPGP packet " + tag + " MUST NOT be part of OpenPGP messages.");
-                    break;
-                case SEIPD:
-                    automaton.next(PushdownAutomaton.InputAlphabet.EncryptedData);
-                    SymmetricEncIntegrityPacket symmetricEncIntegrityPacket = (SymmetricEncIntegrityPacket) inputStream.readPacket();
-                    break;
                 case MOD:
                     ModDetectionCodePacket modDetectionCodePacket = (ModDetectionCodePacket) inputStream.readPacket();
                     break;
                 case EXP_1:
-                    break;
                 case EXP_2:
-                    break;
                 case EXP_3:
-                    break;
                 case EXP_4:
-                    break;
+                    throw new MalformedOpenPgpMessageException("Experimental packet " + tag + " found inside the message.");
             }
         }
     }
 
+    private PGPEncryptedDataList assembleEncryptedDataList(BCPGInputStream inputStream)
+            throws IOException {
+        ByteArrayOutputStream buf = new ByteArrayOutputStream();
+        BCPGOutputStream bcpgOut = new BCPGOutputStream(buf);
+
+        for (SymmetricKeyEncSessionPacket skesk : skeskList) {
+            bcpgOut.write(skesk.getEncoded());
+        }
+        skeskList.clear();
+        for (PublicKeyEncSessionPacket pkesk : pkeskList) {
+            bcpgOut.write(pkesk.getEncoded());
+        }
+        pkeskList.clear();
+
+        SequenceInputStream sqin = new SequenceInputStream(
+                new ByteArrayInputStream(buf.toByteArray()), inputStream);
+
+        PGPEncryptedDataList encryptedDataList = (PGPEncryptedDataList) ImplementationFactory.getInstance()
+                .getPGPObjectFactory(sqin).nextObject();
+        return encryptedDataList;
+    }
+
     private OpenPgpPacket nextTagOrThrow(BCPGInputStream inputStream)
             throws IOException, InvalidOpenPgpPacketException {
         try {
@@ -167,17 +210,13 @@ public class PGPDecryptionStream extends InputStream {
 
     private void popLayer() throws MalformedOpenPgpMessageException {
         if (packetLayers.pop().isNested)
-            automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
+            automaton.next(InputAlphabet.EndOfSequence);
     }
 
     @Override
     public int read() throws IOException {
         if (packetLayers.isEmpty()) {
-            try {
+            automaton.assertValid();
-                automaton.assertValid();
-            } catch (MalformedOpenPgpMessageException e) {
-                throw e.toRuntimeException();
-            }
             return -1;
         }
 
@@ -187,13 +226,10 @@ public class PGPDecryptionStream extends InputStream {
         } catch (IOException e) {
         }
         if (r == -1) {
+            popLayer();
             try {
-                popLayer();
                 walkLayer();
-            } catch (MalformedOpenPgpMessageException e) {
+            } catch (PGPException e) {
-                throw e.toRuntimeException();
-            }
-            catch (PGPException e) {
                 throw new RuntimeException(e);
             }
             return read();
@@ -227,11 +263,15 @@ public class PGPDecryptionStream extends InputStream {
             return new Layer(bcpgIn, true);
         }
 
-        static Layer LiteralMessage(InputStream inputStream) {
+        static Layer literalMessage(InputStream inputStream) {
             return new Layer(inputStream, false);
         }
 
-        static Layer CompressedData(InputStream inputStream) {
+        static Layer compressedData(InputStream inputStream) {
+            return new Layer(inputStream, true);
+        }
+
+        static Layer encryptedData(InputStream inputStream) {
             return new Layer(inputStream, true);
         }
     }

pgpainless-core/src/main/java/org/pgpainless/decryption_verification/OpenPgpMessageInputStream.java

