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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
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package org.pgpainless.decryption_verification;
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import org.bouncycastle.openpgp.PGPCompressedData;
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import org.bouncycastle.openpgp.PGPEncryptedDataList;
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import org.bouncycastle.openpgp.PGPLiteralData;
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import org.bouncycastle.openpgp.PGPOnePassSignatureList;
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import org.bouncycastle.openpgp.PGPSignatureList;
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import org.pgpainless.exception.MalformedOpenPgpMessageException;
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import java.util.Stack;
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import static org.pgpainless.decryption_verification.PushdownAutomaton.StackAlphabet.msg;
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import static org.pgpainless.decryption_verification.PushdownAutomaton.StackAlphabet.ops;
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import static org.pgpainless.decryption_verification.PushdownAutomaton.StackAlphabet.terminus;
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/**
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* Pushdown Automaton to verify the correct syntax of OpenPGP messages during decryption.
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* <p>
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* OpenPGP messages MUST follow certain rules in order to be well-formed.
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* Section §11.3. of RFC4880 specifies a formal grammar for OpenPGP messages.
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* <p>
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* This grammar was transformed into a pushdown automaton, which is implemented below.
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* The automaton only ends up in a valid state ({@link #isValid()} iff the OpenPGP message conformed to the
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* grammar.
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* <p>
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* There are some specialties with this implementation though:
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* Bouncy Castle combines ESKs and Encrypted Data Packets into a single object, so we do not have to
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* handle those manually.
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* <p>
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* Bouncy Castle further combines OnePassSignatures and Signatures into lists, so instead of pushing multiple
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* 'o's onto the stack repeatedly, a sequence of OnePassSignatures causes a single 'o' to be pushed to the stack.
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* The same is true for Signatures.
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* <p>
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* Therefore, a message is valid, even if the number of OnePassSignatures and Signatures does not match.
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* If a message contains at least one OnePassSignature, it is sufficient if there is at least one Signature to
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* not cause a {@link MalformedOpenPgpMessageException}.
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*
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* @see <a href="https://www.rfc-editor.org/rfc/rfc4880#section-11.3">RFC4880 §11.3. OpenPGP Messages</a>
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*/
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public class PushdownAutomaton {
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public enum InputAlphabet {
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/**
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* A {@link PGPLiteralData} packet.
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*/
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LiteralData,
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/**
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* A {@link PGPSignatureList} object.
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*/
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Signatures,
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/**
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* A {@link PGPOnePassSignatureList} object.
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*/
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OnePassSignatures,
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/**
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* A {@link PGPCompressedData} packet.
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* The contents of this packet MUST form a valid OpenPGP message, so a nested PDA is opened to verify
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* its nested packet sequence.
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*/
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CompressedData,
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/**
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* A {@link PGPEncryptedDataList} object.
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* This object combines multiple ESKs and the corresponding Symmetrically Encrypted
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* (possibly Integrity Protected) Data packet.
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*/
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EncryptedData,
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/**
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* Marks the end of a (sub-) sequence.
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* This input is given if the end of an OpenPGP message is reached.
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* This might be the case for the end of the whole ciphertext, or the end of a packet with nested contents
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* (e.g. the end of a Compressed Data packet).
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*/
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EndOfSequence
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}
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public enum StackAlphabet {
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/**
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* OpenPGP Message.
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*/
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msg,
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/**
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* OnePassSignature (in case of BC this represents a OnePassSignatureList).
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*/
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ops,
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/**
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* ESK. Not used, as BC combines encrypted data with their encrypted session keys.
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*/
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esk,
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/**
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* Special symbol representing the end of the message.
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*/
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terminus
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}
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/**
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* Set of states of the automaton.
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* Each state defines its valid transitions in their {@link State#transition(InputAlphabet, PushdownAutomaton)}
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* method.
