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Initial work on Data Signatures chapter

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@ -6,6 +6,47 @@ SPDX-License-Identifier: CC-BY-SA-4.0
(signing_data)= (signing_data)=
# Signatures over data # Signatures over data
One major use of OpenPGP is to create signatures over various pieces of data, for example software packages or emails.
Contrary to alternative solutions like [signify](https://flak.tedunangst.com/post/signify), OpenPGP offers the advantage of allowing for strong authetication due to the binding between certificates and entities.
## Signature types
There are two signature types which are relevant for data signatures:
* 0x00: *Binary Signature* is a universal signature type for binary data. This signature type is typically used for files or data streams.
Binary signatures are calculated over the data "as is", without performing any transformations.
* 0x01: *Text Signature* is used for textual data, such as email bodies.
To calculate a text signature, the data is first transformed by converting line endings into a canonical form (<CR><LF>).
This is done to mitigate issues caused by platform-specific default text encodings.
## Signature schemes
Historically, OpenPGP offers different schemes for signing data:
* *Detached signatures* can be distributed alongside the unaltered data that is being signed.
This method is especially useful for signing software releases and other files that must not be modified by the signing process.
* Inline-signatures using so called One-Pass-Signature packets embed the signed data in an OpenPGP artifact.
This method is usually used with signed and/or encrypted emails.
* The *Cleartext Signature Framework* (CSF) is a deprecated method of signing text, which leaves the original message in a human-readable format.
This method is often used in scenarios where recipient clients might not offer OpenPGP support.
### Detached signatures
A detached signature is produced by calculating an OpenPGP signature over a piece of data.
The resulting OpenPGP signature packet can then be distributed alongside the original data.
### Inline-signatures
Most clients that support OpenPGP for encrypted and/or signed messages make use of inline-signatures.
To produce a signature, the entirety of the data needs to be processed by the producer. This has the consequence, that an application that efficiently emits signed data, can only append the signature at the end of the data stream.
On the other hand, an application that needs to efficiently verify signed data needs to know the signers public key and used hash algorithm before processing the data.
To solve this issue, One-Pass-Signature packets are prefixed to the signed data. Those are small packets containing the fingerprint of the signing key, as well as the used hash algorithm. This is all the information a receiving application needs to know in order to initiate the verification process.
To produce an inline-signed message, the original data is first wrapped in a Literal Data packet, which is prefixed with one or more One-Pass-Signature packets, and affixed with the corresponding signature packets.
The verifying application can read the One-Pass-Signature packets and initiate the verification process.
The literal data can then be processed, such that the signatures at the end of the message can be verified in *one pass*.
TODO: explain nesting of OPSs.
```{admonition} TODO ```{admonition} TODO
:class: warning :class: warning