# A high-level view

## Why OpenPGP?

OpenPGP is a widely recognized IETF-standardized set of cryptographic operations. It is broadly used in securing communications, for example, in encrypted text messages and email, and enjoys a vast ecosystem of libraries, tools, and community support forums. Moreover, its robustness and versatility has made OpenPGP a security choice for other use cases in which encryption is important. These include file transfer applications, password managers, and secure data storage. 

There are other compelling reasons for why you might consider using OpenPGP in your project:

1. **Decentralized trust model**: OpenPGP's decentralization defines mechanisms for authentication that allow individuals and entities to create and manage their own cryptographic identities. Unlike centralized trust models, decentralized trust models empower individuals and entities to manage their own identities, fostering a community-driven web of trust instead of relying on a centralized authority, thus reducing single points of failure.  

2. **End-to-end encryption**: OpenPGP provides a robust framework for implementing end-to-end encryption. Content remains confidential, verifiable, authenticated, and protected against unauthorized access, even when the communication channel itself might be otherwise compromised. Encryption is crucial in a myriad of scenarios, particularly when transmitting sensitive information such as financial data, personal identification information, or proprietary business data.

3. **Anonymity and pseudonymity**: In sensitive and volatile situations where identity protection is crucial, OpenPGP can be used to provide a level of anonymity or pseudonymity that helps protect user identities. For example, OpenPGP has been used alongside other privacy tools, such as Tor and VPNs, to provide secure and anonymous communication for whistleblowers, human rights lawyers, activists in repressive regimes, and journalists, reducing their risks for retaliation and state violence.

4. **Interoperability**: OpenPGP is a a well-structured and standardized protocol, widely adopted by various public and private entities but not tied to any particular vendor's technology. It supports all major operating systems, like Windows, macOS, GNU/Linux, Android, and iOS. Because of standardization, wide adoption, cross-platform compatibility, and adaptability, OpenPGP's interoperability significantly contributes to reducing development time, costs, and technical hurdles.

## A very brief history

The OpenPGP standard has evolved over time, and remains under active development.

(Also see [https://www.openpgp.org/about/history/](https://www.openpgp.org/about/history/))

### Pretty Good Privacy (PGP)

The origins of OpenPGP can be traced back to *Pretty Good Privacy (PGP)*, a software program written by [Phil Zimmermann](https://en.wikipedia.org/wiki/Phil_Zimmermann) and first released in 1991.

The original PGP software played a role in the political struggles sometimes referred to as the ["Crypto Wars"](https://en.wikipedia.org/wiki/Crypto_Wars) (also see ["Crypto: How the Code Rebels Beat the Government Saving Privacy in the Digital" (2002)](https://en.wikipedia.org/wiki/Crypto_(book)), which includes some of PGP's history).

The original PGP software was never under a Free Software license, despite being widely published. [PGP's ownership has changed over the years](https://en.wikipedia.org/wiki/Pretty_Good_Privacy#PGP_Corporation_and_Symantec), and [PGP's scope and suite of products have expanded] (https://en.wikipedia.org/wiki/Pretty_Good_Privacy#PGP_Corporation_encryption_applications).

### Standardizing OpenPGP

While PGP was first developed as commercial software, the owner at the time, PGP Inc., started a standardization effort with the IETF, first publishing [RFC 1991 "PGP Message Exchange Formats"](https://datatracker.ietf.org/doc/html/rfc1991) in August 1996.

In July 1997, a process to produce an open standard under the then new name [OpenPGP](https://en.wikipedia.org/wiki/Pretty_Good_Privacy#OpenPGP) was started, resulting in [RFC 2440 "OpenPGP Message Format"](https://datatracker.ietf.org/doc/html/rfc2440), published November 1998. RFC 2440 describes OpenPGP version 3.

The name OpenPGP can be used freely by implementations, unlike the name PGP, which is a [registered trademark](https://uspto.report/TM/74685229)).

### GnuPG, an early Free Software implementation

[First released 1997-12-20](https://gnupg.org/download/release_notes.html#sec-2-70) by Werner Koch, a German computer programmer, GNU Privacy Guard (GnuPG) is a free and open-source implementation of the OpenPGP standard. 

GnuPG was a major early implementation of OpenPGP. Over the years, the importance of GnuPG has grown significantly as it became a foundational tool for email security, software signing, and more. It played an important (and successful) role in the [release of NSA documents](https://theintercept.com/2014/10/28/smuggling-snowden-secrets/) by [Edward Snowden](https://en.wikipedia.org/wiki/Edward_Snowden).

The GnuPG program binary is called "gpg," thus the names "GnuPG" and "gpg" are often used interchangeably.

Note: The terms "pgp key" and "gpg key" are sometimes used. Since PGP and GnuPG are just two of many existing OpenPGP implementations, the proper term is "OpenPGP key" (or "OpenPGP certificate", more on that [later](certificates_chapter)).

```{admonition} TODO
:class: warning
Heiko, I do not believe the above section on the binary needs to be in this history section. Also, I have added Werner Koch by name to this history, bringing his contribution to the level afforded Zimmerman and Snowden.
```

## The OpenPGP version 4 era

### OpenPGP version 4

In 2007, the IETF published [RFC 4880](https://datatracker.ietf.org/doc/html/rfc4880), which defined version 4 of the OpenPGP standard. At this time, version 4 is the most commonly used version.


