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<!--
SPDX-FileCopyrightText: 2022 Paul Schaub <info@pgpainless.org>
SPDX-License-Identifier: Apache-2.0
-->
# Pushdown Automaton for the OpenPGP Message Format
See [RFC4880 §11.3. OpenPGP Messages](https://www.rfc-editor.org/rfc/rfc4880#section-11.3) for the formal definition.
A simulation of the automaton can be found [here](https://automatonsimulator.com/#%7B%22type%22%3A%22PDA%22%2C%22pda%22%3A%7B%22transitions%22%3A%7B%22start%22%3A%7B%22%22%3A%7B%22%22%3A%5B%7B%22state%22%3A%22s12%22%2C%22stackPushChar%22%3A%22%23%22%7D%5D%2C%22%23%22%3A%5B%5D%7D%7D%2C%22s0%22%3A%7B%22C%22%3A%7B%22M%22%3A%5B%7B%22state%22%3A%22s0%22%2C%22stackPushChar%22%3A%22M%22%7D%5D%7D%2C%22L%22%3A%7B%22M%22%3A%5B%7B%22state%22%3A%22s1%22%2C%22stackPushChar%22%3A%22%22%7D%5D%7D%2C%22S%22%3A%7B%22M%22%3A%5B%7B%22state%22%3A%22s0%22%2C%22stackPushChar%22%3A%22M%22%7D%5D%2C%22o%22%3A%5B%5D%7D%2C%22O%22%3A%7B%22M%22%3A%5B%7B%22state%22%3A%22s9%22%2C%22stackPushChar%22%3A%22o%22%7D%5D%7D%2C%22E%22%3A%7B%22M%22%3A%5B%5D%7D%2C%22p%22%3A%7B%22M%22%3A%5B%7B%22state%22%3A%22s6%22%2C%22stackPushChar%22%3A%22X%22%7D%5D%7D%2C%22s%22%3A%7B%22M%22%3A%5B%7B%22state%22%3A%22s6%22%2C%22stackPushChar%22%3A%22X%22%7D%5D%7D%2C%22I%22%3A%7B%22M%22%3A%5B%7B%22state%22%3A%22s8%22%2C%22stackPushChar%22%3A%22E%22%7D%5D%7D%2C%22J%22%3A%7B%22M%22%3A%5B%7B%22state%22%3A%22s8%22%2C%22stackPushChar%22%3A%22E%22%7D%5D%7D%7D%2C%22s1%22%3A%7B%22%22%3A%7B%22%22%3A%5B%5D%2C%22%23%22%3A%5B%7B%22state%22%3A%22s4%22%2C%22stackPushChar%22%3A%22%22%7D%5D%7D%2C%22S%22%3A%7B%22o%22%3A%5B%7B%22state%22%3A%22s10%22%2C%22stackPushChar%22%3A%22%22%7D%5D%7D%7D%2C%22s6%22%3A%7B%22p%22%3A%7B%22X%22%3A%5B%7B%22state%22%3A%22s6%22%2C%22stackPushChar%22%3A%22X%22%7D%5D%7D%2C%22s%22%3A%7B%22X%22%3A%5B%7B%22state%22%3A%22s6%22%2C%22stackPushChar%22%3A%22X%22%7D%5D%7D%2C%22I%22%3A%7B%22X%22%3A%5B%7B%22state%22%3A%22s8%22%2C%22stackPushChar%22%3A%22E%22%7D%5D%7D%2C%22J%22%3A%7B%22X%22%3A%5B%7B%22state%22%3A%22s8%22%2C%22stackPushChar%22%3A%22E%22%7D%5D%7D%7D%2C%22s8%22%3A%7B%22%22%3A%7B%22E%22%3A%5B%7B%22state%22%3A%22s0%22%2C%22stackPushChar%22%3A%22M%22%7D%5D%7D%7D%2C%22s9%22%3A%7B%22%22%3A%7B%22%22%3A%5B%7B%22state%22%3A%22s0%22%2C%22stackPushChar%22%3A%22M%22%7D%5D%7D%7D%2C%22s10%22%3A%7B%22%22%3A%7B%22%22%3A%5B%5D%2C%22%23%22%3A%5B%7B%22state%22%3A%22s4%22%2C%22stackPushChar%22%3A%22%22%7D%5D%7D%2C%22S%22%3A%7B%22o%22%3A%5B%7B%22state%22%3A%22s10%22%2C%22stackPushChar%22%3A%22%22%7D%5D%7D%7D%2C%22s4%22%3A%7B%22%22%3A%7B%22o%22%3A%5B%5D%7D%7D%2C%22s12%22%3A%7B%22%22%3A%7B%22%22%3A%5B%7B%22state%22%3A%22s0%22%2C%22stackPushChar%22%3A%22M%22%7D%5D%7D%7D%7D%2C%22startState%22%3A%22start%22%2C%22acceptStates%22%3A%5B%22s4%22%5D%7D%2C%22states%22%3A%7B%22start%22%3A%7B%7D%2C%22s12%22%3A%7B%22top%22%3A395.00001525878906%2C%22left%22%3A99%2C%22displayId%22%3A%22Add%20Terminal%22%7D%2C%22s0%22%3A%7B%22top%22%3A259.00001525878906%2C%22left%22%3A162%2C%22displayId%22%3A%22OpenPGP%20Message%22%7D%2C%22s1%22%3A%7B%22top%22%3A304.00001525878906%2C%22left%22%3A524%2C%22displayId%22%3A%22Literal%20Message%22%7D%2C%22s9%22%3A%7B%22top%22%3A476.00001525878906%2C%22left%22%3A282%2C%22displayId%22%3A%22One%20Pass%20Signatures%22%7D%2C%22s6%22%3A%7B%22top%22%3A100%2C%22left%22%3A324%2C%22displayId%22%3A%22ESKs%22%7D%2C%22s8%22%3A%7B%22top%22%3A202%2C%22left%22%3A471%2C%22displayId%22%3A%22Encrypted%20Data%22%7D%2C%22s4%22%3A%7B%22isAccept%22%3Atrue%2C%22top%22%3A381.00001525878906%2C%22left%22%3A832%2C%22displayId%22%3A%22Accept%22%7D%2C%22s10%22%3A%7B%22top%22%3A237.00001525878906%2C%22left%22%3A809%2C%22displayId%22%3A%22Corresponding%20Signatures%22%7D%7D%2C%22transitions%22%3A%5B%7B%22stateA%22%3A%22start%22%2C%22label%22%3A%22%CF%B5%2C%CF%B5%2C%23%22%2C%22stateB%22%3A%22s12%22%7D%2C%7B%22stateA%22%3A%22s0%22%2C%22label%22%3A%22C%2CM%2CM%22%2C%22stateB%22%3A%22s0%22%7D%2C%7B%22stateA%22%3A%22s0%22%2C%22label%22%3A%22L%2CM%2C%CF%B5%22%2C%22stateB%22%3A%22s1%22%7D%2C%7B%22stateA%22%3A%22s0%22%2C%22label%22%3A%22S%2CM%2CM%22%2C%22stateB%22%3A%22s0%22%7D%2C%7B%22stateA%22%3A%22s0%22%2C%22label%22%3A%22O%2CM%2Co%22%2C%22stateB%22%3A%22s9%22%7D%2C%7B%22stateA%22%3A%22s0%22%2C%22label%22%3A%22p%2CM%2CX%22%2C%22stateB%22%3A%22s6%22%7D%2C%7B%22stateA%22%3A%22s0%22%2C%22label%22%3A%22s%2CM%2CX%22%2C%22stateB%22%3A%22s6%22%7D%2C%7B%22stateA%22%3A%22s0%22%2C%22label%22%3A%22I%2CM%2CE%22%2C%22stateB%22%3A%22s8%22%7D%2C%7B%22stateA%22%3A%22s0%22%2C%22label%22%3A%22J%2CM%2CE%22%2C%22stateB%22%3A%22s8%22%7D%2C%7B%22stateA%22%3A%22s1%22%2C%22label%22%3A%22%CF%B5%2C%23%2C%CF%B5%22%2C%22stateB%22%3A%22s4%22%7D%2C%7B%22stateA%22%3A%22s1%22%2C%22label%22%3A%22S%2Co%2C%CF%B5%22%2C%22stateB%22%3A%22s10%22%7D%2C%7B%22stateA%22%3A%22s6%22%2C%22label%22%3A%22p%2CX%2CX%22%2C%22stateB%22%3A%22s6%22%7D%2C%7B%22stateA%22%3A%22s6%22%2C%22label%22%3A%22s%2CX%2CX%22%2C%22stateB%22%3A%22s6%22%7D%2C%7B%22stateA%22%3A%22s6%22%2C%22label%22%3A%22I%2CX%2CE%22%2C%22stateB%22%3A%22s8%22%7D%2C%7B%22stateA%22%3A%22s6%22%2C%22label%22%3A%22J%2CX%2CE%22%2C%22stateB%22%3A%22s8%22%7D%2C%7B%22stateA%22%3A%22s8%22%2C%22label%22%3A%22%CF%B5%2CE%2CM%22%2C%22stateB%22%3A%22s0%22%7D%2C%7B%22stateA%22%3A%22s9%22%2C%22label%22%3A%22%CF%B5%2C%CF%B5%2CM%22%2C%22stateB%22%3A%22s0%22%7D%2C%7B%22stateA%22%3A%22s10%22%2C%22label%22%3A%22%CF%B5%2C%23%2C%CF%B5%22%2C%22stateB%22%3A%22s4%22%7D%2C%7B%22stateA%22%3A%22s10%22%2C%22label%22%3A%22S%2Co%2C%CF%B5%22%2C%22stateB%22%3A%22s10%22%7D%2C%7B%22stateA%22%3A%22s12%22%2C%22label%22%3A%22%CF%B5%2C%CF%B5%2CM%22%2C%22stateB%22%3A%22s0%22%7D%5D%2C%22bulkTests%22%3A%7B%22accept%22%3A%22L%5CnCL%5CnCCL%5CnSL%5CnSSL%5CnSCL%5CnSpICL%5CnOLS%5CnOOLSS%5CnCspIL%5CnppppJCOLS%5CnOspILS%5CnOpJCLS%5CnOCLS%5CnCOCLS%22%2C%22reject%22%3A%22C%5CnO%5CnOL%5CnLS%5CnLL%5CnS%5Cns%5Cnp%5CnOOLS%22%7D%7D).
RFC4880 defines the grammar of OpenPGP messages as follows:
```
OpenPGP Message :- Encrypted Message | Signed Message |
Compressed Message | Literal Message.
Compressed Message :- Compressed Data Packet.
Literal Message :- Literal Data Packet.
ESK :- Public-Key Encrypted Session Key Packet |
Symmetric-Key Encrypted Session Key Packet.
ESK Sequence :- ESK | ESK Sequence, ESK.
Encrypted Data :- Symmetrically Encrypted Data Packet |
Symmetrically Encrypted Integrity Protected Data Packet
Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data.
One-Pass Signed Message :- One-Pass Signature Packet,
OpenPGP Message, Corresponding Signature Packet.
Signed Message :- Signature Packet, OpenPGP Message |
One-Pass Signed Message.
In addition, decrypting a Symmetrically Encrypted Data packet or a
Symmetrically Encrypted Integrity Protected Data packet as well as
decompressing a Compressed Data packet must yield a valid OpenPGP
Message.
