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Merge pull request 'ch4: additional edits by heiko on top of tammi's edits in heiko-ch4' (#148) from heiko-ch4-edit2 into heiko-ch4

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Reviewed-on: https://codeberg.org/openpgp/notes/pulls/148
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book/source/04-certificates.md
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@ -10,6 +10,14 @@ OpenPGP fundamentally hinges on the concept of "OpenPGP certificates," also know
An OpenPGP certificate, by definition, does not contain private key material.
Fundamentally, the effective management of certificates and a thorough grasp of their authentication and trust models are crucial for proficient OpenPGP usage. Although this document offers just a brief overview of these aspects, they form a fundamental part of the broader OpenPGP framework and warrant further study.
- For an in-depth exploration of OpenPGP's private key material, refer to {ref}`private_key_chapter`. This chapter provides essential insights into private key management and security practices.
- The bindings that link the components of a certificate are comprehensively discussed in {ref}`component_signatures_chapter`, offering a deeper understanding of certificate structure and integrity.
- Finally, our chapter {ref}`zoom_certificates` discusses the internal structure of certificates in detail.
## Terminology: Understanding "keys"
The term "(cryptographic) keys" is central to grasping the concept of OpenPGP certificates. However, it can refer to different entities, making it a potentially confusing term. Let's clarify those differences.
@ -28,14 +36,6 @@ In OpenPGP, the term "key" may refer to three distinct layers, each serving a un
The following section will delve into the OpenPGP-specific layers (2 and 3) to provide a clearer understanding of their roles within OpenPGP certificates.
Fundamentally, the effective management of certificates and a thorough grasp of their authentication and trust models are crucial for proficient OpenPGP usage. Although this document offers just a brief overview of these aspects, they form a fundamental part of the broader OpenPGP framework and warrant further study.
– For an in-depth exploration of OpenPGP's private key material, refer to {ref}`private_key_chapter`. This chapter provides essential insights into key management and security practices.
– The bindings that unify the components of a certificate are comprehensively discussed in {ref}`component_signatures_chapter`, offering a deeper understanding of certificate structure and integrity.
– For a detailed analysis of the internal (packet) structure of certificates and keys, our chapter {ref}`zoom_certificates` can serve as an invaluable resource.
## Structure of OpenPGP certificates
An OpenPGP certificate (or "OpenPGP key") is a collection of an arbitrary number of elements[^packets]:
@ -96,9 +96,11 @@ For example, an OpenPGP version 4 certificate with the fingerprint `B3D2 7B09 FB
Historically, even shorter 32-bit identifiers were used, like this: `2455 4239`, or `0x24554239`. Such identifiers still appear in very old documents about PGP. However, [32-bit identifiers have been long deemed unfit for purpose](https://evil32.com/). At one point, 32-bit identifiers were called "short Key ID," while 64-bit identifiers were referred to as "long Key ID."
```{note}
In practice, the fingerprint of a component key, while not theoretically unique, functions effectively as a unique identifier. The use of a [cryptographic hash algorithm](crypto-hash) in generating fingerprints makes the occurrence of two different component keys with the same fingerprint extremely unlikely.
In practice, the fingerprint of a component key, while not theoretically unique, functions effectively as a unique identifier. The use of a [cryptographic hash algorithm](crypto-hash) in generating fingerprints makes the occurrence of two different component keys with the same fingerprint extremely unlikely[^finger-unique].
```
[^finger-unique]: For both OpenPGP version 6 and version 4, the likelihood of accidental occurrence of duplicate fingerprints is negligible when key material is generated based on an acceptable source of entropy. A separate question is if an attacker can purposely craft a second key with the same fingerprint as a given pre-existing component key. With the current state of the art, this is not possible for OpenPGP version 6 and version 4 keys. However, at the time of this writing, the SHA-1-based fingerprints of OpenPGP version 4 are considered insufficiently strong at protecting against the generation of pairs of key material with the same fingerprint.