@@ -0,0 +1,331 @@
+package org.pgpainless.decryption_verification;
+
+import com.sun.tools.javac.code.Attribute;
+import org.bouncycastle.bcpg.BCPGInputStream;
+import org.bouncycastle.bcpg.BCPGOutputStream;
+import org.bouncycastle.bcpg.OnePassSignaturePacket;
+import org.bouncycastle.bcpg.Packet;
+import org.bouncycastle.bcpg.PacketTags;
+import org.bouncycastle.bcpg.SignaturePacket;
+import org.bouncycastle.openpgp.PGPCompressedData;
+import org.bouncycastle.openpgp.PGPEncryptedData;
+import org.bouncycastle.openpgp.PGPEncryptedDataList;
+import org.bouncycastle.openpgp.PGPException;
+import org.bouncycastle.openpgp.PGPLiteralData;
+import org.bouncycastle.openpgp.PGPObjectFactory;
+import org.bouncycastle.openpgp.PGPOnePassSignature;
+import org.bouncycastle.openpgp.PGPOnePassSignatureList;
+import org.bouncycastle.openpgp.PGPPBEEncryptedData;
+import org.bouncycastle.openpgp.PGPPrivateKey;
+import org.bouncycastle.openpgp.PGPPublicKey;
+import org.bouncycastle.openpgp.PGPPublicKeyEncryptedData;
+import org.bouncycastle.openpgp.PGPSecretKey;
+import org.bouncycastle.openpgp.PGPSecretKeyRing;
+import org.bouncycastle.openpgp.PGPSignature;
+import org.bouncycastle.openpgp.PGPSignatureList;
+import org.bouncycastle.openpgp.operator.PBEDataDecryptorFactory;
+import org.bouncycastle.openpgp.operator.PublicKeyDataDecryptorFactory;
+import org.pgpainless.PGPainless;
+import org.pgpainless.algorithm.EncryptionPurpose;
+import org.pgpainless.algorithm.OpenPgpPacket;
+import org.pgpainless.decryption_verification.automaton.InputAlphabet;
+import org.pgpainless.decryption_verification.automaton.PDA;
+import org.pgpainless.exception.MalformedOpenPgpMessageException;
+import org.pgpainless.exception.MessageNotIntegrityProtectedException;
+import org.pgpainless.implementation.ImplementationFactory;
+import org.pgpainless.key.info.KeyRingInfo;
+import org.pgpainless.key.protection.SecretKeyRingProtector;
+import org.pgpainless.key.protection.UnlockSecretKey;
+import org.pgpainless.util.Passphrase;
+import org.pgpainless.util.Tuple;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.ArrayList;
+import java.util.List;
+
+public class OpenPgpMessageInputStream extends InputStream {
+
+    protected final PDA automaton = new PDA();
+    protected final ConsumerOptions options;
+    protected final BCPGInputStream bcpgIn;
+    protected InputStream in;
+
+    private List<PGPSignature> signatures = new ArrayList<>();
+    private List<PGPOnePassSignature> onePassSignatures = new ArrayList<>();
+
+    public OpenPgpMessageInputStream(InputStream inputStream, ConsumerOptions options)
+            throws IOException, PGPException {
+        this.options = options;
+        // TODO: Use BCPGInputStream.wrap(inputStream);
+        if (inputStream instanceof BCPGInputStream) {
+            this.bcpgIn = (BCPGInputStream) inputStream;
+        } else {
+            this.bcpgIn = new BCPGInputStream(inputStream);
+        }
+
+        walk();
+    }
+
+    private void walk() throws IOException, PGPException {
+        loop: while (true) {
+
+            int tag = bcpgIn.nextPacketTag();
+            if (tag == -1) {
+                break loop;
+            }
+
+            OpenPgpPacket nextPacket = OpenPgpPacket.requireFromTag(tag);
+            switch (nextPacket) {
+                case LIT:
+                    automaton.next(InputAlphabet.LiteralData);
+                    PGPLiteralData literalData = new PGPLiteralData(bcpgIn);
+                    in = literalData.getDataStream();
+                    break loop;
+
+                case COMP:
+                    automaton.next(InputAlphabet.CompressedData);
+                    PGPCompressedData compressedData = new PGPCompressedData(bcpgIn);
+                    in = new OpenPgpMessageInputStream(compressedData.getDataStream(), options);
+                    break loop;
+
+                case OPS:
+                    automaton.next(InputAlphabet.OnePassSignatures);
+                    readOnePassSignatures();
+                    break;
+
+                case SIG:
+                    automaton.next(InputAlphabet.Signatures);
+                    readSignatures();
+                    break;
+
+                case PKESK:
+                case SKESK:
+                case SED:
+                case SEIPD:
+                    automaton.next(InputAlphabet.EncryptedData);
+                    PGPEncryptedDataList encDataList = new PGPEncryptedDataList(bcpgIn);
+
+                    // TODO: Replace with !encDataList.isIntegrityProtected()
+                    if (!encDataList.get(0).isIntegrityProtected()) {
+                        throw new MessageNotIntegrityProtectedException();
+                    }
+
+                    SortedESKs esks = new SortedESKs(encDataList);
+
+                    // TODO: try session keys
+
+                    // Try passwords
+                    for (PGPPBEEncryptedData skesk : esks.skesks) {
+                        for (Passphrase passphrase : options.getDecryptionPassphrases()) {
+                            PBEDataDecryptorFactory decryptorFactory = ImplementationFactory.getInstance()
+                                    .getPBEDataDecryptorFactory(passphrase);
+                            try {
+                                InputStream decrypted = skesk.getDataStream(decryptorFactory);
+                                in = new OpenPgpMessageInputStream(decrypted, options);
+                                break loop;
+                            } catch (PGPException e) {
+                                // password mismatch? Try next password
+                            }
+
+                        }
+                    }
+
+                    // Try (known) secret keys
+                    for (PGPPublicKeyEncryptedData pkesk : esks.pkesks) {
+                        long keyId = pkesk.getKeyID();
+                        PGPSecretKeyRing decryptionKeys = getDecryptionKey(keyId);
+                        if (decryptionKeys == null) {
+                            continue;
+                        }
+                        SecretKeyRingProtector protector = options.getSecretKeyProtector(decryptionKeys);
+                        PGPSecretKey decryptionKey = decryptionKeys.getSecretKey(keyId);
+                        PGPPrivateKey privateKey = UnlockSecretKey.unlockSecretKey(decryptionKey, protector);
+
+                        PublicKeyDataDecryptorFactory decryptorFactory = ImplementationFactory.getInstance()
+                                .getPublicKeyDataDecryptorFactory(privateKey);
+                        try {
+                            InputStream decrypted = pkesk.getDataStream(decryptorFactory);
+                            in = new OpenPgpMessageInputStream(decrypted, options);
+                            break loop;
+                        } catch (PGPException e) {
+                            // hm :/
+                        }
+                    }
+
+                    // try anonymous secret keys
+                    for (PGPPublicKeyEncryptedData pkesk : esks.anonPkesks) {
+                        for (Tuple<PGPSecretKeyRing, PGPSecretKey> decryptionKeyCandidate : findPotentialDecryptionKeys(pkesk)) {
+                            SecretKeyRingProtector protector = options.getSecretKeyProtector(decryptionKeyCandidate.getA());
+                            PGPPrivateKey privateKey = UnlockSecretKey.unlockSecretKey(decryptionKeyCandidate.getB(), protector);
+                            PublicKeyDataDecryptorFactory decryptorFactory = ImplementationFactory.getInstance()
+                                    .getPublicKeyDataDecryptorFactory(privateKey);
+
+                            try {
+                                InputStream decrypted = pkesk.getDataStream(decryptorFactory);
+                                in = new OpenPgpMessageInputStream(decrypted, options);
+                                break loop;
+                            } catch (PGPException e) {
+                                // hm :/
+                            }
+                        }
+                    }
+
+                    // TODO: try interactive password callbacks
+
+                    break loop;
+
+                case MARKER:
+                    bcpgIn.readPacket(); // skip marker packet
+                    break;
+
+                case SK:
+                case PK:
+                case SSK:
+                case PSK:
+                case TRUST:
+                case UID:
+                case UATTR:
+
+                case MOD:
+                    break;
+
+                case EXP_1:
+                case EXP_2:
+                case EXP_3:
+                case EXP_4:
+                    break;
+            }
+        }
+    }
+
+    private List<Tuple<PGPSecretKeyRing, PGPSecretKey>> findPotentialDecryptionKeys(PGPPublicKeyEncryptedData pkesk) {
+        int algorithm = pkesk.getAlgorithm();
+        List<Tuple<PGPSecretKeyRing, PGPSecretKey>> decryptionKeyCandidates = new ArrayList<>();
+
+        for (PGPSecretKeyRing secretKeys : options.getDecryptionKeys()) {
+            KeyRingInfo info = PGPainless.inspectKeyRing(secretKeys);
+            for (PGPPublicKey publicKey : info.getEncryptionSubkeys(EncryptionPurpose.ANY)) {
+                if (publicKey.getAlgorithm() == algorithm && info.isSecretKeyAvailable(publicKey.getKeyID())) {
+                    PGPSecretKey candidate = secretKeys.getSecretKey(publicKey.getKeyID());
+                    decryptionKeyCandidates.add(new Tuple<>(secretKeys, candidate));
+                }
+            }
+        }
+        return decryptionKeyCandidates;
+    }
+
+    private PGPSecretKeyRing getDecryptionKey(long keyID) {
+        for (PGPSecretKeyRing secretKeys : options.getDecryptionKeys()) {
+            PGPSecretKey decryptionKey = secretKeys.getSecretKey(keyID);
+            if (decryptionKey == null) {
+                continue;
+            }
+            return secretKeys;
+        }
+        return null;
+    }
+
+    private void readOnePassSignatures() throws IOException {
+        ByteArrayOutputStream buf = new ByteArrayOutputStream();
+        BCPGOutputStream bcpgOut = new BCPGOutputStream(buf);
+        int tag = bcpgIn.nextPacketTag();
+        while (tag == PacketTags.ONE_PASS_SIGNATURE || tag == PacketTags.MARKER) {
+            Packet packet = bcpgIn.readPacket();
+            if (tag == PacketTags.ONE_PASS_SIGNATURE) {
+                OnePassSignaturePacket sigPacket = (OnePassSignaturePacket) packet;
+                sigPacket.encode(bcpgOut);
+            }
+        }
+        bcpgOut.close();
+
+        PGPObjectFactory objectFactory = ImplementationFactory.getInstance().getPGPObjectFactory(buf.toByteArray());
+        PGPOnePassSignatureList signatureList = (PGPOnePassSignatureList) objectFactory.nextObject();
+        for (PGPOnePassSignature ops : signatureList) {
+            onePassSignatures.add(ops);
+        }
+    }
+
+    private void readSignatures() throws IOException {
+        ByteArrayOutputStream buf = new ByteArrayOutputStream();
+        BCPGOutputStream bcpgOut = new BCPGOutputStream(buf);
+        int tag = bcpgIn.nextPacketTag();
+        while (tag == PacketTags.SIGNATURE || tag == PacketTags.MARKER) {
+            Packet packet = bcpgIn.readPacket();
+            if (tag == PacketTags.SIGNATURE) {
+                SignaturePacket sigPacket = (SignaturePacket) packet;
+                sigPacket.encode(bcpgOut);
+            }
+        }
+        bcpgOut.close();
+
+        PGPObjectFactory objectFactory = ImplementationFactory.getInstance().getPGPObjectFactory(buf.toByteArray());
+        PGPSignatureList signatureList = (PGPSignatureList) objectFactory.nextObject();
+        for (PGPSignature signature : signatureList) {
+            signatures.add(signature);
+        }
+    }
+
+    @Override
+    public int read() throws IOException {
+        int r = -1;
+        try {
+            r = in.read();
+        } catch (IOException e) {
+            //
+        }
+        if (r == -1) {
+            if (in instanceof OpenPgpMessageInputStream) {
+                in.close();
+            } else {
+                try {
+                    walk();
+                } catch (PGPException e) {
+                    throw new RuntimeException(e);
+                }
+            }
+        }
+        return r;
+    }
+
+    @Override
+    public void close() throws IOException {
+        try {
+            in.close();
+            // Nested streams (except LiteralData) need to be closed.
+            if (automaton.getState() != PDA.State.LiteralMessage) {
+                automaton.next(InputAlphabet.EndOfSequence);
+                automaton.assertValid();
+            }
+        } catch (IOException e) {
+            //
+        }
+
+        super.close();
+    }
+
+    private static class SortedESKs {
+
+        private List<PGPPBEEncryptedData> skesks = new ArrayList<>();
+        private List<PGPPublicKeyEncryptedData> pkesks = new ArrayList<>();
+        private List<PGPPublicKeyEncryptedData> anonPkesks = new ArrayList<>();
+
+        SortedESKs(PGPEncryptedDataList esks) {
+            for (PGPEncryptedData esk : esks) {
+                if (esk instanceof PGPPBEEncryptedData) {
+                    skesks.add((PGPPBEEncryptedData) esk);
+                } else if (esk instanceof PGPPublicKeyEncryptedData) {
+                    PGPPublicKeyEncryptedData pkesk = (PGPPublicKeyEncryptedData) esk;
+                    if (pkesk.getKeyID() != 0) {
+                        pkesks.add(pkesk);
+                    } else {
+                        anonPkesks.add(pkesk);
+                    }
+                } else {
+                    throw new IllegalArgumentException("Unknown ESK class type.");
+                }
+            }
+        }
+    }
+}

