gep: provisional GEP for certificate pinning (#4078)

Signed-off-by: Norwin Schnyder <[email protected]>

Author: snorwin

geps/gep-4078/index.md

@@ -0,0 +1,25 @@
+# GEP-4078: Certificate Pinning for Client Certificate Validation
+
+* Issue: [#4078](https://github.com/kubernetes-sigs/gateway-api/issues/4078)
+* Status: Provisional
+
+(See [status definitions](../overview.md#gep-states).)
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
+"SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
+document are to be interpreted as described in BCP 14 ([RFC8174]) when, and
+only when, they appear in all capitals, as shown here.
+
+[RFC8174]: https://www.rfc-editor.org/rfc/rfc8174
+
+## What
+Enhance the existing Client Certificate Validation defined in [GEP-91](../gep-91/index.md) by introducing support for certificate pinning. This allows specifying one or more certificate or public key hashes (SPKI) that are considered valid for client connections. During TLS client authentication, the Gateway will validate not only against the configured CAs, but also against the pinned certificates or keys. This provides a mechanism to restrict allowed clients to a narrowly defined set of certificates, even if the CA trust domain is broad.
+
+## Why
+While [GEP-91](../gep-91/index.md) enables client certificate validation against a CA, in practice many operators rely on large organizational CAs or even on public CAs. These often cover many certificates and, as a result, unintentionally expand the trust boundary. This limitation becomes particularly significant when client certificates are used not only for encryption but also for authentication. In these cases, operators may want to restrict access to an explicit set of identities, rather than the entire population served by a CA.  
+In addition, certificate pinning strengthens security by reducing exposure to rogue certificates and mitigating the risk of man-in-the-middle (MITM) attacks.
+
+## Who: Beneficiaries
+* **Application Developers**: Can use certificate pinning as a lightweight alternative to JWTs or other token systems, authenticating users or devices directly through TLS without additional infrastructure.  
+* **Platform Operators/Administrators**: Gain a standardized way to enforce fine-grained client restrictions, even when using broad or public CAs, which improves security.  
+* **Security and Compliance Teams**: Benefit from a certificate-bound authentication mechanism that limits access to explicitly approved identities, aligning with regulatory and organizational policies.

geps/gep-4078/metadata.yaml

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+apiVersion: internal.gateway.networking.k8s.io/v1alpha1
+kind: GEPDetails
+number: 4078
+name: Certificate Pinning for Client Certificate Validation 
+status: Provisional
+# Any authors who contribute to the GEP in any way should be listed here using
+# their GitHub handle.
+authors:
+  - snorwin
+relationships:
+  # obsoletes indicates that a GEP makes the linked GEP obsolete, and completely
+  # replaces that GEP. The obsoleted GEP MUST have its obsoletedBy field
+  # set back to this GEP, and MUST be moved to Declined.
+  obsoletes: {}
+  obsoletedBy: {}
+  # extends indicates that a GEP extends the linked GEP, adding more detail
+  # or additional implementation. The extended GEP MUST have its extendedBy
+  # field set back to this GEP.
+  extends:
+   - number: 91
+     name: Client Certificate Validation for TLS terminating at the Gateway Listener
+  extendedBy: {}
+  # seeAlso indicates other GEPs that are relevant in some way without being
+  # covered by an existing relationship.
+  seeAlso: {}
+# references is a list of hyperlinks to relevant external references.
+# It's intended to be used for storing GitHub discussions, Google docs, etc.
+references: {}
+# featureNames is a list of the feature names introduced by the GEP, if there
+# are any. This will allow us to track which feature was introduced by which GEP.
+# This is the value added to supportedFeatures and the conformance tests, in string form.
+featureNames: {}
+# changelog is a list of hyperlinks to PRs that make changes to the GEP, in
+# ascending date order.
+changelog: {}

geps/gep-91/metadata.yaml

@@ -5,3 +5,29 @@ name: Client Certificate Validation for TLS terminating at the Gateway Listener
 status: Provisional
 authors:
   - arkodg
+relationships:
+  # obsoletes indicates that a GEP makes the linked GEP obsolete, and completely
+  # replaces that GEP. The obsoleted GEP MUST have its obsoletedBy field
+  # set back to this GEP, and MUST be moved to Declined.
+  obsoletes: {}
+  obsoletedBy: {}
+  # extends indicates that a GEP extends the linked GEP, adding more detail
+  # or additional implementation. The extended GEP MUST have its extendedBy
+  # field set back to this GEP.
+  extends:
+   - number: 4078
+     name: Certificate Pinning for Client Certificate Validation 
+  extendedBy: {}
+  # seeAlso indicates other GEPs that are relevant in some way without being
+  # covered by an existing relationship.
+  seeAlso: {}
+# references is a list of hyperlinks to relevant external references.
+# It's intended to be used for storing GitHub discussions, Google docs, etc.
+references: {}
+# featureNames is a list of the feature names introduced by the GEP, if there
+# are any. This will allow us to track which feature was introduced by which GEP.
+# This is the value added to supportedFeatures and the conformance tests, in string form.
+featureNames: {}
+# changelog is a list of hyperlinks to PRs that make changes to the GEP, in
+# ascending date order.
+changelog: {}