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move up and improve section on distribution mechanisms for certificates

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Tammi L. Coles 2024-02-15 21:17:20 +01:00
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@ -233,6 +233,43 @@ A common use case of fingerprints and Key IDs, crucial for day-to-day secure com
While their properties differ, these mechanisms showcase the indispensable role of fingerprints and Key IDs in facilitating secure email exchanges within the OpenPGP ecosystem.
(certificate-distribution)=
## Distribution mechanisms for certificates
The OpenPGP ecosystem employs various mechanisms for the distribution and retrieval of certificates, each with its unique infrastructure, operational model, and set of advantages and challenges. This section explores these mechanisms, providing insights into how they support the broader objectives of certificate management, including security, privacy, and usability.
### Web Key Directory (WKD)
The Web Key Directory (WKD) offers a decentralized solution for the distribution of OpenPGP certificates, enabling domain owners to host OpenPGP keys on their own web servers. This approach provides a direct, user-friendly method for retrieving certificates based on email addresses, aligning with OpenPGP's trust and privacy principles.
- **Decentralization and domain control**: WKD's decentralized framework allows for the hosting of certificates in a well-known location on a webserver, managed by the entity controlling the DNS domain of an email-based identity. This model empowers domain owners with direct control over the certificates associated with their domain, enhancing security and trustworthiness. The decentralization aspect ensures that the reliability and availability of OpenPGP certificates can vary depending on the organization operating each WKD instance, allowing for tailored security practices and policies.
- **Privacy and autonomy**: By facilitating a method of certificate distribution that does not rely on centralized keyserver networks, WKD inherently supports privacy by minimizing exposure to third-party tracking or control. Users and domain owners can manage certificate distribution autonomously, providing a privacy-centric alternative to traditional keyservers.
- **Seamless integration with email systems**: WKD leverages the existing infrastructure of email addresses and domain names for smooth integration with email clients. This supports the automatic discovery and retrieval of certificates, streamlining the process for end-users. Such seamless integration ensures that certificates are always current, making secure communication more accessible and manageable.
### keys.openpgp.org (Hagrid)
Hagrid, the software powering the keys.openpgp.org service, represents a paradigm shift in keyserver design, focusing on verifying user identities and safeguarding privacy. This "verifying" keyserver model is distinctive for its approach to managing identity components associated with OpenPGP certificates.
- **Trust and verification**: Hagrid introduces a rigorous verification process, wherein identity components (including email addresses) are only published after the keyserver has sent a verification email to the address in question and received explicit opt-in consent from the user. This verification mechanism ensures that all published certificates are authentically linked to their claimed identities, significantly mitigating risks of impersonation and unauthorized certificate publication.
- **Centralization and privacy considerations**: Unlike traditional, decentralized keyservers, Hagrid operates on a centralized model, which necessitates trust in the operator to accurately perform verification steps. However, this centralization facilitates a trade-off that enhances privacy: Hagrid prevents the "enumeration" of certificates and identities, meaning third parties cannot simply list or query all email addresses stored in the service's database. This feature is critical for user privacy and control over personal information in the digital space.
- **User control and anti-spam measures**: By implementing a model that publishes identity information solely with user consent, Hagrid empowers users with unparalleled control over their digital identities. This approach not only protects users' privacy but also contributes to a cleaner, more reliable directory of certificates, devoid of spam and irrelevant data. Furthermore, Hagrid's design simplifies the publication of revocations, enabling users to easily update or invalidate their certificates without necessitating the publication of additional identity components.
### SKS-style keyservers: Challenges and solutions
SKS-style keyservers have historically facilitated the exchange of OpenPGP certificates within a distributed, unverified database framework. While instrumental in the OpenPGP ecosystem, this model has encountered significant challenges, particularly related to security and privacy.
- **Security and privacy concerns**: The openness of SKS-style keyservers has exposed them to certificate flooding attacks, where attackers inundate a key with excessive, often malicious, signatures. This not only disrupts operational efficiency but also raises privacy concerns, as these servers indiscriminately distribute third-party certifications and identity packets, potentially without the certificate owner's consent.
- **Adaptive responses and Hockeypuck's role**: In response, the OpenPGP community has sought solutions to these vulnerabilities. Hockeypuck, an advanced keyserver software, exemplifies these efforts by aiming to enhance security and data integrity, directly addressing the limitations of the traditional SKS architecture. One notable discussion within the community ([GitHub issue #136](https://github.com/hockeypuck/hockeypuck/issues/136)) focuses on proposals like HIP-1, aiming to provide key owners with more control over their certificates on keyservers, thus mitigating risks such as certificate flooding.
- **Evolving towards more secure models**: This transition from SKS-style servers to more controlled, secure models signifies a broader shift within the OpenPGP community. Newer mechanisms like WKD and Hagrid illustrate this evolution, offering more privacy-respecting and user-centric approaches to certificate distribution. Hagrid, in particular, introduces a verifying keyserver model that only distributes verified identity information, a stark contrast to the traditional SKS system's approach.
The progression from SKS-style keyservers to innovative solutions like Hockeypuck and Hagrid demonstrates the OpenPGP community's commitment to safeguarding the OpenPGP ecosystem against evolving threats, ensuring a more secure and reliable infrastructure for certificate distribution.
(certificate-freshness)=
## Certificate freshness: Triggering updates with an expiration time
@ -273,17 +310,6 @@ Sequoia additionally certifies these "local, third party, User IDs" with a local
To prevent accidental publication of these local User IDs (e.g. to public keyservers), Sequoia marks these binding signatures as "local" artifacts using [Exportable Certification](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-exportable-certification) subpackets to mark them as non-exportable.
(certificate-distribution)=
## Certificate distribution mechanisms
Different mechanisms for discovering certificates, and updating certificate data exist in the OpenPGP space:
- A *Web Key Directory* service is based on a well-known location on a webserver, serving certificates in a specific format. A WKD server is operated by the entity that controls the DNS domain of an email-based identity of a certificate. This means that WKD is inherently decentralized, and the reliability of OpenPGP certificates may vary depending on the organization that operates a particular WKD instance.
- The *keys.openpgp.org* service is a "verifying" keyserver: the keyserver software only publishes identity components (which include email addresses) after sending a verification email to that address, and receiving opt-in consent by the user of the email address. This service makes a different tradeoff: it is centralized, and relying on it to correctly perform the verification step requires trust in the operator. The tradeoff allows the service to only list identity information with the consent of the owner of that identity, and to prevent "enumeration" of the certificates and identities it stores (that is: third parties cannot obtain a list of email addresses in the service's database). By design, this service allows easy publication of revocations without requiring publication of any identity components.
- *SKS-style keyservers* act as a distributed synchronizing database, which accepts certificate information without verification. The SKS network handles third-party signatures, additional changes to their handling are pending[^hip1].
[^hip1]: <https://github.com/hockeypuck/hockeypuck/issues/136>
One central difference between hockeypuck and hagrid (the software that runs the *keys.openpgp.org* service) is that hockeypuck distributes identity packets and third-party certifications that have indeterminate validity, while hagrid does not.
(keyserver-flooding)=
## Third-party certification flooding