feat(auth): change master password with full vault re-encryption

Adds the canonical PM feature: let the user pick a new master password
and have every entry transparently re-encrypted under the new key,
without ever exposing plaintext to the server.

Backend endpoint: POST /change-master-password
==============================================
Body:
  {
    currentMasterPassword,   verified against current stored hash
    newMasterPassword,       basis for the new hash + new client key
    newSalt,                 64-char hex, client-generated
    entries: [{ id, encrypted_password, iv,
                totp_secret?, totp_iv? }, ...]
  }

Flow:
  1. Authenticate + RequireCSRF (caller already logged in).
  2. RejectIfAccountLocked — pw change is brute-forceable through a
     hijacked session, so it respects the same per-account lockout as
     /login.
  3. Verify currentMasterPassword against the stored hash. Branches on
     hash_algo to handle both legacy 'pbkdf2' and current 'pbkdf2-sha256'.
     Wrong pw → RecordFailedAccountAttempt + audit + 401.
  4. Compute new auth hash = SHA256(PBKDF2(new_pw, new_salt, 600k)),
     always using the current scheme (migration baked in).
  5. ATOMIC transaction:
       UPDATE users SET password_hash, salt, kdf_iterations, hash_algo
       UPDATE vault_entries SET encrypted_password, iv, totp_secret, totp_iv
       (per entry)
     Any failure → rollback, user stays on the old config.
  6. DeleteAllUserSessions — every OTHER session is invalidated so a
     leaked old token can't keep working past the rotation. The current
     caller's session stays valid.
  7. ClearAccountLockout + audit_log entry.
  8. Returns { message, salt, kdfIterations }.

Client
======
New modal in index.html (#changeMasterModal) with three password
fields (current / new / confirm) + inline error display. Added a
"Change master password" button in the Settings panel → Account
section. Escape-key handler routes through it like the other modals.

doChangeMasterPassword():
  1. Local validation: all fields filled, new ≥ 8 chars, new == confirm,
     new ≠ current. Fast failure beats a round trip.
  2. randomHexSalt() → 32 secure random bytes, hex-encoded.
  3. Derive newKey = PBKDF2(new_pw, new_salt, 600k).
  4. Walk state.entries: decrypt password + (optional) TOTP under the
     current key, re-encrypt under newKey with fresh random IVs.
     One decrypt failure aborts the whole change — better than partial
     commit.
  5. POST to /change-master-password.
  6. On success: swap state.salt + state.cryptoKey, persistCryptoKey,
     update sessionStorage, refresh cached ciphertexts in state.entries,
     close modal, toast.
  7. On 401 / 429 / generic error: show inline error in the modal so
     the user can fix and retry without re-typing everything.

Threat model notes
==================
 - The current session token stays valid because the new server hash
   only invalidates OTHER sessions. Self-logout would be needlessly
   disruptive (user already proved knowledge of both pws).
 - Server still sees the old + new master pws transiently in /change-
   master-password. Same trade-off as /login — eliminating it requires
   redesigning to send pre-computed verifiers (SRP-style), tracked
   separately.
 - The salt rotates with the password — best-practice against any
   precomputed dictionary attack tied to the previous salt.

delphi-backend/Handlers/PM.Handler.Auth.pas

@@ -626,11 +626,225 @@ begin
   TJSONHelper.SendOK(AResponse, 'Migration complete');
 end;
 
