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pgpainless/misc/OpenPGPMessageFormat.md
Paul Schaub 36cb6918a4
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Pushdown Automaton for the OpenPGP Message Format

See RFC4880 §11.3. OpenPGP Messages for the formal definition.

A simulation of the automaton can be found here.

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/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

The dotted line indicates a nested transition. For example the arrow Compressed Data,m/m indicates, that the content of the Compressed Data packet itself is an OpenPGP Message.