From 9fcb9782a3be9e84ec761c95bcff2aef65bcf6ed Mon Sep 17 00:00:00 2001 From: Paul Schaub Date: Thu, 8 Sep 2022 18:23:20 +0200 Subject: [PATCH] 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 --- .../PushdownAutomaton.java | 391 ++++++++++++++++++ .../MalformedOpenPgpMessageException.java | 31 ++ .../PushDownAutomatonTest.java | 205 +++++++++ 3 files changed, 627 insertions(+) create mode 100644 pgpainless-core/src/main/java/org/pgpainless/decryption_verification/PushdownAutomaton.java create mode 100644 pgpainless-core/src/main/java/org/pgpainless/exception/MalformedOpenPgpMessageException.java create mode 100644 pgpainless-core/src/test/java/org/pgpainless/decryption_verification/PushDownAutomatonTest.java diff --git a/pgpainless-core/src/main/java/org/pgpainless/decryption_verification/PushdownAutomaton.java b/pgpainless-core/src/main/java/org/pgpainless/decryption_verification/PushdownAutomaton.java new file mode 100644 index 00000000..6930de4b --- /dev/null +++ b/pgpainless-core/src/main/java/org/pgpainless/decryption_verification/PushdownAutomaton.java @@ -0,0 +1,391 @@ +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. + *

+ * OpenPGP messages MUST follow certain rules in order to be well-formed. + * Section §11.3. of RFC4880 specifies a formal grammar for OpenPGP messages. + *

+ * 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. + *

+ * 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. + *

+ * 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. + *

+ * 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 RFC4880 §11.3. OpenPGP Messages + */ +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 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. + *

+ * 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. + *

+ * 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(); + } +} diff --git a/pgpainless-core/src/main/java/org/pgpainless/exception/MalformedOpenPgpMessageException.java b/pgpainless-core/src/main/java/org/pgpainless/exception/MalformedOpenPgpMessageException.java new file mode 100644 index 00000000..07fed365 --- /dev/null +++ b/pgpainless-core/src/main/java/org/pgpainless/exception/MalformedOpenPgpMessageException.java @@ -0,0 +1,31 @@ +// SPDX-FileCopyrightText: 2022 Paul Schaub +// +// 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 RFC4880 §11.3. OpenPGP Messages + */ +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."); + } +} diff --git a/pgpainless-core/src/test/java/org/pgpainless/decryption_verification/PushDownAutomatonTest.java b/pgpainless-core/src/test/java/org/pgpainless/decryption_verification/PushDownAutomatonTest.java new file mode 100644 index 00000000..1bd07308 --- /dev/null +++ b/pgpainless-core/src/test/java/org/pgpainless/decryption_verification/PushDownAutomatonTest.java @@ -0,0 +1,205 @@ +// SPDX-FileCopyrightText: 2022 Paul Schaub +// +// 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()); + } +}