+// ===== POST /change-master-password ==========================================
+// Body: {
+//   currentMasterPassword,   // verified against current stored hash
+//   newMasterPassword,       // basis for new hash + new client AES key
+//   newSalt,                 // 64-char hex, client-generated
+//   entries: [{ id, encrypted_password, iv, totp_secret?, totp_iv? }, ...]
+//                            // entries re-encrypted client-side with the
+//                            // new key (derived from new pw + new salt)
+// }
+//
+// All-or-nothing transaction: verifies current, then in one tx updates the
+// user row (hash + salt + iter count + algo) AND every entry's ciphertext.
+// On any failure the user stays on the old config — they can retry without
+// data loss.
+//
+// Side effects:
+//   - Invalidates ALL other sessions so a leaked old token can't keep
+//     working past the pw change.
+//   - Writes an audit_log entry.
+//
+// The /migrate-kdf endpoint exists for the same "re-encrypt all entries"
+// pattern when the master pw stays the same; this endpoint differs by
+// rotating the salt + pw too.
+procedure HandleChangeMasterPassword(ARequest: TIdHTTPRequestInfo;
+  AResponse: TIdHTTPResponseInfo; const AParams: TArray<string>);
+var
+  LUserId, I: Integer;
+  LBody, LEntry, LObj: TJSONObject;
+  LEntries: TJSONArray;
+  LUser, LCurPwd, LNewPwd, LNewSalt, LStoredHash, LOldSalt, LAlgo, LIP,
+    LComputed, LNewHash: string;
+  LOldIters: Integer;
+  LQ: TFDQuery;
+  LValid: Boolean;
+  LEntryId: Integer;
+  LEncPwd, LIv, LTotpSec, LTotpIv: string;
+begin
+  try
+    LUserId := Authenticate(ARequest, AResponse);
+    RequireCSRF(ARequest, AResponse, LUserId);
+  except
+    on ESessionRejected do Exit;
+  end;
+
+  LIP := GetClientIP(ARequest);
+
+  LBody := TJSONHelper.ReadBody(ARequest);
+  try
+    LCurPwd  := LBody.GetValue<string>('currentMasterPassword', '');
+    LNewPwd  := LBody.GetValue<string>('newMasterPassword',     '');
+    LNewSalt := LBody.GetValue<string>('newSalt',               '');
+    LEntries := LBody.GetValue<TJSONArray>('entries');
+
+    // Input validation. Length 64 = 32 raw bytes in hex, matches the salt
+    // format produced by RandomHex(32) and client-side randomHexSalt().
+    if (Length(LCurPwd) < 1) or (Length(LNewPwd) < 8) then
+    begin
+      TJSONHelper.SendError(AResponse, 400,
+        'New master password must be at least 8 characters');
+      Exit;
+    end;
+    if Length(LNewSalt) <> 64 then
+    begin
+      TJSONHelper.SendError(AResponse, 400, 'Invalid newSalt length');
+      Exit;
+    end;
+    if LEntries = nil then
+    begin
+      TJSONHelper.SendError(AResponse, 400, 'Missing entries array');
+      Exit;
+    end;
+
+    DB.Lock;
+    try
+      // Step 1: load current state.
+      LQ := TFDQuery.Create(nil);
+      try
+        LQ.Connection := DB.Connection;
+        LQ.SQL.Text :=
+          'SELECT username, password_hash, salt, hash_algo, kdf_iterations ' +
+          'FROM users WHERE id = :uid';
+        LQ.ParamByName('uid').AsInteger := LUserId;
+        LQ.Open;
+        if LQ.IsEmpty then
+        begin
+          TJSONHelper.SendError(AResponse, 401, 'User not found');
+          Exit;
+        end;
+        LUser       := LQ.FieldByName('username').AsString;
+        LStoredHash := LQ.FieldByName('password_hash').AsString;
+        LOldSalt    := LQ.FieldByName('salt').AsString;
+        LAlgo       := LQ.FieldByName('hash_algo').AsString;
+        LOldIters   := LQ.FieldByName('kdf_iterations').AsInteger;
+        if LAlgo = '' then LAlgo := HASH_ALGO_LEGACY;
+        if LOldIters <= 0 then LOldIters := PBKDF2_ITERATIONS;
+      finally
+        LQ.Free;
+      end;
+
+      // Lockout protection on the pw change itself (same threat model as
+      // /login — attacker with a hijacked session shouldn't be able to
+      // brute-force the current pw to swap it for one they know).
+      if RejectIfAccountLocked(AResponse, LUser) then Exit;
+
+      // Step 2: verify the CURRENT master pw.
+      LValid := False;
+      if SameText(LAlgo, HASH_ALGO_LEGACY) then
+      begin
+        LComputed := PBKDF2_SHA256_Hex(LCurPwd, LOldSalt, LOldIters);
+        LValid := ConstantTimeEquals(LComputed, LStoredHash);
+      end
+      else if SameText(LAlgo, HASH_ALGO_CURRENT) then
+      begin
+        LComputed := ComputeAuthHashCurrent(LCurPwd, LOldSalt, LOldIters);
+        LValid := ConstantTimeEquals(LComputed, LStoredHash);