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*/
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public enum State {
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OpenPgpMessage {
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@Override
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State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
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StackAlphabet stackItem = automaton.popStack();
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if (stackItem != msg) {
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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switch (input) {
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case LiteralData:
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return LiteralMessage;
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case Signatures:
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automaton.pushStack(msg);
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return OpenPgpMessage;
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case OnePassSignatures:
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automaton.pushStack(ops);
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automaton.pushStack(msg);
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return OpenPgpMessage;
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case CompressedData:
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return CompressedMessage;
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case EncryptedData:
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return EncryptedMessage;
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case EndOfSequence:
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default:
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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}
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},
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LiteralMessage {
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@Override
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State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
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StackAlphabet stackItem = automaton.popStack();
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switch (input) {
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case Signatures:
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if (stackItem == ops) {
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return CorrespondingSignature;
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} else {
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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case EndOfSequence:
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if (stackItem == terminus && automaton.stack.isEmpty()) {
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return Valid;
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} else {
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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case LiteralData:
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case OnePassSignatures:
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case CompressedData:
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case EncryptedData:
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default:
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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}
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},
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CompressedMessage {
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@Override
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State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
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StackAlphabet stackItem = automaton.popStack();
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switch (input) {
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case Signatures:
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if (stackItem == ops) {
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return CorrespondingSignature;
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} else {
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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case EndOfSequence:
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if (stackItem == terminus && automaton.stack.isEmpty()) {
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return Valid;
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} else {
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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case LiteralData:
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case OnePassSignatures:
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case CompressedData:
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case EncryptedData:
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default:
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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}
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},
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EncryptedMessage {
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@Override
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State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
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StackAlphabet stackItem = automaton.popStack();
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switch (input) {
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case Signatures:
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if (stackItem == ops) {
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return CorrespondingSignature;
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} else {
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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case EndOfSequence:
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if (stackItem == terminus && automaton.stack.isEmpty()) {
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return Valid;
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} else {
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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case LiteralData:
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case OnePassSignatures:
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case CompressedData:
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case EncryptedData:
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default:
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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}
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},
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CorrespondingSignature {
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@Override
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State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
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StackAlphabet stackItem = automaton.popStack();
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if (stackItem == terminus && input == InputAlphabet.EndOfSequence && automaton.stack.isEmpty()) {
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return Valid;
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} else {
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throw new MalformedOpenPgpMessageException(this, input, stackItem);
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}
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}
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},
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Valid {
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@Override
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State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException {
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throw new MalformedOpenPgpMessageException(this, input, null);
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}
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},
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;
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/**
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* Pop the automatons stack and transition to another state.
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* If no valid transition from the current state is available given the popped stack item and input symbol,
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* a {@link MalformedOpenPgpMessageException} is thrown.
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* Otherwise, the stack is manipulated according to the valid transition and the new state is returned.
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*
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* @param input input symbol
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* @param automaton automaton
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* @return new state of the automaton
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* @throws MalformedOpenPgpMessageException in case of an illegal input symbol
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*/
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abstract State transition(InputAlphabet input, PushdownAutomaton automaton) throws MalformedOpenPgpMessageException;
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}
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private final Stack<StackAlphabet> stack = new Stack<>();
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private State state;
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// Some OpenPGP packets have nested contents (e.g. compressed / encrypted data).
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PushdownAutomaton nestedSequence = null;
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public PushdownAutomaton() {
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state = State.OpenPgpMessage;
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stack.push(terminus);
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stack.push(msg);
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}
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/**
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* Process the next input packet.
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*
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* @param input input
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* @throws MalformedOpenPgpMessageException in case the input packet is illegal here
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*/
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public void next(InputAlphabet input) throws MalformedOpenPgpMessageException {
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_next(input);
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}
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/**
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* Process the next input packet.
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* This method returns true, iff the given input triggered a successful closing of this PDAs nested PDA.
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* <p>
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* This is for example the case, if the current packet is a Compressed Data packet which contains a
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* valid nested OpenPGP message and the last input was {@link InputAlphabet#EndOfSequence} indicating the
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* end of the Compressed Data packet.
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* <p>
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* If the input triggered this PDAs nested PDA to close its nested PDA, this method returns false
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* in order to prevent this PDA from closing its nested PDA prematurely.
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*
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* @param input input
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* @return true if this just closed its nested sequence, false otherwise
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* @throws MalformedOpenPgpMessageException if the input is illegal
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*/
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private boolean _next(InputAlphabet input) throws MalformedOpenPgpMessageException {
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if (nestedSequence != null) {
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boolean sequenceInNestedSequenceWasClosed = nestedSequence._next(input);
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if (sequenceInNestedSequenceWasClosed) return false; // No need to close out nested sequence too.
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} else {
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// make a state transition in this automaton
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state = state.transition(input, this);
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// If the processed packet contains nested sequence, open nested automaton for it
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if (input == InputAlphabet.CompressedData || input == InputAlphabet.EncryptedData) {
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nestedSequence = new PushdownAutomaton();
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}
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}
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if (input != InputAlphabet.EndOfSequence) {
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return false;
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}
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// Close nested sequence if needed
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boolean nestedIsInnerMost = nestedSequence != null && nestedSequence.isInnerMost();
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if (nestedIsInnerMost) {
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if (nestedSequence.isValid()) {
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// Close nested sequence
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nestedSequence = null;
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return true;
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} else {
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throw new MalformedOpenPgpMessageException("Climbing up nested message validation failed." +
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" Automaton for current nesting level is not in valid state: " + nestedSequence.getState() + " " + nestedSequence.stack.peek() + " (Input was " + input + ")");
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}
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}
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return false;
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}
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/**
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* Return the current state of the PDA.
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*
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* @return state
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*/
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private State getState() {
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return state;
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}
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/**
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* Return true, if the PDA is in a valid state (the OpenPGP message is valid).
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*
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* @return true if valid, false otherwise
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*/
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public boolean isValid() {
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return getState() == State.Valid && stack.isEmpty();
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}
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/**
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* Pop an item from the stack.
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*
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* @return stack item
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*/
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private StackAlphabet popStack() {
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return stack.pop();
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}
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/**
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* Push an item onto the stack.
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*
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* @param item item
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*/
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private void pushStack(StackAlphabet item) {
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stack.push(item);
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}
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/**
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* Return true, if this packet sequence has no nested sequence.
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* A nested sequence is for example the content of a Compressed Data packet.
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*
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* @return true if PDA is innermost, false if it has a nested sequence
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*/
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private boolean isInnerMost() {
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return nestedSequence == null;
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}
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@Override
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public String toString() {
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StringBuilder out = new StringBuilder("State: ").append(state)
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.append(", Stack (asc.): ").append(stack)
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.append('\n');
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if (nestedSequence != null) {
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// recursively call toString() on nested PDAs and indent their representation
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String nestedToString = nestedSequence.toString();
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String[] lines = nestedToString.split("\n");
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for (int i = 0; i < lines.length; i++) {
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String nestedLine = lines[i];
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out.append(i == 0 ? "⤷ " : " ") // indent nested PDA
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.append(nestedLine)
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.append('\n');
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}
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}
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return out.toString();
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}
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}
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// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
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//
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// SPDX-License-Identifier: Apache-2.0
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package org.pgpainless.exception;
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import org.bouncycastle.openpgp.PGPException;
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import org.pgpainless.decryption_verification.PushdownAutomaton;
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/**
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* Exception that gets thrown if the OpenPGP message is malformed.
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* Malformed messages are messages which do not follow the grammar specified in the RFC.
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*
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* @see <a href="https://www.rfc-editor.org/rfc/rfc4880#section-11.3">RFC4880 §11.3. OpenPGP Messages</a>
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*/
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public class MalformedOpenPgpMessageException extends PGPException {
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public MalformedOpenPgpMessageException(String message) {
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super(message);
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}
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public MalformedOpenPgpMessageException(String message, MalformedOpenPgpMessageException cause) {
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super(message, cause);
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}
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public MalformedOpenPgpMessageException(PushdownAutomaton.State state,
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PushdownAutomaton.InputAlphabet input,
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PushdownAutomaton.StackAlphabet stackItem) {
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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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}
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}
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// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
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//
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// SPDX-License-Identifier: Apache-2.0
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package org.pgpainless.decryption_verification;
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import org.junit.jupiter.api.Test;
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import org.pgpainless.exception.MalformedOpenPgpMessageException;
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import static org.junit.jupiter.api.Assertions.assertThrows;
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import static org.junit.jupiter.api.Assertions.assertTrue;
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public class PushDownAutomatonTest {
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/**
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* MSG is valid.
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*
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* @throws MalformedOpenPgpMessageException fail
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*/
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@Test
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public void testSimpleLiteralMessageIsValid() throws MalformedOpenPgpMessageException {
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PushdownAutomaton automaton = new PushdownAutomaton();
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automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
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automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
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assertTrue(automaton.isValid());
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}
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/**
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* OPS MSG SIG is valid.
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*
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* @throws MalformedOpenPgpMessageException fail
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*/
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@Test
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public void testSimpleOpsSignedMesssageIsValid() throws MalformedOpenPgpMessageException {
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PushdownAutomaton automaton = new PushdownAutomaton();
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automaton.next(PushdownAutomaton.InputAlphabet.OnePassSignatures);
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automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
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automaton.next(PushdownAutomaton.InputAlphabet.Signatures);
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automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
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assertTrue(automaton.isValid());
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}
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/**
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* SIG MSG is valid.
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*
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* @throws MalformedOpenPgpMessageException fail
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*/
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@Test
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public void testSimplePrependSignedMessageIsValid() throws MalformedOpenPgpMessageException {
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PushdownAutomaton automaton = new PushdownAutomaton();
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automaton.next(PushdownAutomaton.InputAlphabet.Signatures);
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automaton.next(PushdownAutomaton.InputAlphabet.LiteralData);
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automaton.next(PushdownAutomaton.InputAlphabet.EndOfSequence);
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assertTrue(automaton.isValid());
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}
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/**
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* OPS COMP(MSG) SIG is valid.
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*
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* @throws MalformedOpenPgpMessageException fail
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*/
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@Test
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public void testOPSSignedCompressedMessageIsValid() throws MalformedOpenPgpMessageException {
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PushdownAutomaton automaton = new PushdownAutomaton();
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automaton.next(PushdownAutomaton.InputAlphabet.OnePassSignatures);
|
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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());
|
||||
}
|
||||
}
|
||||
Loading…
Reference in a new issue