Kotlin conversion: PDA
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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.syntax_check;
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import org.pgpainless.exception.MalformedOpenPgpMessageException;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import javax.annotation.Nonnull;
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import javax.annotation.Nullable;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.List;
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import java.util.Stack;
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import static org.pgpainless.decryption_verification.syntax_check.StackSymbol.msg;
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import static org.pgpainless.decryption_verification.syntax_check.StackSymbol.terminus;
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/**
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* Pushdown Automaton for validating context-free languages.
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* In PGPainless, this class is used to validate OpenPGP message packet sequences against the allowed syntax.
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*
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* @see <a href="https://www.rfc-editor.org/rfc/rfc4880#section-11.3">OpenPGP Message Syntax</a>
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*/
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public class PDA {
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private static final Logger LOGGER = LoggerFactory.getLogger(PDA.class);
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// right now we implement what rfc4880 specifies.
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// TODO: Consider implementing what we proposed here:
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// https://mailarchive.ietf.org/arch/msg/openpgp/uepOF6XpSegMO4c59tt9e5H1i4g/
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private final Syntax syntax;
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private final Stack<StackSymbol> stack = new Stack<>();
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private final List<InputSymbol> inputs = new ArrayList<>(); // Track inputs for debugging / error reporting
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private State state;
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/**
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* Default constructor which initializes the PDA to work with the {@link OpenPgpMessageSyntax}.
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*/
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public PDA() {
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this(new OpenPgpMessageSyntax(), State.OpenPgpMessage, terminus, msg);
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}
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/**
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* Construct a PDA with a custom {@link Syntax}, initial {@link State} and initial {@link StackSymbol StackSymbols}.
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*
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* @param syntax syntax
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* @param initialState initial state
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* @param initialStack zero or more initial stack items (get pushed onto the stack in order of appearance)
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*/
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public PDA(@Nonnull Syntax syntax, @Nonnull State initialState, @Nonnull StackSymbol... initialStack) {
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this.syntax = syntax;
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this.state = initialState;
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for (StackSymbol symbol : initialStack) {
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pushStack(symbol);
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}
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}
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/**
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* Process the next {@link InputSymbol}.
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* This will either leave the PDA in the next state, or throw a {@link MalformedOpenPgpMessageException} if the
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* input symbol is rejected.
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*
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* @param input input symbol
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* @throws MalformedOpenPgpMessageException if the input symbol is rejected
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*/
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public void next(@Nonnull InputSymbol input)
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throws MalformedOpenPgpMessageException {
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StackSymbol stackSymbol = popStack();
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try {
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Transition transition = syntax.transition(state, input, stackSymbol);
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state = transition.getNewState();
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for (StackSymbol item : transition.getPushedItems()) {
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pushStack(item);
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}
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inputs.add(input);
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} catch (MalformedOpenPgpMessageException e) {
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MalformedOpenPgpMessageException wrapped = new MalformedOpenPgpMessageException(
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"Malformed message: After reading packet sequence " + Arrays.toString(inputs.toArray()) +
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", token '" + input + "' is not allowed." +
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"\nNo transition from state '" + state + "' with stack " + Arrays.toString(stack.toArray()) +
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(stackSymbol != null ? "||'" + stackSymbol + "'." : "."), e);
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LOGGER.debug("Invalid input '" + input + "'", wrapped);
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throw wrapped;
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}
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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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public @Nonnull State getState() {
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return state;
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}
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/**
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* Peek at the stack, returning the topmost stack item without changing the stack.
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*
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* @return topmost stack item, or null if stack is empty
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*/
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public @Nullable StackSymbol peekStack() {
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if (stack.isEmpty()) {
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return null;
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}
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return stack.peek();
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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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* Throw a {@link MalformedOpenPgpMessageException} if the pda is not in a valid state right now.