+      end;
+      if not LValid then
+      begin
+        RecordFailedAccountAttempt(LUser, LIP);
+        LogAudit(LUserId, 'failed_change_password', LIP);
+        TJSONHelper.SendError(AResponse, 401, 'Current password is incorrect');
+        Exit;
+      end;
+
+      // Step 3: compute the new auth hash with the new salt + target iters.
+      LNewHash := ComputeAuthHashCurrent(LNewPwd, LNewSalt, PBKDF2_ITERATIONS_TARGET);
+
+      // Step 4: atomic transaction — user row + every entry's ciphertext.
+      DB.Connection.StartTransaction;
+      try
+        LQ := TFDQuery.Create(nil);
+        try
+          LQ.Connection := DB.Connection;
+          LQ.SQL.Text :=
+            'UPDATE users SET ' +
+            '  password_hash  = :h, ' +
+            '  salt           = :s, ' +
+            '  kdf_iterations = :it, ' +
+            '  hash_algo      = :algo ' +
+            'WHERE id = :uid';
+          LQ.ParamByName('h').AsString    := LNewHash;
+          LQ.ParamByName('s').AsString    := LNewSalt;
+          LQ.ParamByName('it').AsInteger  := PBKDF2_ITERATIONS_TARGET;
+          LQ.ParamByName('algo').AsString := HASH_ALGO_CURRENT;
+          LQ.ParamByName('uid').AsInteger := LUserId;
+          LQ.ExecSQL;
+        finally
+          LQ.Free;
+        end;
+
+        LQ := TFDQuery.Create(nil);
+        try
+          LQ.Connection := DB.Connection;
+          LQ.SQL.Text :=
+            'UPDATE vault_entries SET ' +
+            '  encrypted_password = :ep, iv = :iv, ' +
+            '  totp_secret = :ts, totp_iv = :tiv, ' +
+            '  updated_at = CURRENT_TIMESTAMP ' +
+            'WHERE id = :id AND user_id = :uid';
+
+          for I := 0 to LEntries.Count - 1 do
+          begin
+            LEntry := LEntries.Items[I] as TJSONObject;
+            LEntryId := LEntry.GetValue<Integer>('id', 0);
+            LEncPwd  := LEntry.GetValue<string>('encrypted_password', '');
+            LIv      := LEntry.GetValue<string>('iv', '');
+            LTotpSec := LEntry.GetValue<string>('totp_secret', '');
+            LTotpIv  := LEntry.GetValue<string>('totp_iv', '');
+            if (LEntryId <= 0) or (LEncPwd = '') or (LIv = '') then
+              raise Exception.CreateFmt('Invalid entry payload at index %d', [I]);
+
+            LQ.ParamByName('id').AsInteger  := LEntryId;
+            LQ.ParamByName('uid').AsInteger := LUserId;
+            LQ.ParamByName('ep').AsString   := LEncPwd;
+            LQ.ParamByName('iv').AsString   := LIv;
+            // TOTP fields are optional per entry — clear when empty so
+            // existing-NULL rows don't get stomped with empty strings.
+            if LTotpSec = '' then LQ.ParamByName('ts').Clear
+            else                   LQ.ParamByName('ts').AsString := LTotpSec;
+            if LTotpIv = '' then LQ.ParamByName('tiv').Clear
+            else                  LQ.ParamByName('tiv').AsString := LTotpIv;
+            LQ.ExecSQL;
+          end;
+        finally
+          LQ.Free;
+        end;
+
+        DB.Connection.Commit;
+      except
+        DB.Connection.Rollback;
+        raise;
+      end;
+    finally
+      DB.Unlock;
+    end;
+
+    // Step 5: invalidate every other session for this user. The CURRENT
+    // session token is still valid — caller stays logged in.
+    DeleteAllUserSessions(LUserId);
+  finally
+    LBody.Free;
+  end;
+
+  ClearAccountLockout(LUser);
+  LogAudit(LUserId, 'change_master_password', LIP);
+
+  LObj := TJSONObject.Create;
+  LObj.AddPair('message', 'Master password changed');
+  LObj.AddPair('salt',          LNewSalt);
+  LObj.AddPair('kdfIterations', TJSONNumber.Create(PBKDF2_ITERATIONS_TARGET));
+  TJSONHelper.SendJSON(AResponse, LObj);
+end;
+
 initialization
-  Router.Register('POST', '/register',     HandleRegister);
+  Router.Register('POST', '/register',                 HandleRegister);
-  Router.Register('POST', '/login',        HandleLogin);
+  Router.Register('POST', '/login',                    HandleLogin);
-  Router.Register('POST', '/logout',       HandleLogout);
+  Router.Register('POST', '/logout',                   HandleLogout);
-  Router.Register('POST', '/reauth',       HandleReauth);
+  Router.Register('POST', '/reauth',                   HandleReauth);
-  Router.Register('POST', '/migrate-kdf',  HandleMigrateKdf);
+  Router.Register('POST', '/migrate-kdf',              HandleMigrateKdf);
+  Router.Register('POST', '/change-master-password',   HandleChangeMasterPassword);
 
