Merge branch 'heiko-ch12' into draft

book/source/12-compression.md

@@ -5,6 +5,29 @@ SPDX-License-Identifier: CC-BY-SA-4.0
 
 # Compression
 
-## Zooming in: Packet structure
+Optional compression of data is one element of OpenPGP's composable functionality. Compression within OpenPGP can be convenient to applications.
 
-### Decompression yields a 'wrapped' openpgp packet stream
+In one use case, this functionality is particularly helpful: When encrypting a message, the encrypted output is by definition high-entropy, and cannot be compressed anymore - even if the plaintext message was low-entropy, and could have been compressed well (like, for example, a text-file).
+
+
+This means that to use whatever potential for compression exists, the message must be compressed *before* encryption. OpenPGP offers an integrated compression mechanism to make this convenient (otherwise, messages would need to be compressed and decompressed before and after encryption, to achieve the same space-efficiency). 
+
+## Decompression yields a 'wrapped' OpenPGP packet stream
+
+Compression in OpenPGP is a simple mechanism: A [Compressed Data packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-compressed-data-packet-type) acts as a compressed container for a series of OpenPGP packets.
+
+The compressed data packet consists of the specification of which compression algorithm is used, followed by a compressed representation of the contained data.
+
+The series of OpenPGP packets inside the Compressed Data packet can be handled like any stream of OpenPGP packets.
+
+## Typical usage
+
+Compressed data packets are often used inside [encrypted data packets](encryption_chapter), or wrapping the data of an [inline-signed message](inline_signature).
+
+---
+
+```{admonition} TODO
+:class: warning
+
+link to a packet dump example that shows compression
+```