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*
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* @throws MalformedOpenPgpMessageException if the pda is not in an acceptable state
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*/
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public void assertValid() throws MalformedOpenPgpMessageException {
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if (!isValid()) {
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throw new MalformedOpenPgpMessageException("Pushdown Automaton is not in an acceptable state: " + toString());
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}
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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 StackSymbol popStack() {
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if (stack.isEmpty()) {
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return null;
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}
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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(StackSymbol item) {
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stack.push(item);
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}
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@Override
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public String toString() {
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return "State: " + state + " Stack: " + stack;
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}
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}
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@ -0,0 +1,120 @@
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// SPDX-FileCopyrightText: 2023 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.syntax_check
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import org.pgpainless.exception.MalformedOpenPgpMessageException
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import org.slf4j.LoggerFactory
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/**
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* Pushdown Automaton for validating context-free languages.
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* In PGPainless, this class is used to validate OpenPGP message packet sequences against the allowed syntax.
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*
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* See [OpenPGP Message Syntax](https://www.rfc-editor.org/rfc/rfc4880#section-11.3)
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*/
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class PDA constructor(
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private val syntax: Syntax,
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private val stack: ArrayDeque<StackSymbol>,
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private val inputs: MutableList<InputSymbol>,
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private var state: State
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) {
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/**
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* Construct a PDA with a custom [Syntax], initial [State] and initial [StackSymbols][StackSymbol].
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*
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* @param syntax syntax
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* @param initialState initial state
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* @param initialStack zero or more initial stack items (get pushed onto the stack in order of appearance)
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*/
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constructor(syntax: Syntax, initialState: State, vararg initialStack: StackSymbol): this (
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syntax, ArrayDeque(initialStack.toList().reversed()), mutableListOf(), initialState)
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/**
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* Default constructor which initializes the PDA to work with the [OpenPgpMessageSyntax].
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*/
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constructor(): this(OpenPgpMessageSyntax(), State.OpenPgpMessage, StackSymbol.terminus, StackSymbol.msg)
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/**
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* Process the next [InputSymbol].
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* This will either leave the PDA in the next state, or throw a [MalformedOpenPgpMessageException] if the
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* input symbol is rejected.
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*
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* @param input input symbol
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* @throws MalformedOpenPgpMessageException if the input symbol is rejected
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*/
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fun next(input: InputSymbol) {
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val stackSymbol = popStack()
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try {
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val transition = syntax.transition(state, input, stackSymbol)
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state = transition.newState
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for (item in transition.pushedItems) {
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pushStack(item)
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}
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inputs.add(input)
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} catch (e: MalformedOpenPgpMessageException) {
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val stackFormat = if (stackSymbol != null) {
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"${stack.joinToString()}||$stackSymbol"
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} else {
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stack.joinToString()
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}
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val wrapped = MalformedOpenPgpMessageException(
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"Malformed message: After reading packet sequence ${inputs.joinToString()}, token '$input' is not allowed.\n" +
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"No transition from state '$state' with stack $stackFormat", e)
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LOGGER.debug("Invalid input '$input'", wrapped)
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throw wrapped
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}
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}
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/**
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* Peek at the stack, returning the topmost stack item without changing the stack.
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*
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* @return topmost stack item, or null if stack is empty
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*/
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fun peekStack(): StackSymbol? = stack.firstOrNull()
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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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fun isValid(): Boolean = state == State.Valid && stack.isEmpty()
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/**
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* Throw a [MalformedOpenPgpMessageException] if the pda is not in a valid state right now.
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*
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* @throws MalformedOpenPgpMessageException if the pda is not in an acceptable state
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*/
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fun assertValid() {
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if (!isValid()) {
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throw MalformedOpenPgpMessageException("Pushdown Automaton is not in an acceptable state: ${toString()}")
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}
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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 fun popStack(): StackSymbol? {
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return stack.removeFirstOrNull()
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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 fun pushStack(item: StackSymbol) {
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stack.addFirst(item)
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}
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override fun toString(): String {
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return "State: $state Stack: ${stack.joinToString()}"
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}
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companion object {
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@JvmStatic
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private val LOGGER = LoggerFactory.getLogger(PDA::class.java)
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}
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}
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