 end.

index.html

@@ -370,6 +370,9 @@
                 <p style="font-size:12px;color:var(--text-dim);margin:0 0 8px">
                     Signed in as <b id="settingUser"></b>
                 </p>
+                <button class="btn btn-ghost btn-sm" id="changeMasterBtn">
+                    <svg><use href="#i-lock"/></svg> Change master password
+                </button>
             </div>
         </div>
     </aside>
@@ -402,6 +405,44 @@
     </div>
 </div>
 
+<!-- ============================================================ -->
+<!-- MODAL: Change Master Password                                 -->
+<!-- ============================================================ -->
+<div id="changeMasterModal" class="modal is-hidden" role="dialog" aria-modal="true">
+    <div class="modal-backdrop" data-close></div>
+    <div class="modal-panel modal-panel-sm">
+        <header class="modal-header">
+            <h3>Change master password</h3>
+            <button class="icon-btn" data-close><svg><use href="#i-x"/></svg></button>
+        </header>
+        <form id="changeMasterForm" class="modal-body" autocomplete="off">
+            <p style="margin:0 0 12px;color:var(--text-dim);font-size:13px;line-height:1.5">
+                Your vault will be re-encrypted with the new key.
+                All other sessions will be signed out.
+            </p>
+            <label class="field">
+                <span>Current master password</span>
+                <input id="cmCurrentPwd" type="password" required>
+            </label>
+            <label class="field">
+                <span>New master password (min 8 chars)</span>
+                <input id="cmNewPwd" type="password" required minlength="8">
+            </label>
+            <label class="field">
+                <span>Confirm new master password</span>
+                <input id="cmConfirmPwd" type="password" required>
+            </label>
+            <p id="cmError" style="margin:8px 0 0;color:#dc2626;font-size:12px;display:none"></p>
+        </form>
+        <footer class="modal-footer">
+            <button type="button" class="btn btn-ghost" data-close>Cancel</button>
+            <button type="submit" form="changeMasterForm" class="btn btn-primary" id="cmConfirmBtn">
+                <svg><use href="#i-check"/></svg> Change password
+            </button>
+        </footer>
+    </div>
+</div>
+
 </div>
 
