# Introducing OpenPGP

This documentation project seeks to introduce OpenPGP concepts and functionalities to software developers who wish to use it in their projects.

This document describes [OpenPGP version 6](https://datatracker.ietf.org/doc/draft-ietf-openpgp-crypto-refresh/), with occasional remarks about differences to earlier versions.

This text is *not* intended as a guide for end-users of OpenPGP-related software.

## What is OpenPGP?

OpenPGP is an open standard for cryptographic operations. OpenPGP supports the secure delivery of files and messages between a sender and a recipient as well as verification of the sender. OpenPGP is an outgrowth of the ["Pretty Good Privacy (PGP)"](https://en.wikipedia.org/wiki/Pretty_Good_Privacy) encryption program and has many widely used and interoperable implementations.
{::comment}
Heiko, let's be sure to create our own page on interoperability instead of linking to this 
{:/comment}

## Who is the audience for this document?

Three groups of people interact with OpenPGP:

1. End-users, who use software that contains OpenPGP functionality (e.g., the Thunderbird email software)
2. Software developers who build applications that contain OpenPGP functionality
3. Implementers of libraries or software that handles the processing of internal OpenPGP data structures

This document is focused on software developers who use OpenPGP functionality in their software projects. It is not intended for end-users. 
{::comment}
Heiko, we should elaborate a bit on why here 
{:/comment}
Thus, this text describes OpenPGP at the "library-level," teaching concepts that will help software developers get started as a user of any implementation
(e.g., OpenPGP JS, Sequoia PGP).

### Requirements

We presuppose solid knowledge of software development concepts and of general cryptographic concepts.

OpenPGP is a system based on well-understood cryptographic building blocks.
We describe the properties of the OpenPGP system and how to use it.

### A companion for the OpenPGP RFC

```
The RFC explains lots of details (which bit goes where) that are crucial
for implementers, but unimportant for software developers who use OpenPGP
through a library.
```

The [OpenPGP RFC](https://datatracker.ietf.org/doc/draft-ietf-openpgp-crypto-refresh/)
defines *"the message formats used in OpenPGP"* to *"provide encryption with
public-key or symmetric cryptographic algorithms, digital signatures,
compression and key management"*.

The RFC, as a standards document, is mainly aimed at the third group:
Implementers of software that handles internal OpenPGP data structures.
In that context, the nitty-gritty of which bit of data goes where is crucial.

For software developers using OpenPGP through a library, however, it is not.
This document describes OpenPGP concepts at the "library" level of abstraction,
and ignores most details about how OpenPGP artifacts are encoded at the lowest level.

The idea is to go over various common OpenPGP artifacts, as they are
currently used, to get an overview.

### Covering versions

We will mainly cover v6 of OpenPGP, but occasionally point out
differences to previous versions.

Version 4 of OpenPGP will remain relevant for a number of years,
and some OpenPGP version 3 artifacts are still in use as of this writing (in 2023).

For example, the RFC states that implementations MAY accept version 3 signatures.
Handling version 3 artifacts is relevant in some contexts, where dealing with
historical OpenPGP material is required.

Where differences between versions may be relevant to application developers,
we will point them out.