pgpainless-core/src/main/java/org/pgpainless/decryption_verification/automaton/InputAlphabet.java

@@ -0,0 +1,41 @@
+package org.pgpainless.decryption_verification.automaton;
+
+import org.bouncycastle.openpgp.PGPCompressedData;
+import org.bouncycastle.openpgp.PGPEncryptedDataList;
+import org.bouncycastle.openpgp.PGPLiteralData;
+import org.bouncycastle.openpgp.PGPOnePassSignatureList;
+import org.bouncycastle.openpgp.PGPSignatureList;
+
+public enum InputAlphabet {
+    /**
+     * A {@link PGPLiteralData} packet.
+     */
+    LiteralData,
+    /**
+     * A {@link PGPSignatureList} object.
+     */
+    Signatures,
+    /**
+     * A {@link PGPOnePassSignatureList} object.
+     */
+    OnePassSignatures,
+    /**
+     * A {@link PGPCompressedData} packet.
+     * The contents of this packet MUST form a valid OpenPGP message, so a nested PDA is opened to verify
+     * its nested packet sequence.
+     */
+    CompressedData,
+    /**
+     * A {@link PGPEncryptedDataList} object.
+     * This object combines multiple ESKs and the corresponding Symmetrically Encrypted
+     * (possibly Integrity Protected) Data packet.
+     */
+    EncryptedData,
+    /**
+     * Marks the end of a (sub-) sequence.
+     * This input is given if the end of an OpenPGP message is reached.
+     * This might be the case for the end of the whole ciphertext, or the end of a packet with nested contents
+     * (e.g. the end of a Compressed Data packet).
+     */
+    EndOfSequence
+}

pgpainless-core/src/main/java/org/pgpainless/decryption_verification/automaton/NestingPDA.java

@@ -1,17 +1,12 @@
-package org.pgpainless.decryption_verification;
+package org.pgpainless.decryption_verification.automaton;
 
-import org.bouncycastle.openpgp.PGPCompressedData;
-import org.bouncycastle.openpgp.PGPEncryptedDataList;
-import org.bouncycastle.openpgp.PGPLiteralData;
-import org.bouncycastle.openpgp.PGPOnePassSignatureList;
-import org.bouncycastle.openpgp.PGPSignatureList;
 import org.pgpainless.exception.MalformedOpenPgpMessageException;
 
 import java.util.Stack;
 
-import static org.pgpainless.decryption_verification.PushdownAutomaton.StackAlphabet.msg;
+import static org.pgpainless.decryption_verification.automaton.StackAlphabet.msg;
-import static org.pgpainless.decryption_verification.PushdownAutomaton.StackAlphabet.ops;
+import static org.pgpainless.decryption_verification.automaton.StackAlphabet.ops;
-import static org.pgpainless.decryption_verification.PushdownAutomaton.StackAlphabet.terminus;
+import static org.pgpainless.decryption_verification.automaton.StackAlphabet.terminus;
 
 /**
  * Pushdown Automaton to verify the correct syntax of OpenPGP messages during decryption.
@@ -37,71 +32,18 @@ import static org.pgpainless.decryption_verification.PushdownAutomaton.StackAlph
  *
  * @see <a href="https://www.rfc-editor.org/rfc/rfc4880#section-11.3">RFC4880 §11.3. OpenPGP Messages</a>
  */
-public class PushdownAutomaton {
+public class NestingPDA {
-
-    public enum InputAlphabet {
-        /**
-         * A {@link PGPLiteralData} packet.
-         */
-        LiteralData,
-        /**
-         * A {@link PGPSignatureList} object.
-         */
-        Signatures,
-        /**
-         * A {@link PGPOnePassSignatureList} object.
-         */
-        OnePassSignatures,
-        /**
-         * A {@link PGPCompressedData} packet.
-         * The contents of this packet MUST form a valid OpenPGP message, so a nested PDA is opened to verify
-         * its nested packet sequence.
-         */
-        CompressedData,
-        /**
-         * A {@link PGPEncryptedDataList} object.
-         * This object combines multiple ESKs and the corresponding Symmetrically Encrypted
-         * (possibly Integrity Protected) Data packet.
-         */
-        EncryptedData,
-        /**
-         * Marks the end of a (sub-) sequence.
-         * This input is given if the end of an OpenPGP message is reached.
-         * This might be the case for the end of the whole ciphertext, or the end of a packet with nested contents
-         * (e.g. the end of a Compressed Data packet).
-         */
-        EndOfSequence
-    }
-
-    public enum StackAlphabet {
-        /**
-         * OpenPGP Message.
-         */
-        msg,
-        /**
-         * OnePassSignature (in case of BC this represents a OnePassSignatureList).
-         */
-        ops,
-        /**
-         * ESK. Not used, as BC combines encrypted data with their encrypted session keys.
-         */
-        esk,
-        /**
-         * Special symbol representing the end of the message.
-         */
-        terminus
-    }
 
     /**
      * Set of states of the automaton.
-     * Each state defines its valid transitions in their {@link State#transition(InputAlphabet, PushdownAutomaton)}
+     * Each state defines its valid transitions in their {@link State#transition(InputAlphabet, NestingPDA)}
      * method.
      */
     public enum State {
 
         OpenPgpMessage {
             @Override
-            State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
+            State transition(InputAlphabet input, NestingPDA automaton) throws MalformedOpenPgpMessageException {
                 StackAlphabet stackItem = automaton.popStack();
                 if (stackItem != msg) {
                     throw new MalformedOpenPgpMessageException(this, input, stackItem);
@@ -135,7 +77,7 @@ public class PushdownAutomaton {
 
         LiteralMessage {
             @Override
-            State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
+            State transition(InputAlphabet input, NestingPDA automaton) throws MalformedOpenPgpMessageException {
                 StackAlphabet stackItem = automaton.popStack();
                 switch (input) {
 
@@ -165,7 +107,7 @@ public class PushdownAutomaton {
 
         CompressedMessage {
             @Override
-            State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
+            State transition(InputAlphabet input, NestingPDA automaton) throws MalformedOpenPgpMessageException {
                 StackAlphabet stackItem = automaton.popStack();
                 switch (input) {
                     case Signatures:
@@ -194,7 +136,7 @@ public class PushdownAutomaton {
 
         EncryptedMessage {
             @Override
-            State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
+            State transition(InputAlphabet input, NestingPDA automaton) throws MalformedOpenPgpMessageException {
                 StackAlphabet stackItem = automaton.popStack();
                 switch (input) {
                     case Signatures:
@@ -223,7 +165,7 @@ public class PushdownAutomaton {
 
