feat(totp): 4-part defense against permanent lockout

- credential-manager.js: add diagnose(key) method that distinguishes
  ok | missing | unreadable | corrupt instead of silently returning null
- crypto-utils.js: silent fallback to .encryption-key.bak when primary
  can't decrypt existing credentials; first-run bootstrap writes .bak;
  rotateKey() backs up old key before swap
- routes/auth/totp.js: new public /api/v1/totp/recovery-info endpoint
  returns {status, isSetUp, hint} so UI can show meaningful errors
- middleware.js: add /totp/recovery-info to PUBLIC_ROUTES so the
  locked-out user can read the diagnostic without being logged in

dashcaddy-api/credential-manager.js

@@ -319,6 +319,53 @@ class CredentialManager {
       : data.value;
   }
 
+  /**
+   * Retrieve a credential with diagnostic info on failure.
+   *
+   * Used by the TOTP recovery flow: when a user is locked out and the secret
+   * can't be decrypted (e.g. encryption key was rotated by a container
+   * recreate), we need to distinguish "no secret was ever set" from "secret
+   * is on disk but unreadable" so the UI can show a useful next step.
+   *
+   * Status codes:
+   *   'ok'                    — value decrypted / returned as-is
+   *   'missing'               — key is not present in the store at all
+   *   'unreadable'            — key is present but decryption failed (key mismatch / corruption)
+   *   'malformed'             — entry exists but value is not in expected encrypted format
+   *
+   * @param {string} key - Credential identifier
+   * @returns {Promise<{ status: string, value: string|null, error?: string }>}
+   */
+  async diagnose(key) {
+    try {
+      const credentials = await this.loadCredentialsFile();
+      const data = credentials[key];
+      if (!data) return { status: 'missing', value: null };
+
+      if (!cryptoUtils.isEncrypted(data.value)) {
+        // Plaintext entry — return as-is
+        return { status: 'ok', value: data.value };
+      }
+
+      try {
+        const decrypted = cryptoUtils.decrypt(data.value);
+        return { status: 'ok', value: decrypted };
+      } catch (decryptErr) {
+        // Most common cause: the encryption key on disk is different from
+        // the key that originally encrypted this entry (rotated by a
+        // container recreate that didn't preserve CREDENTIALS_FILE env).
+        console.warn(
+          `[CredentialManager] '${key}' is present but cannot be decrypted ` +
+          `(likely encryption-key mismatch): ${decryptErr.message}`
+        );
+        return { status: 'unreadable', value: null, error: decryptErr.message };
+      }
+    } catch (err) {
+      console.error(`[CredentialManager] diagnose('${key}') failed:`, err.message);
+      return { status: 'malformed', value: null, error: err.message };
+    }
+  }
+
   async deleteFromFile(key) {
     await this._lockedUpdate(credentials => {
       delete credentials[key];

dashcaddy-api/crypto-utils.js

@@ -66,6 +66,31 @@ function loadOrCreateKey() {
       if (keyData.length >= 64) {
         encryptionKey = Buffer.from(keyData, 'hex');
         console.log('[Crypto] Loaded encryption key from file');
+        // First-run bootstrap: if .bak doesn't exist yet, write the current
+        // key to it. This ensures the silent recovery path is available from
+        // the very next restart without requiring an explicit rotateKey().
+        if (!fs.existsSync(KEY_FILE + '.bak')) {
+          try {
+            fs.writeFileSync(KEY_FILE + '.bak', keyData, { mode: 0o600 });
+            console.log(`[Crypto] Seeded ${KEY_FILE}.bak with current key for future fallback`);
+          } catch (e) {
+            console.warn('[Crypto] Could not seed .bak key file:', e.message);
+          }
+        }
+        // Try fallback to .bak key if primary can't decrypt existing credentials.
+        // This handles the "container recreate rotated the key" case where the
+        // backup key on disk is the ORIGINAL key that can still read the
+        // bind-mounted /app/data/credentials.json written before the upgrade.
+        if (fs.existsSync(KEY_FILE + '.bak')) {
+          try {
+            const backupData = fs.readFileSync(KEY_FILE + '.bak', 'utf8').trim();
+            if (backupData.length >= 64) {
+              encryptionKey = tryFallbackToBackupKey(Buffer.from(keyData, 'hex'), Buffer.from(backupData, 'hex'));
+            }
+          } catch (e) {
+            console.warn('[Crypto] Could not check backup key:', e.message);
+          }
+        }
         return encryptionKey;
       }
       // File exists but key is invalid/empty - will generate new one below
@@ -89,6 +114,64 @@ function loadOrCreateKey() {
   return encryptionKey;
 }
 