### Multiple major implementations

Today, multiple implementations of OpenPGP play important roles:

- Proton Mail, which provides email encryption services for a large number of users, uses and maintains [OpenPGP.js](https://openpgpjs.org/) as well as [GopenPGP](https://gopenpgp.org/), an OpenPGP wrapper library written in golang.
- The Mozilla Thunderbird email software uses [RNP](https://www.rnpgp.org/), its C++ implementation of OpenPGP.
- The RPM Package Manager software includes an OpenPGP backend based on [Sequoia PGP](https://sequoia-pgp.org/), a modern OpenPGP implementation written in Rust. The Fedora operating system, Fedora Linux 38, [uses Sequoia PGP in rpm](https://sequoia-pgp.org/blog/2023/04/27/rpm-sequoia/).

## The road ahead

### OpenPGP version 6

As of this writing (in 2023), [version 6 of OpenPGP](https://datatracker.ietf.org/doc/draft-ietf-openpgp-crypto-refresh/) is approaching publication as an RFC.
The [IETF OpenPGP working group](https://datatracker.ietf.org/wg/openpgp/about/#autoid-1) is focused on updating the cryptographic mechanisms, adding new algorithms, and the deprecation of obsolete algorithms.

This document describes OpenPGP version 6, while pointing out differences to previous versions that are relevant to application developers.

Significant support for OpenPGP version 6 has already been achieved for multiple implementations, including:

- [GopenPGP](https://github.com/ProtonMail/gopenpgp/tree/v3),
- [OpenPGP.js](https://github.com/openpgpjs/openpgpjs/tree/v6),
- [PGPainless](https://github.com/pgpainless/pgpainless/milestone/6),
- [PGPy](https://github.com/dkg/PGPy/tree/dkg/crypto-refresh),
- [Sequoia PGP](https://gitlab.com/sequoia-pgp/sequoia/-/tree/crypto-refresh).

### Post-Quantum Cryptography in OpenPGP

There is [ongoing work](https://datatracker.ietf.org/doc/draft-wussler-openpgp-pqc/) to standardize and add support for post-quantum public-key algorithms in OpenPGP. This project is funded by the [german "BSI"](https://en.wikipedia.org/wiki/Federal_Office_for_Information_Security). Goals include adding support for post-quantum cryptography to Thunderbird and GnuPG. A [presentation](https://datatracker.ietf.org/meeting/113/materials/slides-113-openpgp-a-post-quantum-approach-for-openpgp-00) was given at [IETF 113](https://datatracker.ietf.org/meeting/113/session/openpgp/).

## Concepts

### Certificates/Keys

Use of OpenPGP is centered around cryptographic keys.

In OpenPGP, bare cryptographic keys are combined with additional metadata into "OpenPGP certificates," which are a relatively complex data structure (OpenPGP certificates are also often called "OpenPGP keys").

An OpenPGP certificate can evolve over time, with components being added, expiring, or being marked as invalid.

See the chapter about [OpenPGP certificates](certificates_chapter) for details, and internal structure, and the chapter about [private keys](private_key_chapter) for handling of private key material in OpenPGP.

Other important topics around certificates are their management, authentication, and trust models. We will only touch on those, in this document.

### High-Level operations

With OpenPGP it's possible to:

- [Encrypt](encryption_chapter) and [Decrypt](decryption_chapter) Messages
- [Sign](signing_data) and [Verify](verification_chapter) Data
- [Issue and examine Statements](certifications_chapter) about Keys and Identities (to perform CA-like functionality)

### Building blocks

To perform these high-level operations, a set of [established cryptographic mechanisms](cyrptography_chapter) are used as building blocks, and combined into OpenPGP's format, which additionally deals with identities and their verification.

(interop_section)=
## Interoperability

OpenPGP was standardized in 1997 to encourage development of interoperable implementations. This has already been a success early on, but in recent years, there has been [much development of new implementations](major_implementations).

Historically, interoperability has only been tested in an adhoc manner. Since 2019, the Sequoia project is maintaining and operating the ["OpenPGP interoperability test suite"](https://tests.sequoia-pgp.org/), for more rigorous and systematic testing. The test suite has identified numerous [issues](https://gitlab.com/sequoia-pgp/openpgp-interoperability-test-suite#hall-of-fame).

## Zooming in: Internal structure of OpenPGP data

OpenPGP data is internally structured as "packets." We'll look into examples of this internal structure throughout the following chapters.

Getting familiar with the internal format of OpenPGP data is a good way to get familiar with the [RFC](https://datatracker.ietf.org/doc/draft-ietf-openpgp-crypto-refresh/), and it may also come in handy for debugging issues.

Gaining some familiarity with the internal structure of OpenPGP data will also help us to read the OpenPGP RFC, which describes the internal structure of OpenPGP packets in full detail.

(Most of the time, however, we will look at OpenPGP artifacts at a higher level of abstraction.)