```
This grammar can be translated into a [pushdown automaton](https://en.wikipedia.org/wiki/Pushdown_automaton) with
the following graphical representation:
```mermaid
graph LR
start((start)) -- "ε,ε/m#" --> pgpmsg((OpenPGP Message))
pgpmsg -- "Literal Data,m/ε" --> literal((Literal Message))
literal -- "ε,#/ε" --> accept((Valid))
literal -- "Signature,o/ε" --> sig4ops((Corresponding Signature))
sig4ops -- "Signature,o/ε" --> sig4ops
sig4ops -- "ε,#/ε" --> accept
pgpmsg -- "OnePassSignature,m/o" --> ops((One-Pass-Signed Message))
ops -- "ε,ε/m" --> pgpmsg
pgpmsg -- "Signature,m/ε" --> signed((Signed Message))
signed -- "ε,ε/m" --> pgpmsg
pgpmsg -- "Compressed Data,m/ε" --> comp((Compressed Message))
comp -. "ε,ε/m" .-> pgpmsg
comp -- "ε,#/ε" --> accept
comp -- "Signature,o/ε" --> sig4ops
pgpmsg -- "SKESK|PKESK,m/k" --> esks((ESKs))
pgpmsg -- "Sym. Enc. (Int. Prot.) Data,m/ε" --> enc
esks -- "SKESK|PKESK,k/k" --> esks
esks -- "Sym. Enc. (Int. Prot.) Data,k/ε" --> enc((Encrypted Message))
enc -. "ε,ε/m" .-> pgpmsg
enc -- "ε,#/ε" --> accept
enc -- "Signature,o/ε" --> sig4ops
accept -- "ε,ε/ε" --> accept
```
Formally, the PDA is defined as $M = (\mathcal{Q}, \Sigma, \Upgamma, \delta, q_0, Z, F)$, where
* $\mathcal{Q}$ is a finite set of states
* $\Sigma$ is a finite set which is called the input alphabet
* $\Upgamma$ is a finite set which is called the stack alphabet
* $\delta$ is a finite set of $\mathcal{Q}\times(\Sigma\cup\textbraceleft\epsilon\textbraceright)\times\Upgamma\times\mathcal{Q}\times\Upgamma^*$, the transition relation
* $q_0\in\mathcal{Q}$ is the start state
* $Z\in\Upgamma$ is the initial stack symbol
* $F\subseteq\mathcal{Q}$ is the set of accepting states
In our diagram, the initial state is $q_0 = \text{start}$.
The initial stack symbol is $Z=\epsilon$ (TODO: Make it `#`?).
The set of accepting states is $F=\textbraceleft\text{valid}\textbraceright$.
$\delta$ is defined by the transitions shown in the graph diagram.
The input alphabet $\Sigma$ consists of the following OpenPGP packets:
* $\text{Literal Data}$: Literal Data Packet
* $\text{Signature}$: Signature Packet
* $\text{OnePassSignature}$: One-Pass-Signature Packet
* $\text{Compressed Data}$: Compressed Data Packet
* $\text{SKESK}$: Symmetric-Key Encrypted Session Key Packet
* $\text{PKESK}$: Public-Key Encrypted Session Key Packet
* $\text{Sym. Enc. Data}$: Symmetrically Encrypted Data Packet
* $\text{Sym. Enc. Int. Prot. Data}$: Symmetrically Encrypted Integrity Protected Data Packet
Additionally, $\epsilon$ is used to transition without reading OpenPGP packets or popping the stack.
The following stack alphabet $\Upgamma$ is used:
* $m$: OpenPGP Message
* $o$: One-Pass-Signature packet.
* $k$: Encrypted Session Key
* `#`: Terminal for valid OpenPGP messages
Note: The standards document states, that Marker Packets shall be ignored as well.
For the sake of readability, those transitions are omitted here.
The dotted line indicates a nested transition.
For example, the transition $(\text{Compressed Message}, \epsilon, \epsilon, \text{OpenPGP Message}, m)$ indicates, that the content of the
Compressed Data packet itself is an OpenPGP Message.
|
