keys: add derivation_version to DerivedKey

[marctrem]

Summary

Adds a version discriminator to splitsecure.keys.v1.DerivedKey:

uint32 derivation_version = 4;

• Version 0 is the legacy derivation construction. Because proto3 scalars default to 0, every already-stored DerivedKey reads 0 and keeps deriving byte-identically — no data migration.
• Version 1 will be a FIPS 186-5 §A.2.2 construction (implemented downstream in priv).
• Consumers must reject an unknown version before deriving, instead of falling back to a default.

Field number 4 was unclaimed and the message has no reserved ranges.

Verification

• ./bufgen.sh (buf format + lint + generate); generated Go and TS committed
• buf breaking --against '.git#branch=main' passes
• go build ./... passes

🤖 Generated with Claude Code

https://claude.ai/code/session_01TyAMGaLYMykDnnCA5ZNKsS

monday: 12937366421

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Aug 31, 2026, 7:38 PM

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Run ID: b7fa6e6f-b872-448f-9155-bc82901a2109

📥 Commits

Reviewing files that changed from the base of the PR and between 131902a and 78e8332.

⛔ Files ignored due to path filters (2)

• gen/es/proto/splitsecure/keys/v1/derived_key_pb.ts is excluded by !**/gen/**, !gen/**
• gen/go/proto/splitsecure/keys/v1/derived_key.pb.go is excluded by !**/*.pb.go, !**/gen/**, !gen/**

📒 Files selected for processing (1)

• proto/splitsecure/keys/v1/derived_key.proto

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📜 Recent review details

🔇 Additional comments (1)

proto/splitsecure/keys/v1/derived_key.proto (1)

11-14: LGTM!

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Walkthrough

The public DerivedKey message now includes a derivation_version field. Version 0 identifies the legacy construction. Consumers must reject unknown versions.

Changes

Derived key contract

Layer / File(s)	Summary
Add derivation version field proto/splitsecure/keys/v1/derived_key.proto	Adds DerivedKey.derivation_version as a uint32 field. Defines version 0 as legacy and requires rejection of unknown versions.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to 78e83

This adds a backward-compatible derivation version field while preserving legacy behavior for existing keys; no actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding derivation_version to DerivedKey.
Description check	✅ Passed	The description directly explains the new field, legacy compatibility, version handling, validation, and verification results.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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