Two-tree integration: credential storage schema update

[ejmockler]

Summary

The IndexedDB credential schema must be extended to store two-tree credential data: cell ID, dual Merkle paths (user tree + cell map tree), district array, and the two tree roots.

Severity: Required for two-tree deployment
Source: Two-Tree Architecture Spec
Related voter-protocol tracking: Phase 5 remaining items

Schema Changes

Current Credential Schema

{
  user_secret: encrypted,
  district_id: string,
  authority_level: number,
  salt: bigint,
  merkle_path: { siblings: string[], pathIndices: number[] },
  merkle_root: string,
  leaf_index: number,
  expires_at: number,
  credential_type: 'single-tree'  // NEW: type discriminator
}

New Two-Tree Credential Schema

{
  user_secret: encrypted,
  cell_id: encrypted,              // More sensitive than district_id
  salt: bigint,
  districts: bigint[24],           // Cell's district memberships
  user_tree_path: { siblings: string[], pathIndices: number[] },
  user_tree_root: string,
  cell_map_path: { siblings: string[], pathBits: number[], attempt: number },
  cell_map_root: string,
  leaf_index: number,
  expires_at: number,
  credential_type: 'two-tree'
}

Migration Strategy

• Existing single-tree credentials remain valid until expires_at
• New credentials created as two-tree type
• Credential loading code checks credential_type and routes to appropriate prover
• IndexedDB schema version bump with backward-compatible migration

Pitfalls

• cell_id must be encrypted at rest (finer geographic granularity than district)
• districts[24] array must match the circuit's expected slot count exactly
• Cell map path includes pathBits and attempt (SMT-specific) — different from standard Merkle pathIndices
• Ensure credential_type discriminator is always present (old credentials without it default to single-tree)

Acceptance Criteria

• IndexedDB schema version bumped
• Two-tree credential type fully supported
• Backward-compatible: old credentials still loadable
• cell_id encrypted with same Web Crypto scheme as user_secret
• Migration function for schema upgrade