+/**
+ * If the primary key fails to decrypt any existing credentials, try the backup
+ * key. This is the silent recovery path: if a container recreate replaced
+ * .encryption-key with a fresh one but left .encryption-key.bak (the previous
+ * key), the old key can still decrypt the bind-mounted credentials.json and
+ * the user stays logged in without ever noticing.
+ *
+ * Called only at startup when both key files exist. Returns the working key
+ * (either primary or backup). If neither works, returns the primary (existing
+ * behavior — `retrieve()` will surface "unreadable" via credential-manager.diagnose).
+ *
+ * @param {Buffer} primaryKey - key from .encryption-key
+ * @param {Buffer} backupKey  - key from .encryption-key.bak
+ * @returns {Buffer} the key that should be used
+ */
+function tryFallbackToBackupKey(primaryKey, backupKey) {
+  const CREDENTIALS_FILE = process.env.CREDENTIALS_FILE ||
+    require('path').join(__dirname, 'credentials.json');
+  if (!fs.existsSync(CREDENTIALS_FILE)) return primaryKey;
+
+  let credentials;
+  try {
+    credentials = JSON.parse(fs.readFileSync(CREDENTIALS_FILE, 'utf8'));
+  } catch {
+    return primaryKey;
+  }
+
+  // Find the first encrypted entry to probe
+  const probeEntry = Object.values(credentials).find(v => v && v.value && isEncrypted(v.value));
+  if (!probeEntry) return primaryKey;
+
+  const tryDecrypt = (key) => {
+    const parts = probeEntry.value.split(':');
+    if (parts.length !== 3) return false;
+    try {
+      const iv = Buffer.from(parts[0], 'base64');
+      const tag = Buffer.from(parts[1], 'base64');
+      const ct = Buffer.from(parts[2], 'base64');
+      const decipher = crypto.createDecipheriv(ALGORITHM, key, iv);
+      decipher.setAuthTag(tag);
+      Buffer.concat([decipher.update(ct), decipher.final()]);
+      return true;
+    } catch { return false; }
+  };
+
+  if (tryDecrypt(primaryKey)) return primaryKey;
+  if (tryDecrypt(backupKey)) {
+    console.warn(
+      '[Crypto] Primary encryption key failed to decrypt credentials; ' +
+      'fell back to .encryption-key.bak. The current primary key was set ' +
+      'without preserving the original. Consider rotating the key explicitly ' +
+      'via the credential-manager API to avoid this warning next restart.'
+    );
+    return backupKey;
+  }
+  return primaryKey; // neither works — credential-manager.diagnose() will report 'unreadable'
+}
+
 /**
  * Encrypt sensitive data
  * @param {string|object} data - Data to encrypt (strings or objects)
@@ -276,6 +359,17 @@ function rotateKey() {
   const oldKey = loadOrCreateKey(); // Ensure we have the current key loaded
   const newKey = generateKey();
 
