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Add Pushdown Automaton for checking OpenPGP message syntax

The automaton implements what is described in
https://github.com/pgpainless/pgpainless/blob/main/misc/OpenPGPMessageFormat.md

However, some differences exist to adopt it to BouncyCastle

Part of #237
This commit is contained in:
Paul Schaub 2022-09-08 18:23:20 +02:00
parent 95ba4e46ca
commit 9fcb9782a3
3 changed files with 627 additions and 0 deletions

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package org.pgpainless.decryption_verification;
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.PushdownAutomaton.StackAlphabet.ops;
import static org.pgpainless.decryption_verification.PushdownAutomaton.StackAlphabet.terminus;
/**
* Pushdown Automaton to verify the correct syntax of OpenPGP messages during decryption.
* <p>
* OpenPGP messages MUST follow certain rules in order to be well-formed.
* Section §11.3. of RFC4880 specifies a formal grammar for OpenPGP messages.
* <p>
* This grammar was transformed into a pushdown automaton, which is implemented below.
* The automaton only ends up in a valid state ({@link #isValid()} iff the OpenPGP message conformed to the
* grammar.
* <p>
* There are some specialties with this implementation though:
* Bouncy Castle combines ESKs and Encrypted Data Packets into a single object, so we do not have to
* handle those manually.
* <p>
* Bouncy Castle further combines OnePassSignatures and Signatures into lists, so instead of pushing multiple
* 'o's onto the stack repeatedly, a sequence of OnePassSignatures causes a single 'o' to be pushed to the stack.
* The same is true for Signatures.
* <p>
* Therefore, a message is valid, even if the number of OnePassSignatures and Signatures does not match.
* If a message contains at least one OnePassSignature, it is sufficient if there is at least one Signature to
* not cause a {@link MalformedOpenPgpMessageException}.
*
* @see <a href="https://www.rfc-editor.org/rfc/rfc4880#section-11.3">RFC4880 §11.3. OpenPGP Messages</a>
*/
public class PushdownAutomaton {
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)}
* method.
*/
public enum State {
OpenPgpMessage {
@Override
State transition(InputAlphabet input, PushdownAutomaton 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, PushdownAutomaton 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, PushdownAutomaton 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, PushdownAutomaton 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, PushdownAutomaton 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, PushdownAutomaton 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, PushdownAutomaton 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;
public PushdownAutomaton() {
state = State.OpenPgpMessage;
stack.push(terminus);
stack.push(msg);
}
/**
* Process the next input packet.
*
* @param input input
* @throws MalformedOpenPgpMessageException in case the input packet is illegal here
*/
public void next(InputAlphabet input) throws MalformedOpenPgpMessageException {
_next(input);
}
/**
* Process the next input packet.
* This method returns true, iff the given input triggered a successful closing of this PDAs nested PDA.
* <p>
* This is for example the case, if the current packet is a Compressed Data packet which contains a
* valid nested OpenPGP message and the last input was {@link InputAlphabet#EndOfSequence} indicating the
* end of the Compressed Data packet.
* <p>
* If the input triggered this PDAs nested PDA to close its nested PDA, this method returns false
* in order to prevent this PDA from closing its nested PDA prematurely.
*
* @param input input
* @return true if this just closed its nested sequence, false otherwise
* @throws MalformedOpenPgpMessageException if the input is illegal
*/
private boolean _next(InputAlphabet input) throws MalformedOpenPgpMessageException {
if (nestedSequence != null) {
boolean sequenceInNestedSequenceWasClosed = nestedSequence._next(input);
if (sequenceInNestedSequenceWasClosed) return false; // No need to close out nested sequence too.
} else {
// make a state transition in this automaton
state = state.transition(input, this);
// If the processed packet contains nested sequence, open nested automaton for it
if (input == InputAlphabet.CompressedData || input == InputAlphabet.EncryptedData) {
nestedSequence = new PushdownAutomaton();
}
}
if (input != InputAlphabet.EndOfSequence) {
return false;
}
// Close nested sequence if needed
boolean nestedIsInnerMost = nestedSequence != null && nestedSequence.isInnerMost();
if (nestedIsInnerMost) {
if (nestedSequence.isValid()) {
// Close nested sequence
nestedSequence = null;
return true;
} else {
throw new MalformedOpenPgpMessageException("Climbing up nested message validation failed." +
" Automaton for current nesting level is not in valid state: " + nestedSequence.getState() + " " + nestedSequence.stack.peek() + " (Input was " + input + ")");
}
}
return false;
}
/**
* Return the current state of the PDA.
*
* @return state
*/
private 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();
}
/**
* 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);
}
/**
* Return true, if this packet sequence has no nested sequence.
* A nested sequence is for example the content of a Compressed Data packet.
*
* @return true if PDA is innermost, false if it has a nested sequence
*/
private boolean isInnerMost() {
return nestedSequence == null;
}
@Override
public String toString() {
StringBuilder out = new StringBuilder("State: ").append(state)
.append(", Stack (asc.): ").append(stack)
.append('\n');
if (nestedSequence != null) {
// recursively call toString() on nested PDAs and indent their representation
String nestedToString = nestedSequence.toString();
String[] lines = nestedToString.split("\n");
for (int i = 0; i < lines.length; i++) {
String nestedLine = lines[i];
out.append(i == 0 ? "" : " ") // indent nested PDA
.append(nestedLine)
.append('\n');
}
}
return out.toString();
}
}

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// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
//
// SPDX-License-Identifier: Apache-2.0
package org.pgpainless.exception;
import org.bouncycastle.openpgp.PGPException;
import org.pgpainless.decryption_verification.PushdownAutomaton;
/**
* Exception that gets thrown if the OpenPGP message is malformed.
* Malformed messages are messages which do not follow the grammar specified in the RFC.
*
* @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 MalformedOpenPgpMessageException(String message) {
super(message);
}
public MalformedOpenPgpMessageException(String message, MalformedOpenPgpMessageException cause) {
super(message, cause);
}
public MalformedOpenPgpMessageException(PushdownAutomaton.State state,
PushdownAutomaton.InputAlphabet input,
PushdownAutomaton.StackAlphabet stackItem) {
this("Invalid input: There is no legal transition from state '" + state + "' for input '" + input + "' when '" + stackItem + "' is on top of the stack.");
}
}

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// 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());
}
}