feat: webauthn prf passkey unlock engine as alternative wrap of the linking key

src/lnurlcash/storage/passkeySlots.ts

@@ -0,0 +1,64 @@
+// Passkey-slot persistence: one localStorage record holding every passkey
+// wrap of the linking key (see passkeys.ts). Slots are public metadata plus
+// AES-GCM wrapped keys - a wrapped blob is useless without the passkey's
+// authenticator, so this sits next to the plaintext registries. Read/write
+// are exported bare; callers serialize read-modify-write cycles with
+// withStorageLock, same convention as bearers.ts.
+
+// the encrypted half of a slot: the linking key under a passkey wrap key
+export type PasskeyWrap = {
+  hkdfSalt: string // hex, 16 bytes - per-slot HKDF salt
+  iv: string // hex, 12 bytes
+  wrappedKey: string // hex, AES-GCM ciphertext of the 32-byte linking key
+}
+
+export type PasskeySlot = PasskeyWrap & {
+  credentialId: string // hex of the raw WebAuthn credential id
+  createdAt: number
+  name?: string // optional holder label ('laptop', 'phone', ...)
+}
+
+export const PASSKEY_SLOTS_STORAGE_KEY = 'sattle_passkey_slots'
+
+// strict shape check, same spirit as keys.ts's isValidStoredSecret:
+// localStorage content is not trustworthy input (hand-edited, restored
+// backups), so slots are validated before use
+const isValidPasskeySlot = (slot: unknown): slot is PasskeySlot => {
+  if (typeof slot !== 'object' || slot === null) return false
+  const s = slot as Record<string, unknown>
+  return (
+    typeof s.credentialId === 'string' &&
+    s.credentialId.length > 0 &&
+    s.credentialId.length % 2 === 0 &&
+    /^[0-9a-f]+$/i.test(s.credentialId) &&
+    typeof s.hkdfSalt === 'string' &&
+    /^[0-9a-f]{32}$/i.test(s.hkdfSalt) &&
+    typeof s.iv === 'string' &&
+    /^[0-9a-f]{24}$/i.test(s.iv) &&
+    typeof s.wrappedKey === 'string' &&
+    s.wrappedKey.length > 0 &&
+    s.wrappedKey.length % 2 === 0 &&
+    /^[0-9a-f]+$/i.test(s.wrappedKey) &&
+    typeof s.createdAt === 'number' &&
+    (s.name === undefined || typeof s.name === 'string')
+  )
+}
+
+// malformed entries are dropped, not thrown on - one corrupted slot must
+// not take the remaining passkeys down with it
+export const readPasskeySlots = (): PasskeySlot[] => {
+  const raw = localStorage.getItem(PASSKEY_SLOTS_STORAGE_KEY)
+  if (!raw) return []
+  try {
+    const parsed: unknown = JSON.parse(raw)
+    return Array.isArray(parsed) ? parsed.filter(isValidPasskeySlot) : []
+  } catch {
+    return []
+  }
+}
+
+export const hasPasskeySlots = (): boolean => readPasskeySlots().length > 0
+
+export const writePasskeySlots = (slots: PasskeySlot[]): void => {
+  localStorage.setItem(PASSKEY_SLOTS_STORAGE_KEY, JSON.stringify(slots))
+}