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Kotlin conversion: Syntax checking
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10 changed files with 157 additions and 254 deletions
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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.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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public enum InputSymbol {
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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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Signature,
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/**
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* A {@link PGPOnePassSignatureList} object.
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*/
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OnePassSignature,
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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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// 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 javax.annotation.Nonnull;
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import javax.annotation.Nullable;
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/**
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* This class describes the syntax for OpenPGP messages as specified by rfc4880.
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*
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* @see <a href="https://www.rfc-editor.org/rfc/rfc4880#section-11.3">
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* rfc4880 - §11.3. OpenPGP Messages</a>
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* @see <a href="https://blog.jabberhead.tk/2022/09/14/using-pushdown-automata-to-verify-packet-sequences/">
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* Blog post about theoretic background and translation of grammar to PDA syntax</a>
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* @see <a href="https://blog.jabberhead.tk/2022/10/26/implementing-packet-sequence-validation-using-pushdown-automata/">
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* Blog post about practically implementing the PDA for packet syntax validation</a>
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*/
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public class OpenPgpMessageSyntax implements Syntax {
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@Override
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public @Nonnull Transition transition(@Nonnull State from, @Nonnull InputSymbol input, @Nullable StackSymbol stackItem)
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throws MalformedOpenPgpMessageException {
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switch (from) {
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case OpenPgpMessage:
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return fromOpenPgpMessage(input, stackItem);
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case LiteralMessage:
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return fromLiteralMessage(input, stackItem);
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case CompressedMessage:
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return fromCompressedMessage(input, stackItem);
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case EncryptedMessage:
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return fromEncryptedMessage(input, stackItem);
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case Valid:
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return fromValid(input, stackItem);
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}
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throw new MalformedOpenPgpMessageException(from, input, stackItem);
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}
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@Nonnull
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Transition fromOpenPgpMessage(@Nonnull InputSymbol input, @Nullable StackSymbol stackItem)
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throws MalformedOpenPgpMessageException {
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if (stackItem != StackSymbol.msg) {
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throw new MalformedOpenPgpMessageException(State.OpenPgpMessage, input, stackItem);
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}
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switch (input) {
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case LiteralData:
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return new Transition(State.LiteralMessage);
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case Signature:
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return new Transition(State.OpenPgpMessage, StackSymbol.msg);
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case OnePassSignature:
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return new Transition(State.OpenPgpMessage, StackSymbol.ops, StackSymbol.msg);
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case CompressedData:
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return new Transition(State.CompressedMessage);
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case EncryptedData:
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return new Transition(State.EncryptedMessage);
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case EndOfSequence:
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default:
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throw new MalformedOpenPgpMessageException(State.OpenPgpMessage, input, stackItem);
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}
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}
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@Nonnull
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Transition fromLiteralMessage(@Nonnull InputSymbol input, @Nullable StackSymbol stackItem)
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throws MalformedOpenPgpMessageException {
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switch (input) {
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case Signature:
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if (stackItem == StackSymbol.ops) {
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return new Transition(State.LiteralMessage);
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}
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break;
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case EndOfSequence:
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if (stackItem == StackSymbol.terminus) {
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return new Transition(State.Valid);
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}
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break;
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}
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throw new MalformedOpenPgpMessageException(State.LiteralMessage, input, stackItem);
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}
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@Nonnull
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Transition fromCompressedMessage(@Nonnull InputSymbol input, @Nullable StackSymbol stackItem)
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throws MalformedOpenPgpMessageException {
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switch (input) {
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case Signature:
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if (stackItem == StackSymbol.ops) {
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return new Transition(State.CompressedMessage);
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}
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break;
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case EndOfSequence:
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if (stackItem == StackSymbol.terminus) {
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return new Transition(State.Valid);
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}
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break;
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}
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throw new MalformedOpenPgpMessageException(State.CompressedMessage, input, stackItem);
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}
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@Nonnull
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Transition fromEncryptedMessage(@Nonnull InputSymbol input, @Nullable StackSymbol stackItem)
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throws MalformedOpenPgpMessageException {
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switch (input) {
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case Signature:
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if (stackItem == StackSymbol.ops) {
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return new Transition(State.EncryptedMessage);
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}
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break;
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case EndOfSequence:
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if (stackItem == StackSymbol.terminus) {
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return new Transition(State.Valid);
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}
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break;
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}
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throw new MalformedOpenPgpMessageException(State.EncryptedMessage, input, stackItem);
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}
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@Nonnull
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Transition fromValid(@Nonnull InputSymbol input, @Nullable StackSymbol stackItem)
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throws MalformedOpenPgpMessageException {
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if (input == InputSymbol.EndOfSequence) {
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// allow subsequent read() calls.
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return new Transition(State.Valid);
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}
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// There is no applicable transition rule out of Valid
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throw new MalformedOpenPgpMessageException(State.Valid, input, stackItem);
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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.syntax_check;
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public enum StackSymbol {
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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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* 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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// 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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/**
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* Set of states of the automaton.
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*/
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public enum State {
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OpenPgpMessage,
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LiteralMessage,
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CompressedMessage,
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EncryptedMessage,
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Valid
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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.syntax_check;
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import org.pgpainless.exception.MalformedOpenPgpMessageException;
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import javax.annotation.Nonnull;
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import javax.annotation.Nullable;
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/**
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* This interface can be used to define a custom syntax for the {@link PDA}.
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*/
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public interface Syntax {
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/**
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* Describe a transition rule from {@link State} <pre>from</pre> for {@link InputSymbol} <pre>input</pre>
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* with {@link StackSymbol} <pre>stackItem</pre> from the top of the {@link PDA PDAs} stack.
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* The resulting {@link Transition} contains the new {@link State}, as well as a list of
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* {@link StackSymbol StackSymbols} that get pushed onto the stack by the transition rule.
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* If there is no applicable rule, a {@link MalformedOpenPgpMessageException} is thrown, since in this case
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* the {@link InputSymbol} must be considered illegal.
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*
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* @param from current state of the PDA
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* @param input input symbol
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* @param stackItem item that got popped from the top of the stack
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* @return applicable transition rule containing the new state and pushed stack symbols
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* @throws MalformedOpenPgpMessageException if there is no applicable transition rule (the input symbol is illegal)
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*/
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@Nonnull Transition transition(@Nonnull State from, @Nonnull InputSymbol input, @Nullable StackSymbol stackItem)
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throws MalformedOpenPgpMessageException;
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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.syntax_check;
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import javax.annotation.Nonnull;
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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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/**
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* Result of applying a transition rule.
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* Transition rules can be described by implementing the {@link Syntax} interface.
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*/
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public class Transition {
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private final List<StackSymbol> pushedItems = new ArrayList<>();
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private final State newState;
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public Transition(@Nonnull State newState, @Nonnull StackSymbol... pushedItems) {
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this.newState = newState;
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this.pushedItems.addAll(Arrays.asList(pushedItems));
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}
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/**
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* Return the new {@link State} that is reached by applying the transition.
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*
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* @return new state
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*/
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@Nonnull
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public State getNewState() {
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return newState;
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}
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/**
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* Return a list of {@link StackSymbol StackSymbols} that are pushed onto the stack
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* by applying the transition.
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* The list contains items in the order in which they are pushed onto the stack.
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* The list may be empty.
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*
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* @return list of items to be pushed onto the stack
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*/
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@Nonnull
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public List<StackSymbol> getPushedItems() {
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return new ArrayList<>(pushedItems);
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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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/**
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* Pushdown Automaton to verify validity of packet sequences according to the OpenPGP Message format.
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*/
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package org.pgpainless.decryption_verification.syntax_check;
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