         CorrespondingSignature {
             @Override
-            State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
+            State transition(InputAlphabet input, NestingPDA automaton) throws MalformedOpenPgpMessageException {
                 StackAlphabet stackItem = automaton.popStack();
                 if (stackItem == terminus && input == InputAlphabet.EndOfSequence && automaton.stack.isEmpty()) {
                     return Valid;
@@ -235,7 +177,7 @@ public class PushdownAutomaton {
 
         Valid {
             @Override
-            State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
+            State transition(InputAlphabet input, NestingPDA automaton) throws MalformedOpenPgpMessageException {
                 throw new MalformedOpenPgpMessageException(this, input, null);
             }
         },
@@ -252,15 +194,15 @@ public class PushdownAutomaton {
          * @return new state of the automaton
          * @throws MalformedOpenPgpMessageException in case of an illegal input symbol
          */
-        abstract State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException;
+        abstract State transition(InputAlphabet input, NestingPDA automaton) throws MalformedOpenPgpMessageException;
     }
 
     private final Stack<StackAlphabet> stack = new Stack<>();
     private State state;
     // Some OpenPGP packets have nested contents (e.g. compressed / encrypted data).
-    PushdownAutomaton nestedSequence = null;
+    NestingPDA nestedSequence = null;
 
-    public PushdownAutomaton() {
+    public NestingPDA() {
         state = State.OpenPgpMessage;
         stack.push(terminus);
         stack.push(msg);
@@ -301,7 +243,7 @@ public class PushdownAutomaton {
 
             // If the processed packet contains nested sequence, open nested automaton for it
             if (input == InputAlphabet.CompressedData || input == InputAlphabet.EncryptedData) {
-                nestedSequence = new PushdownAutomaton();
+                nestedSequence = new NestingPDA();
             }
         }
 

pgpainless-core/src/main/java/org/pgpainless/decryption_verification/automaton/PDA.java

@@ -0,0 +1,237 @@
+package org.pgpainless.decryption_verification.automaton;
+
+import org.pgpainless.exception.MalformedOpenPgpMessageException;
+
+import java.util.Stack;
+
+import static org.pgpainless.decryption_verification.automaton.StackAlphabet.msg;
+import static org.pgpainless.decryption_verification.automaton.StackAlphabet.ops;
+import static org.pgpainless.decryption_verification.automaton.StackAlphabet.terminus;
+
+public class PDA {
+    /**
+     * Set of states of the automaton.
+     * Each state defines its valid transitions in their {@link NestingPDA.State#transition(InputAlphabet, NestingPDA)}
+     * method.
+     */
+    public enum State {
+
+        OpenPgpMessage {
+            @Override
+            State transition(InputAlphabet input, PDA automaton) throws MalformedOpenPgpMessageException {
+                StackAlphabet stackItem = automaton.popStack();
+                if (stackItem != msg) {
+                    throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                }
+                switch (input) {
+
+                    case LiteralData:
+                        return LiteralMessage;
+
+                    case Signatures:
+                        automaton.pushStack(msg);
+                        return OpenPgpMessage;
+
+                    case OnePassSignatures:
+                        automaton.pushStack(ops);
+                        automaton.pushStack(msg);
+                        return OpenPgpMessage;
+
+                    case CompressedData:
+                        return CompressedMessage;
+
+                    case EncryptedData:
+                        return EncryptedMessage;
+
+                    case EndOfSequence:
+                    default:
+                        throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                }
+            }
+        },
+
+        LiteralMessage {
+            @Override
+            State transition(InputAlphabet input, PDA automaton) throws MalformedOpenPgpMessageException {
+                StackAlphabet stackItem = automaton.popStack();
+                switch (input) {
+
+                    case Signatures:
+                        if (stackItem == ops) {
+                            return CorrespondingSignature;
+                        } else {
+                            throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                        }
+
+                    case EndOfSequence:
+                        if (stackItem == terminus && automaton.stack.isEmpty()) {
+                            return Valid;
+                        } else {
+                            throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                        }
+
+                    case LiteralData:
+                    case OnePassSignatures:
+                    case CompressedData:
+                    case EncryptedData:
+                    default:
+                        throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                }
+            }
+        },
+
+        CompressedMessage {
+            @Override
+            State transition(InputAlphabet input, PDA automaton) throws MalformedOpenPgpMessageException {
+                StackAlphabet stackItem = automaton.popStack();
+                switch (input) {
+                    case Signatures:
+                        if (stackItem == ops) {
+                            return CorrespondingSignature;
+                        } else {
+                            throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                        }
+
+                    case EndOfSequence:
+                        if (stackItem == terminus && automaton.stack.isEmpty()) {
+                            return Valid;
+                        } else {
+                            throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                        }
+
+                    case LiteralData:
+                    case OnePassSignatures:
+                    case CompressedData:
+                    case EncryptedData:
+                    default:
+                        throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                }
+            }
+        },
+
+        EncryptedMessage {
+            @Override
+            State transition(InputAlphabet input, PDA automaton) throws MalformedOpenPgpMessageException {
+                StackAlphabet stackItem = automaton.popStack();
+                switch (input) {
+                    case Signatures:
+                        if (stackItem == ops) {
+                            return CorrespondingSignature;
+                        } else {
+                            throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                        }
+
+                    case EndOfSequence:
+                        if (stackItem == terminus && automaton.stack.isEmpty()) {
+                            return Valid;
+                        } else {
+                            throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                        }
+
+                    case LiteralData:
+                    case OnePassSignatures:
+                    case CompressedData:
+                    case EncryptedData:
+                    default:
+                        throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                }
+            }
+        },
+
+        CorrespondingSignature {
+            @Override
+            State transition(InputAlphabet input, PDA automaton) throws MalformedOpenPgpMessageException {
+                StackAlphabet stackItem = automaton.popStack();
+                if (stackItem == terminus && input == InputAlphabet.EndOfSequence && automaton.stack.isEmpty()) {
+                    return Valid;
+                } else {
+                    throw new MalformedOpenPgpMessageException(this, input, stackItem);
+                }
+            }
+        },
+
+        Valid {
+            @Override
+            State transition(InputAlphabet input, PDA automaton) throws MalformedOpenPgpMessageException {
+                throw new MalformedOpenPgpMessageException(this, input, null);
+            }
+        },
+        ;
+
+        /**
+         * Pop the automatons stack and transition to another state.
+         * If no valid transition from the current state is available given the popped stack item and input symbol,
+         * a {@link MalformedOpenPgpMessageException} is thrown.
+         * Otherwise, the stack is manipulated according to the valid transition and the new state is returned.
+         *
+         * @param input     input symbol
+         * @param automaton automaton
+         * @return new state of the automaton
+         * @throws MalformedOpenPgpMessageException in case of an illegal input symbol
+         */
+        abstract State transition(InputAlphabet input, PDA automaton) throws MalformedOpenPgpMessageException;
+    }
+
+    private final Stack<StackAlphabet> stack = new Stack<>();
+    private State state;
+
+    public PDA() {
+        state = State.OpenPgpMessage;
+        stack.push(terminus);
+        stack.push(msg);
+    }
+
+    public void next(InputAlphabet input) throws MalformedOpenPgpMessageException {
+        State old = state;
+        StackAlphabet stackItem = stack.isEmpty() ? null : stack.peek();
+        state = state.transition(input, this);
+        System.out.println("Transition from " + old + " to " + state + " via " + input + " with stack " + stackItem);
+    }
+
+    /**
+     * Return the current state of the PDA.
+     *
+     * @return state
+     */
+    public State getState() {
+        return state;
+    }
+
+    /**
+     * Return true, if the PDA is in a valid state (the OpenPGP message is valid).
+     *
+     * @return true if valid, false otherwise
+     */
+    public boolean isValid() {
+        return getState() == State.Valid && stack.isEmpty();
+    }
+
+    public void assertValid() throws MalformedOpenPgpMessageException {
+        if (!isValid()) {
+            throw new MalformedOpenPgpMessageException("Pushdown Automaton is not in an acceptable state: " + toString());
+        }
+    }
+
+    /**
+     * Pop an item from the stack.
+     *
+     * @return stack item
+     */
+    private StackAlphabet popStack() {
+        return stack.pop();
+    }
+
+    /**
+     * Push an item onto the stack.
+     *
+     * @param item item
+     */
+    private void pushStack(StackAlphabet item) {
+        stack.push(item);
+    }
+
+    @Override
+    public String toString() {
+        return "State: " + state + " Stack: " + stack;
+    }
+}

pgpainless-core/src/main/java/org/pgpainless/decryption_verification/automaton/StackAlphabet.java

