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Add section on well-formedness

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Paul Schaub 2023-11-24 23:23:43 +01:00
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book/source/09-verification.md
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@ -32,6 +32,18 @@ This is especially the case with signatures created by dedicated signing subkeys
* **Revocation**:
Lastly, signatures can be invalidated by revocations.
### Well-formedness of signatures
There is a number of criteria, that a signature must fulfill in order to be considered well-formed:
- Each signature MUST have a signature creation time subpacket in its hashed subpacket area. A signature with only a unhashed creation time - or none at all - is not well-formed.
- The signature cannot be older than the key that issued it.
- Analogous, a signature with a creation time in the future needs to be rejected as well.
- A well-formed signature needs to carry an Issuer Fingerprint subpacket, or an Issuer KeyID subpacket.
It is generally recommended to place those in the hashed area of the signature, but a receiving implementation may also accept signatures which only contain unhashed copies of these subpackets.
- A signature disqualifies as well-formed, if it contains subpackets unknown to the implementation, which are marked as critical.
Unknown subpackets which are not marked as critical do not have an effect on whether the signature is well-formed.
- The same applies to notations. Critical, unknown notations result render the signature malformed.
(temporal-validity)=
### Temporal validity