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Elaborate how the signature is calculated

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Heiko Schaefer 2023-11-03 18:00:46 +01:00
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book/source/06-signatures.md
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@ -53,7 +53,7 @@ In this chapter, we discuss the general principles of OpenPGP signatures, which
For more detail about specific types of signatures, see the chapters {ref}`signing_data` and {ref}`component_signatures_chapter`, respectively.
## Structure of an OpenPGP signature
## Structure of an OpenPGP signature packet
As outlined above, an OpenPGP signature is a composite data structure, which combines:
@ -67,7 +67,18 @@ As outlined above, an OpenPGP signature is a composite data structure, which com
Structure of an OpenPGP signature packet
```
The cryptographic signature is calculated by its issuer. It certifies a hash digest, which in turn combines a set of input data. The exact input data depends on the signature type. Roughly: the hash digest is over the elements that the OpenPGP signature makes a statement about, combined with the metadata in the OpenPGP signature packet itself. More on this later.
### Generation and validation of cryptographic signatures in OpenPGP
The central element of an OpenPGP signature packet is the raw cryptographic signature it contains. This cryptographic signature is calculated by the issuer of that signature packet.
In abstract terms, the cryptographic signature certifies a set of input data ("signature over"): The signer is making a statement about that set of input data. That statement is encoded as the signature packet.
If we look more closely, the cryptographic signature is actually calculated in two steps:
1. A hash digest is calculated from the set of input data.
2. The signature is calculated for this hash digest.
The exact input data depends on the signature type. However, the input data always comprises the information that the signature makes a statement about. It includes the metadata in the OpenPGP signature packet itself.
(signature_subpackets)=
## Signature subpackets