@@ -0,0 +1,20 @@
+package org.pgpainless.decryption_verification.automaton;
+
+public enum StackAlphabet {
+    /**
+     * OpenPGP Message.
+     */
+    msg,
+    /**
+     * OnePassSignature (in case of BC this represents a OnePassSignatureList).
+     */
+    ops,
+    /**
+     * ESK. Not used, as BC combines encrypted data with their encrypted session keys.
+     */
+    esk,
+    /**
+     * Special symbol representing the end of the message.
+     */
+    terminus
+}

pgpainless-core/src/main/java/org/pgpainless/exception/MalformedOpenPgpMessageException.java

@@ -4,8 +4,10 @@
 
 package org.pgpainless.exception;
 
-import org.bouncycastle.openpgp.PGPException;
+import org.pgpainless.decryption_verification.automaton.InputAlphabet;
-import org.pgpainless.decryption_verification.PushdownAutomaton;
+import org.pgpainless.decryption_verification.automaton.NestingPDA;
+import org.pgpainless.decryption_verification.automaton.PDA;
+import org.pgpainless.decryption_verification.automaton.StackAlphabet;
 
 /**
  * Exception that gets thrown if the OpenPGP message is malformed.
@@ -13,7 +15,7 @@ import org.pgpainless.decryption_verification.PushdownAutomaton;
  *
  * @see <a href="https://www.rfc-editor.org/rfc/rfc4880#section-11.3">RFC4880 §11.3. OpenPGP Messages</a>
  */
-public class MalformedOpenPgpMessageException extends PGPException {
+public class MalformedOpenPgpMessageException extends RuntimeException {
 
     public MalformedOpenPgpMessageException(String message) {
         super(message);
@@ -23,20 +25,17 @@ public class MalformedOpenPgpMessageException extends PGPException {
         super(message, cause);
     }
 
-    public MalformedOpenPgpMessageException(PushdownAutomaton.State state,
+    public MalformedOpenPgpMessageException(NestingPDA.State state,
-                                            PushdownAutomaton.InputAlphabet input,
+                                            InputAlphabet input,
-                                            PushdownAutomaton.StackAlphabet stackItem) {
+                                            StackAlphabet stackItem) {
         this("Invalid input: There is no legal transition from state '" + state + "' for input '" + input + "' when '" + stackItem + "' is on top of the stack.");
     }
 
-    public RTE toRuntimeException() {
+    public MalformedOpenPgpMessageException(PDA.State state, InputAlphabet input, StackAlphabet stackItem) {
-        return new RTE(this);
+        this("Invalid input: There is no legal transition from state '" + state + "' for input '" + input + "' when '" + stackItem + "' is on top of the stack.");
     }
 
-    public static class RTE extends RuntimeException {
+    public MalformedOpenPgpMessageException(String message, PDA automaton) {
-
+        super(message + automaton.toString());
-        public RTE(MalformedOpenPgpMessageException e) {
-            super(e);
-        }
     }
 }

pgpainless-core/src/main/java/org/pgpainless/key/info/KeyRingInfo.java

@@ -1100,29 +1100,33 @@ public class KeyRingInfo {
 
         List<PGPPublicKey> signingKeys = getSigningSubkeys();
         for (PGPPublicKey pk : signingKeys) {
-            PGPSecretKey sk = getSecretKey(pk.getKeyID());
+            return isSecretKeyAvailable(pk.getKeyID());
-            if (sk == null) {
-                // Missing secret key
-                continue;
-            }
-            S2K s2K = sk.getS2K();
-            // Unencrypted key
-            if (s2K == null) {
-                return true;
-            }
-
-            // Secret key on smart-card
-            int s2kType = s2K.getType();
-            if (s2kType >= 100 && s2kType <= 110) {
-                continue;
-            }
-            // protected secret key
-            return true;
         }
         // No usable secret key found
         return false;
     }
 
+    public boolean isSecretKeyAvailable(long keyId) {
+        PGPSecretKey sk = getSecretKey(keyId);
+        if (sk == null) {
+            // Missing secret key
+            return false;
+        }
+        S2K s2K = sk.getS2K();
+        // Unencrypted key
+        if (s2K == null) {
+            return true;
+        }
+
+        // Secret key on smart-card
+        int s2kType = s2K.getType();
+        if (s2kType >= 100 && s2kType <= 110) {
+            return false;
+        }
+        // protected secret key
+        return true;
+    }
+
     private KeyAccessor getKeyAccessor(@Nullable String userId, long keyID) {
         if (getPublicKey(keyID) == null) {
             throw new NoSuchElementException("No subkey with key id " + Long.toHexString(keyID) + " found on this key.");

pgpainless-core/src/test/java/org/pgpainless/decryption_verification/OpenPgpMessageInputStreamTest.java

@@ -0,0 +1,86 @@
+package org.pgpainless.decryption_verification;
+
+import org.bouncycastle.bcpg.ArmoredInputStream;
+import org.bouncycastle.openpgp.PGPException;
+import org.bouncycastle.util.io.Streams;
+import org.junit.jupiter.api.Test;
+import org.pgpainless.exception.MalformedOpenPgpMessageException;
+import org.pgpainless.util.ArmoredInputStreamFactory;
+import org.pgpainless.util.Passphrase;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.pgpainless.decryption_verification.PGPDecryptionStreamTest.COMP;
+import static org.pgpainless.decryption_verification.PGPDecryptionStreamTest.COMP_COMP_LIT;
+import static org.pgpainless.decryption_verification.PGPDecryptionStreamTest.COMP_LIT;
+import static org.pgpainless.decryption_verification.PGPDecryptionStreamTest.LIT;
+import static org.pgpainless.decryption_verification.PGPDecryptionStreamTest.LIT_LIT;
+import static org.pgpainless.decryption_verification.PGPDecryptionStreamTest.PASSPHRASE;
+import static org.pgpainless.decryption_verification.PGPDecryptionStreamTest.PLAINTEXT;
+import static org.pgpainless.decryption_verification.PGPDecryptionStreamTest.SENC_LIT;
+import static org.pgpainless.decryption_verification.PGPDecryptionStreamTest.SIG_LIT;
+
+public class OpenPgpMessageInputStreamTest {
+
+    @Test
+    public void testProcessLIT() throws IOException, PGPException {
+        String plain = process(LIT, ConsumerOptions.get());
+        assertEquals(PLAINTEXT, plain);
+    }
+
+    @Test
+    public void testProcessLIT_LIT_fails() {
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> process(LIT_LIT, ConsumerOptions.get()));
+    }
+
+    @Test
+    public void testProcessCOMP_LIT() throws PGPException, IOException {
+        String plain = process(COMP_LIT, ConsumerOptions.get());
+        assertEquals(PLAINTEXT, plain);
+    }
+
+    @Test
+    public void testProcessCOMP_fails() {
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> process(COMP, ConsumerOptions.get()));
+    }
+
+    @Test
+    public void testProcessCOMP_COMP_LIT() throws PGPException, IOException {
+        String plain = process(COMP_COMP_LIT, ConsumerOptions.get());
+        assertEquals(PLAINTEXT, plain);
+    }
+
+    @Test
+    public void testProcessSIG_LIT() throws PGPException, IOException {
+        String plain = process(SIG_LIT, ConsumerOptions.get());
+        assertEquals(PLAINTEXT, plain);
+    }
+
+    @Test
+    public void testProcessSENC_LIT() throws PGPException, IOException {
+        String plain = process(SENC_LIT, ConsumerOptions.get().addDecryptionPassphrase(Passphrase.fromPassword(PASSPHRASE)));
+        assertEquals(PLAINTEXT, plain);
+    }
+
+    private String process(String armoredMessage, ConsumerOptions options) throws PGPException, IOException {
+        OpenPgpMessageInputStream in = get(armoredMessage, options);
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        Streams.pipeAll(in, out);
+        in.close();
+        return out.toString();
+    }
+
+    private OpenPgpMessageInputStream get(String armored, ConsumerOptions options) throws IOException, PGPException {
+        ByteArrayInputStream bytesIn = new ByteArrayInputStream(armored.getBytes(StandardCharsets.UTF_8));
+        ArmoredInputStream armorIn = ArmoredInputStreamFactory.get(bytesIn);
+        OpenPgpMessageInputStream pgpIn = new OpenPgpMessageInputStream(armorIn, options);
+        return pgpIn;
+    }
+}

pgpainless-core/src/test/java/org/pgpainless/decryption_verification/PGPDecryptionStreamTest.java