### Primary key
The OpenPGP primary key is a component key that serves a distinct, central role in an OpenPGP certificate:
@ -147,7 +149,7 @@ For further conventions on User IDs, refer to the document [draft-dkg-openpgp-us
**Split User IDs**
One proposed variant for encoding identities in User IDs is to use ["split User IDs"](https://dkg.fifthhorseman.net/blog/2021-dkg-openpgp-transition.html#split-user-ids). Although currently uncommon, there are currently no significant technical barriers to implementing this format[^dkg-split].
One proposed variant for encoding identities in User IDs is to use ["split User IDs"](https://dkg.fifthhorseman.net/blog/2021-dkg-openpgp-transition.html#split-user-ids). Although uncommon, there are currently no significant technical barriers to implementing this format[^dkg-split].
[^dkg-split]: Historically, the OpenPGP ecosystem faced challenges in this context. For further details, refer to Daniel Kahn Gillmor's January 2019 article, ["What were Separated User IDs"](https://dkg.fifthhorseman.net/blog/2019-dkg-openpgp-transition.html#what-were-separated-user-ids).
@ -181,7 +183,7 @@ In very abstract terms, the primary key of a certificate acts as a root of trust
By binding components using digital signatures, recipients of an OpenPGP certificate need only validate the authenticity of the primary key to use for their communication partner. Traditionally, this is done by manually verifying the *fingerprint* of the primary key. Once the validity of the primary key is confirmed, the validity of the remaining components can be automatically assessed by the user's OpenPGP software. Generally, components are valid parts of a certificate if there is a statement signed by the certificate's primary key endorsing this validity.
## Metadata capabilities, preferences, and storage
## Metadata in certificates
OpenPGP certificates, their component keys, and identities possess metadata that is not stored within the components it pertains to. Instead, this metadata is stored within signature packets, which are integral to the structure of an OpenPGP certificate.
@ -191,7 +193,7 @@ Key attributes, such as capabilities (like *signing* or *encryption*) and expira
- **Subkey metadata** is defined within the [subkey binding signature](binding_subkeys) that links the subkey to the certificate.
- **User ID metadata** is is associated via the [certifying self-signature](bind_ident) that links the identity to the certificate.
- **Identity component metadata** is associated via the [certifying self-signature](bind_ident) that links the identity (usually in the form of a User ID) to the certificate.
It is crucial to note that the components of an OpenPGP certificate remain static after their creation. The use of signatures to store metadata allows for subsequent modifications without altering the original components. For instance, a certificate holder can update the expiration time of a component by issuing a new, superseding signature.
@ -218,13 +220,13 @@ Notably, in many algorithms, encryption and signing-related functionalities (i.e
### Algorithm preferences and feature signaling
OpenPGP demonstrates significant ["cryptographic agility"](https://en.wikipedia.org/wiki/Cryptographic_agility). It doesn't rely on a single fixed set of algorithms. Instead, it defines a suite of cryptographic primitives from which users (or their applications) can choose.
OpenPGP incorporates significant ["cryptographic agility"](https://en.wikipedia.org/wiki/Cryptographic_agility). It doesn't rely on a single fixed set of algorithms. Instead, it defines a suite of cryptographic primitives from which users (or their applications) can choose.
This agility facilitates the easy adoption of new cryptographic primitives into the standard, allowing for a seamless transition. Users can gradually migrate to new cryptographic mechanisms without disruption.
However, this approach requires that OpenPGP software determine the cryptographic mechanisms that a set of communication partners can handle and prefer. OpenPGP employs several mechanisms for this purpose, which allow negotiation between sender and recipient. It's important to note that OpenPGP is not an online scheme; thus, this negotiation is effectively one-way. The active party interprets the preferences expressed in the certificate of the passive party.
Key negotiation mechanisms in OpenPGP include:
Negotiation mechanisms in OpenPGP include:
- [Preferred hash algorithms](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#preferred-hashes-subpacket)
- [Preferred symmetric ciphers for v1 SEIPD](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#preferred-v1-seipd)