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edit introduction and rationale of certificate minimization

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book/source/adv/certificates.md
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@ -130,23 +130,21 @@ One mechanism that addresses a part of this issue is *expiration*: By setting th
(minimization)=
## Certificate minimization
Certificate minimization is the practice of presenting a partial view of a certificate by filtering out some of its components.
Certificate minimization involves selectively filtering out components of a certificate that are not essential for its intended use-case or to mitigate specific security concerns. This practice aims to enhance performance – particularly for client software – address security vulnerabilities like certificate flooding, and protect user privacy.
Filtering out some elements of a certificate can serve various purposes:
### Rationale and techniques for minimization
- Omitting unnecessary components for specific use-cases. For example, email clients need encryption, signing and certification component keys, but not authentication subkeys, which are used, e.g., for SSH connections.
- Omitting third-party certifications if they are not required for a use-case. ["Certificate flooding,"](https://dkg.fifthhorseman.net/blog/openpgp-certificate-flooding.html) for example, can lead to consumer software rejecting a certificate entirely. Filtering out third-party User ID certifications on import can mitigate this.
- Sometimes, a certificate organically grows so big that the user software [has problems handling it](https://www.reddit.com/r/GnuPG/comments/bp23p4/my_key_is_too_large/).
The strategy behind certificate minimization focuses on creating a streamlined certificate by removing elements not required for its specific application. This approach not only boosts operational efficiency and client software performance but also safeguards OpenPGP communications against various threats. By filtering which components to retain or omit, the process can serve distinct purposes:
### Elements that can be omitted as part of a minimization process
- **Omitting unnecessary components**: In contexts such as email encryption, only the keys necessary for encryption, signing, and certification are retained, excluding those like authentication subkeys that are irrelevant to the primary use-case.
- **Omitting third-party certifications**: Proactively filtering these out upon import can prevent the overload of a certificate with excessive certifications, a common tactic in ["certificate flooding,"](https://dkg.fifthhorseman.net/blog/openpgp-certificate-flooding.html) that is designed to render a certificate unusable. This is particularly relevant when certificates grow organically large, to the point that user software [encounters difficulties handling them](https://www.reddit.com/r/GnuPG/comments/bp23p4/my_key_is_too_large/).
There are different types of elements that can be omitted during minimization:
Additionally, specific elements of a certificate can be selectively omitted during the minimization process to tailor the certificate to its use-case, improve manageability, and ensure software compatibility:
- Subkeys (along with signatures on those subkeys)
- Identity components (along with both their self-signatures and third-party signatures)
- Signatures, by themselves:
- Self-signatures that have been superseded by newer self-signatures for the same purpose
- Third-party certifications
- **Subkeys and their signatures**: Subkeys not used for the current application, along with their associated signatures, can be omitted.
- **Identity components and outdated signatures**: Both self-signatures that have been superseded by newer versions and third-party signatures on identity components that are not necessary can be excluded. This helps keep the certificate lean and focused on its current application, reducing bloat and enhancing performance.
Through these targeted techniques, certificate minimization serves to enhance the practical usability of certificates in various environments and protect against potential security threats and privacy concerns. It strikes a careful balance, maintaining the OpenPGP trust framework while optimizing certificates for efficiency and specific operational contexts.
### Minimization in applications