@@ -12,6 +12,8 @@ import org.bouncycastle.openpgp.PGPSignature;
 import org.bouncycastle.util.io.Streams;
 import org.junit.jupiter.api.Test;
 import org.pgpainless.PGPainless;
+import org.pgpainless.algorithm.CompressionAlgorithm;
+import org.pgpainless.encryption_signing.EncryptionOptions;
 import org.pgpainless.encryption_signing.EncryptionResult;
 import org.pgpainless.encryption_signing.EncryptionStream;
 import org.pgpainless.encryption_signing.ProducerOptions;
@@ -19,6 +21,7 @@ import org.pgpainless.encryption_signing.SigningOptions;
 import org.pgpainless.exception.MalformedOpenPgpMessageException;
 import org.pgpainless.key.protection.SecretKeyRingProtector;
 import org.pgpainless.util.ArmoredInputStreamFactory;
+import org.pgpainless.util.Passphrase;
 
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
@@ -33,7 +36,7 @@ import static org.junit.jupiter.api.Assertions.assertThrows;
 
 public class PGPDecryptionStreamTest {
 
-    private static final String KEY = "-----BEGIN PGP PRIVATE KEY BLOCK-----\n" +
+    public static final String KEY = "-----BEGIN PGP PRIVATE KEY BLOCK-----\n" +
             "Version: PGPainless\n" +
             "Comment: DA05 848F 37D4 68E6 F982  C889 7A70 1FC6 904D 3F4C\n" +
             "Comment: Alice <alice@pgpainless.org>\n" +
@@ -58,9 +61,10 @@ public class PGPDecryptionStreamTest {
             "=THgv\n" +
             "-----END PGP PRIVATE KEY BLOCK-----";
 
-    private static final String PLAINTEXT = "Hello, World!\n";
+    public static final String PLAINTEXT = "Hello, World!\n";
+    public static final String PASSPHRASE = "sw0rdf1sh";
 
-    private static final String LIT = "" +
+    public static final String LIT = "" +
             "-----BEGIN PGP MESSAGE-----\n" +
             "Version: PGPainless\n" +
             "\n" +
@@ -68,7 +72,7 @@ public class PGPDecryptionStreamTest {
             "=WGju\n" +
             "-----END PGP MESSAGE-----";
 
-    private static final String LIT_LIT = "" +
+    public static final String LIT_LIT = "" +
             "-----BEGIN PGP MESSAGE-----\n" +
             "Version: BCPG v1.71\n" +
             "\n" +
@@ -76,7 +80,7 @@ public class PGPDecryptionStreamTest {
             "=A91Q\n" +
             "-----END PGP MESSAGE-----";
 
-    private static final String COMP_LIT = "" +
+    public static final String COMP_LIT = "" +
             "-----BEGIN PGP MESSAGE-----\n" +
             "Version: BCPG v1.71\n" +
             "\n" +
@@ -84,7 +88,7 @@ public class PGPDecryptionStreamTest {
             "=ZYDg\n" +
             "-----END PGP MESSAGE-----";
 
-    private static final String COMP = "" +
+    public static final String COMP = "" +
             "-----BEGIN PGP MESSAGE-----\n" +
             "Version: BCPG v1.71\n" +
             "\n" +
@@ -92,7 +96,7 @@ public class PGPDecryptionStreamTest {
             "=MDzg\n" +
             "-----END PGP MESSAGE-----";
 
-    private static final String COMP_COMP_LIT = "" +
+    public static final String COMP_COMP_LIT = "" +
             "-----BEGIN PGP MESSAGE-----\n" +
             "Version: BCPG v1.71\n" +
             "\n" +
@@ -101,7 +105,7 @@ public class PGPDecryptionStreamTest {
             "=K9Zl\n" +
             "-----END PGP MESSAGE-----";
 
-    private static final String SIG_LIT = "" +
+    public static final String SIG_LIT = "" +
             "-----BEGIN PGP MESSAGE-----\n" +
             "Version: BCPG v1.71\n" +
             "\n" +
@@ -111,6 +115,15 @@ public class PGPDecryptionStreamTest {
             "=WKPn\n" +
             "-----END PGP MESSAGE-----";
 
+    public static final String SENC_LIT = "" +
+            "-----BEGIN PGP MESSAGE-----\n" +
+            "Version: PGPainless\n" +
+            "\n" +
+            "jA0ECQMCuZ0qHNXWnGhg0j8Bdm1cxV65sYb7jDgb4rRMtdNpQ1dC4UpSYuk9YWS2\n" +
+            "DpNEijbX8b/P1UOK2kJczNDADMRegZuLEI+dNsBnJjk=\n" +
+            "=i4Y0\n" +
+            "-----END PGP MESSAGE-----";
+
     @Test
     public void genLIT() throws IOException {
         ArmoredOutputStream armorOut = new ArmoredOutputStream(System.out);
@@ -125,7 +138,7 @@ public class PGPDecryptionStreamTest {
     public void processLIT() throws IOException, PGPException {
         ByteArrayInputStream bytesIn = new ByteArrayInputStream(LIT.getBytes(StandardCharsets.UTF_8));
         ArmoredInputStream armorIn = ArmoredInputStreamFactory.get(bytesIn);
-        PGPDecryptionStream decIn = new PGPDecryptionStream(armorIn);
+        MessageDecryptionStream decIn = new MessageDecryptionStream(armorIn, ConsumerOptions.get());
 
         ByteArrayOutputStream out = new ByteArrayOutputStream();
         Streams.pipeAll(decIn, out);
@@ -152,10 +165,10 @@ public class PGPDecryptionStreamTest {
     public void processLIT_LIT() throws IOException, PGPException {
         ByteArrayInputStream bytesIn = new ByteArrayInputStream(LIT_LIT.getBytes(StandardCharsets.UTF_8));
         ArmoredInputStream armorIn = ArmoredInputStreamFactory.get(bytesIn);
-        PGPDecryptionStream decIn = new PGPDecryptionStream(armorIn);
+        MessageDecryptionStream decIn = new MessageDecryptionStream(armorIn, ConsumerOptions.get());
 
         ByteArrayOutputStream out = new ByteArrayOutputStream();
-        assertThrows(MalformedOpenPgpMessageException.RTE.class, () -> Streams.pipeAll(decIn, out));
+        assertThrows(MalformedOpenPgpMessageException.class, () -> Streams.pipeAll(decIn, out));
     }
 
     @Test
@@ -175,7 +188,7 @@ public class PGPDecryptionStreamTest {
     public void processCOMP_LIT() throws IOException, PGPException {
         ByteArrayInputStream bytesIn = new ByteArrayInputStream(COMP_LIT.getBytes(StandardCharsets.UTF_8));
         ArmoredInputStream armorIn = ArmoredInputStreamFactory.get(bytesIn);
-        PGPDecryptionStream decIn = new PGPDecryptionStream(armorIn);
+        MessageDecryptionStream decIn = new MessageDecryptionStream(armorIn, ConsumerOptions.get());
 
