120 lines
4.0 KiB
Kotlin
120 lines
4.0 KiB
Kotlin
// 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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} |