docs: ADR-037 revision, versioned envelope and key ring

[ivanball]

Revises ADR-037 (field-level encryption at rest) to record the redesigned EncryptedStringConverter, and rebuilds docs/.

What the revision records

• Versioned storage envelope. Base64 of [key version (1)][nonce (12)][ciphertext][tag (16)]. The previous un-versioned [nonce][ciphertext][tag] layout is gone with no legacy decode path, which is affordable exactly because this ADR has recorded zero adopted columns since 2026-07-06. That window closes at the first adopted column.
• Key ring. A new constructor over IReadOnlyDictionary<byte, byte[]> plus a nominated current version, validated once (non-null, non-empty, no null entry, every key exactly 32 bytes, current version present) and defensively copied to a FrozenDictionary. The single-key byte[] constructor stays, as sugar for a one-entry ring at version 1.
• The version byte is authenticated. It travels as AES-GCM associated data on both encrypt and decrypt, so the tag covers it: rewriting it fails decryption even when the substituted version maps to the same key.
• Rotation story. Add a new key as current while keeping the old registered, deploy, re-encrypt in the background at any pace, retire the old version. The old "no rotation story" and "no key identifier or version in the layout" trade-offs are rewritten to match.
• Stateless and context-free stance. Version resolution is data-driven from the envelope, never from the DbContext; per-tenant and per-request key selection stay deliberately out of scope for a value converter (that needs a SaveChanges interceptor or application-layer encryption).
• Per-value overhead moves from 28 bytes to 29 before Base64. Test coverage noted as 11 cases to 21. Every EncryptedStringConverter.cs citation rebased against the branch source. Honest latent posture preserved: still zero production columns.

Source

The code change lands via MMCA.Common PR #247 (ivanball/MMCA.Common#247), unreleased; it ships in the next framework release, and the revision note says so.

Notes

• docs-src/ edit and the regenerated docs/adr/037-field-level-encryption-at-rest.html, assets/data/search-index.json and sitemap.xml are committed together (freshness gate).
• The ADR index README.md was left untouched per instructions; its summary cell for 037 still describes the old nonce+ct+tag layout and will need a follow-up.
• Do not merge without confirmation: a push to main publishes the live site.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KwnaQjfkHbHqoMm4Pec3oE