         ByteArrayOutputStream out = new ByteArrayOutputStream();
         Streams.pipeAll(decIn, out);
@@ -198,8 +211,8 @@ public class PGPDecryptionStreamTest {
     public void processCOMP() throws IOException {
         ByteArrayInputStream bytesIn = new ByteArrayInputStream(COMP.getBytes(StandardCharsets.UTF_8));
         ArmoredInputStream armorIn = ArmoredInputStreamFactory.get(bytesIn);
-        assertThrows(MalformedOpenPgpMessageException.RTE.class, () -> {
+        assertThrows(MalformedOpenPgpMessageException.class, () -> {
-            PGPDecryptionStream decIn = new PGPDecryptionStream(armorIn);
+            MessageDecryptionStream decIn = new MessageDecryptionStream(armorIn, ConsumerOptions.get());
             Streams.drain(decIn);
         });
     }
@@ -228,7 +241,7 @@ public class PGPDecryptionStreamTest {
     public void processCOMP_COMP_LIT() throws PGPException, IOException {
         ByteArrayInputStream bytesIn = new ByteArrayInputStream(COMP_COMP_LIT.getBytes(StandardCharsets.UTF_8));
         ArmoredInputStream armorIn = ArmoredInputStreamFactory.get(bytesIn);
-        PGPDecryptionStream decIn = new PGPDecryptionStream(armorIn);
+        MessageDecryptionStream decIn = new MessageDecryptionStream(armorIn, ConsumerOptions.get());
 
         ByteArrayOutputStream out = new ByteArrayOutputStream();
         Streams.pipeAll(decIn, out);
@@ -279,7 +292,35 @@ public class PGPDecryptionStreamTest {
     public void processSIG_LIT() throws IOException, PGPException {
         ByteArrayInputStream bytesIn = new ByteArrayInputStream(SIG_LIT.getBytes(StandardCharsets.UTF_8));
         ArmoredInputStream armorIn = ArmoredInputStreamFactory.get(bytesIn);
-        PGPDecryptionStream decIn = new PGPDecryptionStream(armorIn);
+        MessageDecryptionStream decIn = new MessageDecryptionStream(armorIn, ConsumerOptions.get());
+
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        Streams.pipeAll(decIn, out);
+        decIn.close();
+
+        System.out.println(out);
+    }
+
+    @Test
+    public void genSENC_LIT() throws PGPException, IOException {
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        EncryptionStream enc = PGPainless.encryptAndOrSign()
+                .onOutputStream(out)
+                .withOptions(ProducerOptions.encrypt(EncryptionOptions.get()
+                                .addPassphrase(Passphrase.fromPassword(PASSPHRASE)))
+                        .overrideCompressionAlgorithm(CompressionAlgorithm.UNCOMPRESSED));
+        enc.write(PLAINTEXT.getBytes(StandardCharsets.UTF_8));
+        enc.close();
+
+        System.out.println(out);
+    }
+
+    @Test
+    public void processSENC_LIT() throws IOException, PGPException {
+        ByteArrayInputStream bytesIn = new ByteArrayInputStream(SENC_LIT.getBytes(StandardCharsets.UTF_8));
+        ArmoredInputStream armorIn = ArmoredInputStreamFactory.get(bytesIn);
+        MessageDecryptionStream decIn = new MessageDecryptionStream(armorIn, ConsumerOptions.get()
+                .addDecryptionPassphrase(Passphrase.fromPassword(PASSPHRASE)));
 
         ByteArrayOutputStream out = new ByteArrayOutputStream();
         Streams.pipeAll(decIn, out);

pgpainless-core/src/test/java/org/pgpainless/decryption_verification/PushDownAutomatonTest.java

@@ -1,205 +0,0 @@
-// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
-//
-// SPDX-License-Identifier: Apache-2.0
-
-package org.pgpainless.decryption_verification;
-
-import org.junit.jupiter.api.Test;
-import org.pgpainless.exception.MalformedOpenPgpMessageException;
-
-import static org.junit.jupiter.api.Assertions.assertThrows;
-import static org.junit.jupiter.api.Assertions.assertTrue;
-
-public class PushDownAutomatonTest {
-
-    /**
-     * MSG is valid.
-     *
-     * @throws MalformedOpenPgpMessageException fail
-     */
-    @Test
-    public void testSimpleLiteralMessageIsValid() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-
-        assertTrue(automaton.isValid());
-    }
-
-    /**
-     * OPS MSG SIG is valid.
-     *
-     * @throws MalformedOpenPgpMessageException fail
-     */
-    @Test
-    public void testSimpleOpsSignedMesssageIsValid() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.OnePassSignatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
-        automaton.next(PushdownAutomaton.InputAlphabet.Signatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-
-        assertTrue(automaton.isValid());
-    }
-
-    /**
-     * SIG MSG is valid.
-     *
-     * @throws MalformedOpenPgpMessageException fail
-     */
-    @Test
-    public void testSimplePrependSignedMessageIsValid() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.Signatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-
-        assertTrue(automaton.isValid());
-    }
-
-    /**
-     * OPS COMP(MSG) SIG is valid.
-     *
-     * @throws MalformedOpenPgpMessageException fail
-     */
-    @Test
-    public void testOPSSignedCompressedMessageIsValid() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.OnePassSignatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.CompressedData);
-        automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-        automaton.next(PushdownAutomaton.InputAlphabet.Signatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-
-        assertTrue(automaton.isValid());
-    }
-
-    /**
-     * OPS ENC(COMP(COMP(MSG))) SIG is valid.
-     *
-     * @throws MalformedOpenPgpMessageException fail
-     */
-    @Test
-    public void testOpsSignedEncryptedCompressedCompressedMessageIsValid() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.OnePassSignatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.EncryptedData);
-        automaton.next(PushdownAutomaton.InputAlphabet.CompressedData);
-        automaton.next(PushdownAutomaton.InputAlphabet.CompressedData);
-
-        automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
-
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-        automaton.next(PushdownAutomaton.InputAlphabet.Signatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-
-        assertTrue(automaton.isValid());
-    }
-
-    /**
-     * MSG SIG is invalid.
-     *
-     * @throws MalformedOpenPgpMessageException fail
-     */
-    @Test
-    public void testLiteralPlusSigsFails() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
-        assertThrows(MalformedOpenPgpMessageException.class,
-                () -> automaton.next(PushdownAutomaton.InputAlphabet.Signatures));
-    }
-
-    /**
-     * MSG MSG is invalid.
-     *
-     * @throws MalformedOpenPgpMessageException fail
-     */
-    @Test
-    public void testTwoLiteralDataPacketsFails() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
-        assertThrows(MalformedOpenPgpMessageException.class,
-                () -> automaton.next(PushdownAutomaton.InputAlphabet.LiteralData));
-    }
-
-    /**
-     * OPS COMP(MSG MSG) SIG is invalid (two literal packets are illegal).
-     *
-     * @throws MalformedOpenPgpMessageException fail
-     */
-    @Test
-    public void testOPSSignedMessageWithTwoLiteralDataPacketsFails() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.OnePassSignatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.CompressedData);
-        automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
-        assertThrows(MalformedOpenPgpMessageException.class,
-                () -> automaton.next(PushdownAutomaton.InputAlphabet.LiteralData));
-    }
-
-    /**
-     * OPS COMP(MSG) MSG SIG is invalid.
-     *
-     * @throws MalformedOpenPgpMessageException fail
-     */
-    @Test
-    public void testOPSSignedMessageWithTwoLiteralDataPacketsFails2() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.OnePassSignatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.CompressedData);
-        automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-        assertThrows(MalformedOpenPgpMessageException.class,
-                () -> automaton.next(PushdownAutomaton.InputAlphabet.LiteralData));
-    }
-
-    /**
-     * OPS COMP(MSG SIG) is invalid (MSG SIG does not form valid nested message).
-     *
-     * @throws MalformedOpenPgpMessageException fail
-     */
-    @Test
-    public void testCorrespondingSignaturesOfOpsSignedMessageAreLayerFurtherDownFails() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.OnePassSignatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.CompressedData);
-        automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
-        assertThrows(MalformedOpenPgpMessageException.class,
-                () -> automaton.next(PushdownAutomaton.InputAlphabet.Signatures));
-    }
-
-    /**
-     * Empty COMP is invalid.
-     */
-    @Test
-    public void testEmptyCompressedDataIsInvalid() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.CompressedData);
-        assertThrows(MalformedOpenPgpMessageException.class,
-                () -> automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence));
-    }
-
-    @Test
-    public void testOPSSignedEncryptedCompressedOPSSignedMessageIsValid() throws MalformedOpenPgpMessageException {
-        PushdownAutomaton automaton = new PushdownAutomaton();
-        automaton.next(PushdownAutomaton.InputAlphabet.OnePassSignatures);
-
-        automaton.next(PushdownAutomaton.InputAlphabet.EncryptedData);
-        automaton.next(PushdownAutomaton.InputAlphabet.OnePassSignatures);
-
-        automaton.next(PushdownAutomaton.InputAlphabet.CompressedData);
-        automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-
-        automaton.next(PushdownAutomaton.InputAlphabet.Signatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-
-        automaton.next(PushdownAutomaton.InputAlphabet.Signatures);
-        automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
-
-        assertTrue(automaton.isValid());
-    }
-}