+  // Save the OLD key to .bak BEFORE swapping the primary. This gives the
+  // startup-time fallback a way to recover the previous key if a future
+  // restart loses the new one (e.g. another accidental recreate). The .bak
+  // file is overwritten on each rotate so it always holds the previous key,
+  // not an ever-accumulating history.
+  try {
+    fs.writeFileSync(KEY_FILE + '.bak', oldKey.toString('hex'), { mode: 0o600 });
+  } catch (error) {
+    console.warn(`[Crypto] Could not save backup key to ${KEY_FILE}.bak:`, error.message);
+  }
+
   try {
     fs.writeFileSync(KEY_FILE, newKey.toString('hex'), { mode: 0o600 });
   } catch (error) {

dashcaddy-api/middleware.js

@@ -283,6 +283,7 @@ module.exports = function configureMiddleware(app, {
     { path: '/probe/',                   prefix: true },
     { path: '/api/v1/tailscale/',        prefix: true },
     { path: '/api/v1/totp/config',        exact: true,  method: 'GET' },
+    { path: '/api/v1/totp/recovery-info', exact: true,  method: 'GET' },
     { path: '/api/v1/totp/verify',        exact: true },
     { path: '/api/v1/totp/setup',         exact: true,  method: 'POST' },
     { path: '/api/v1/totp/verify-setup',  exact: true,  method: 'POST' },

dashcaddy-api/routes/auth/totp.js

@@ -37,6 +37,66 @@ module.exports = function({ authManager, credentialManager, totpConfig, saveTotp
     });
   }, 'totp-config-get'));
 
+  // Recovery diagnostic (public, no auth required).
+  //
+  // Returns information a locked-out user needs to choose a recovery path:
+  //   - whether TOTP is configured at all (isSetUp)
+  //   - whether the stored secret is readable by the current encryption key
+  //   - a human-readable hint matching the situation
+  //
+  // Status values:
+  //   'not_configured'  — no TOTP setup yet, user should set it up
+  //   'healthy'         — secret present and decryptable, normal login
+  //   'unreadable'      — secret on disk but can't decrypt (key rotated)
+  //   'corrupt'         — entry exists but value is malformed
+  //
+  // This route never returns the secret itself — only metadata about it.
+  router.get('/totp/recovery-info', asyncHandler(async (req, res) => {
+    if (!ctx.totpConfig.isSetUp) {
+      return res.json({
+        success: true,
+        status: 'not_configured',
+        isSetUp: false,
+        hint: 'TOTP has not been set up on this server yet. Open settings to configure it.'
+      });
+    }
+
+    const diag = await ctx.credentialManager.diagnose('totp.secret');
+    if (diag.status === 'ok') {
+      return res.json({
+        success: true,
+        status: 'healthy',
+        isSetUp: true,
+        hint: 'TOTP is configured and the stored secret is readable. Enter your authenticator code to log in.'
+      });
+    }
+    if (diag.status === 'unreadable') {
+      return res.json({
+        success: true,
+        status: 'unreadable',
+        isSetUp: true,
+        hint: 'Your stored TOTP secret is on disk but cannot be decrypted — this usually means the encryption key changed during an upgrade. ' +
+              'If you saved your Base32 secret when you first set up TOTP, paste it below to restore access. ' +
+              'Otherwise you will need SSH access to the server to recover or rotate the key.'
+      });
+    }
+    if (diag.status === 'missing') {
+      // Config says isSetUp:true but no secret in store — corrupted config state
+      return res.json({
+        success: true,
+        status: 'corrupt',
+        isSetUp: true,
+        hint: 'TOTP is marked as configured but the secret is missing. Set up TOTP again with a fresh secret.'
+      });
+    }
+    return res.json({
+      success: true,
+      status: 'corrupt',
+      isSetUp: true,
+      hint: 'TOTP storage is in an unexpected state. ' + (diag.error || '')
+    });
+  }, 'totp-recovery-info'));
+
   // Generate new TOTP secret + QR code
   router.post('/totp/setup', asyncHandler(async (req, res) => {
     const { authenticator } = require('otplib');