pgpainless-core/src/test/java/org/pgpainless/decryption_verification/automaton/NestingPDATest.java

@@ -0,0 +1,205 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.decryption_verification.automaton;
+
+import org.junit.jupiter.api.Test;
+import org.pgpainless.exception.MalformedOpenPgpMessageException;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class NestingPDATest {
+
+    /**
+     * MSG is valid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testSimpleLiteralMessageIsValid() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.LiteralData);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        assertTrue(automaton.isValid());
+    }
+
+    /**
+     * OPS MSG SIG is valid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testSimpleOpsSignedMesssageIsValid() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.OnePassSignatures);
+        automaton.next(InputAlphabet.LiteralData);
+        automaton.next(InputAlphabet.Signatures);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        assertTrue(automaton.isValid());
+    }
+
+    /**
+     * SIG MSG is valid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testSimplePrependSignedMessageIsValid() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.Signatures);
+        automaton.next(InputAlphabet.LiteralData);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        assertTrue(automaton.isValid());
+    }
+
+    /**
+     * OPS COMP(MSG) SIG is valid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testOPSSignedCompressedMessageIsValid() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.OnePassSignatures);
+        automaton.next(InputAlphabet.CompressedData);
+        automaton.next(InputAlphabet.LiteralData);
+        automaton.next(InputAlphabet.EndOfSequence);
+        automaton.next(InputAlphabet.Signatures);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        assertTrue(automaton.isValid());
+    }
+
+    /**
+     * OPS ENC(COMP(COMP(MSG))) SIG is valid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testOpsSignedEncryptedCompressedCompressedMessageIsValid() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.OnePassSignatures);
+        automaton.next(InputAlphabet.EncryptedData);
+        automaton.next(InputAlphabet.CompressedData);
+        automaton.next(InputAlphabet.CompressedData);
+
+        automaton.next(InputAlphabet.LiteralData);
+
+        automaton.next(InputAlphabet.EndOfSequence);
+        automaton.next(InputAlphabet.EndOfSequence);
+        automaton.next(InputAlphabet.EndOfSequence);
+        automaton.next(InputAlphabet.Signatures);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        assertTrue(automaton.isValid());
+    }
+
+    /**
+     * MSG SIG is invalid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testLiteralPlusSigsFails() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.LiteralData);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> automaton.next(InputAlphabet.Signatures));
+    }
+
+    /**
+     * MSG MSG is invalid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testTwoLiteralDataPacketsFails() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.LiteralData);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> automaton.next(InputAlphabet.LiteralData));
+    }
+
+    /**
+     * OPS COMP(MSG MSG) SIG is invalid (two literal packets are illegal).
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testOPSSignedMessageWithTwoLiteralDataPacketsFails() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.OnePassSignatures);
+        automaton.next(InputAlphabet.CompressedData);
+        automaton.next(InputAlphabet.LiteralData);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> automaton.next(InputAlphabet.LiteralData));
+    }
+
+    /**
+     * OPS COMP(MSG) MSG SIG is invalid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testOPSSignedMessageWithTwoLiteralDataPacketsFails2() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.OnePassSignatures);
+        automaton.next(InputAlphabet.CompressedData);
+        automaton.next(InputAlphabet.LiteralData);
+        automaton.next(InputAlphabet.EndOfSequence);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> automaton.next(InputAlphabet.LiteralData));
+    }
+
+    /**
+     * OPS COMP(MSG SIG) is invalid (MSG SIG does not form valid nested message).
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testCorrespondingSignaturesOfOpsSignedMessageAreLayerFurtherDownFails() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.OnePassSignatures);
+        automaton.next(InputAlphabet.CompressedData);
+        automaton.next(InputAlphabet.LiteralData);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> automaton.next(InputAlphabet.Signatures));
+    }
+
+    /**
+     * Empty COMP is invalid.
+     */
+    @Test
+    public void testEmptyCompressedDataIsInvalid() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.CompressedData);
+        assertThrows(MalformedOpenPgpMessageException.class,
+                () -> automaton.next(InputAlphabet.EndOfSequence));
+    }
+
+    @Test
+    public void testOPSSignedEncryptedCompressedOPSSignedMessageIsValid() throws MalformedOpenPgpMessageException {
+        NestingPDA automaton = new NestingPDA();
+        automaton.next(InputAlphabet.OnePassSignatures);
+
+        automaton.next(InputAlphabet.EncryptedData);
+        automaton.next(InputAlphabet.OnePassSignatures);
+
+        automaton.next(InputAlphabet.CompressedData);
+        automaton.next(InputAlphabet.LiteralData);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        automaton.next(InputAlphabet.Signatures);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        automaton.next(InputAlphabet.Signatures);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        assertTrue(automaton.isValid());
+    }
+}

pgpainless-core/src/test/java/org/pgpainless/decryption_verification/automaton/PDATest.java

@@ -0,0 +1,75 @@
+package org.pgpainless.decryption_verification.automaton;
+
+import org.junit.jupiter.api.Test;
+import org.pgpainless.exception.MalformedOpenPgpMessageException;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class PDATest {
+
+
+    /**
+     * MSG is valid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testSimpleLiteralMessageIsValid() throws MalformedOpenPgpMessageException {
+        PDA automaton = new PDA();
+        automaton.next(InputAlphabet.LiteralData);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        assertTrue(automaton.isValid());
+    }
+
+    /**
+     * OPS MSG SIG is valid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testSimpleOpsSignedMesssageIsValid() throws MalformedOpenPgpMessageException {
+        PDA automaton = new PDA();
+        automaton.next(InputAlphabet.OnePassSignatures);
+        automaton.next(InputAlphabet.LiteralData);
+        automaton.next(InputAlphabet.Signatures);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        assertTrue(automaton.isValid());
+    }
+
+
+    /**
+     * SIG MSG is valid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testSimplePrependSignedMessageIsValid() throws MalformedOpenPgpMessageException {
+        PDA automaton = new PDA();
+        automaton.next(InputAlphabet.Signatures);
+        automaton.next(InputAlphabet.LiteralData);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        assertTrue(automaton.isValid());
+    }
+
+
+    /**
+     * OPS COMP(MSG) SIG is valid.
+     *
+     * @throws MalformedOpenPgpMessageException fail
+     */
+    @Test
+    public void testOPSSignedCompressedMessageIsValid() throws MalformedOpenPgpMessageException {
+        PDA automaton = new PDA();
+        automaton.next(InputAlphabet.OnePassSignatures);
+        automaton.next(InputAlphabet.CompressedData);
+        // Here would be a nested PDA for the LiteralData packet
+        automaton.next(InputAlphabet.Signatures);
+        automaton.next(InputAlphabet.EndOfSequence);
+
+        assertTrue(automaton.isValid());
+    }
+
+}