 <!-- ============================================================ -->

js/app.js

@@ -2409,6 +2409,151 @@ function closeReauth(ok) {
     }
 }
 
+// ============================================================
+// CHANGE MASTER PASSWORD
+// ============================================================
+//
+// Two-step flow:
+//   1. Modal collects current pw + new pw (×2) + validates locally.
+//   2. Client re-encrypts every entry under the new key (derived from
+//      new pw + freshly-generated salt) and sends the whole payload to
+//      /change-master-password. Server verifies current pw, then commits
+//      user row + all entry ciphertexts in one transaction.
+// On success: state.salt + state.cryptoKey are swapped, the user stays
+// logged in (current session preserved), all OTHER sessions invalidated.
+
+// 64-char hex salt — matches the format the server expects and the shape
+// produced by Delphi RandomHex(32). Crypto-secure RNG.
+function randomHexSalt() {
+    const bytes = crypto.getRandomValues(new Uint8Array(32));
+    let hex = '';
+    for (const b of bytes) hex += b.toString(16).padStart(2, '0');
+    return hex;
+}
+
+function openChangeMasterModal() {
+    $('#cmCurrentPwd').value = '';
+    $('#cmNewPwd').value     = '';
+    $('#cmConfirmPwd').value = '';
+    const errEl = $('#cmError');
+    if (errEl) { errEl.textContent = ''; errEl.style.display = 'none'; }
+    $('#changeMasterModal').classList.remove('is-hidden');
+    setTimeout(() => $('#cmCurrentPwd').focus(), 50);
+}
+
+function closeChangeMasterModal() {
+    $('#changeMasterModal').classList.add('is-hidden');
+}
+
+function showCmError(msg) {
+    const el = $('#cmError');
+    if (!el) return;
+    el.textContent = msg;
+    el.style.display = '';
+}
+
+async function doChangeMasterPassword() {
+    const curPwd  = $('#cmCurrentPwd').value;
+    const newPwd  = $('#cmNewPwd').value;
+    const confPwd = $('#cmConfirmPwd').value;
+
+    // Local validation. Server enforces these too, but failing fast saves
+    // a round trip + leaves the modal open so the user can fix and retry.
+    if (!curPwd || !newPwd || !confPwd) return showCmError('All fields are required');
+    if (newPwd.length < 8)              return showCmError('New password must be at least 8 characters');
+    if (newPwd !== confPwd)             return showCmError('New password and confirmation do not match');
+    if (newPwd === curPwd)              return showCmError('New password must differ from the current one');
+
+    // Disable the confirm button so a double-click doesn't fire two
+    // re-encryption passes in parallel.
+    const btn = $('#cmConfirmBtn');
+    if (btn) btn.disabled = true;
+    try {
+        // Step 1: generate the new salt and derive the new AES key.
+        const newSalt = randomHexSalt();
+        const newKey  = await deriveKey(newPwd, newSalt, 600000);
+
+        // Step 2: re-encrypt every entry's password AND every entry's TOTP
+        // secret (if present) under the new key. The current state.cryptoKey
+        // still decrypts the existing ciphertext.
+        const encrypted = [];
+        for (const e of state.entries) {
+            const plain = await decryptPwd(e.encrypted_password, e.iv);
+            if (plain === '[ERROR]') {
+                throw new Error('Could not decrypt entry id=' + e.id);
+            }
+            // Swap the key around encryptPwd so it picks up the new one.
+            const oldKey = state.cryptoKey;
+            state.cryptoKey = newKey;
+            try {
+                const re = await encryptPwd(plain);
+                let totpEnc = '', totpIv = '';
+                if (e.totp_secret && e.totp_iv) {
+                    state.cryptoKey = oldKey;
+                    const plainTotp = await decryptTotpSecret(e.totp_secret, e.totp_iv);
+                    state.cryptoKey = newKey;
+                    if (plainTotp !== '[ERROR]') {
+                        const t = await encryptPwd(plainTotp);
+                        totpEnc = t.encrypted;
+                        totpIv  = t.iv;
+                    }
+                }
+                encrypted.push({
+                    id:                 e.id,
+                    encrypted_password: re.encrypted,
+                    iv:                 re.iv,
+                    totp_secret:        totpEnc,
+                    totp_iv:            totpIv,
+                });
+            } finally {
+                state.cryptoKey = oldKey; // restore until server confirms
+            }
+        }
+
+        // Step 3: send the atomic request. Server verifies the current pw,
+        // updates the user row, swaps every entry's ciphertext, returns
+        // the new salt + iter count.
+        const r = await api('/change-master-password', {
+            method: 'POST',
+            headers: authHeaders({ 'Content-Type': 'application/json' }),
+            body: JSON.stringify({
+                currentMasterPassword: curPwd,
+                newMasterPassword:     newPwd,
+                newSalt:               newSalt,
+                entries:               encrypted,
+            }),
+        });
+
+        // Step 4: server committed → switch the in-memory key & salt, refresh
+        // the cached ciphertexts, persist for F5 survival.
+        state.salt      = r.salt || newSalt;
+        state.cryptoKey = newKey;
+        await persistCryptoKey();
+        sessionStorage.setItem('salt', state.salt);
+        for (let i = 0; i < state.entries.length; i++) {
+            const nc = encrypted[i];
+            state.entries[i].encrypted_password = nc.encrypted_password;
+            state.entries[i].iv                 = nc.iv;
+            state.entries[i].totp_secret        = nc.totp_secret || null;
+            state.entries[i].totp_iv            = nc.totp_iv     || null;
+        }
+
+        closeChangeMasterModal();
+        toast('Master password changed · other sessions signed out');
+    } catch (err) {
+        if (err.status === 401) {
+            showCmError('Current password is incorrect');
+        } else if (err.status === 429 && err.body && err.body.retry_after) {
+            showCmError('Account locked, try again in ' +
+                Math.ceil(err.body.retry_after / 60) + ' min');
+        } else {
+            showCmError('Failed: ' + (err.message || 'unknown error'));
+        }
+    } finally {
+        if (btn) btn.disabled = false;
+    }
+}
+
 // ============================================================
 // ENCRYPTED EXPORT CONTAINER
 // ============================================================
@@ -3184,6 +3329,13 @@ async function init() {
     });
     $('#exportBtn').addEventListener('click', doExport);
     $('#importBtn').addEventListener('click', doImport);
+    $('#changeMasterBtn').addEventListener('click', openChangeMasterModal);
+    $('#changeMasterForm').addEventListener('submit', e => {
+        e.preventDefault();
+        doChangeMasterPassword();
+    });
+    $$('#changeMasterModal [data-close]').forEach(b =>
+        b.addEventListener('click', closeChangeMasterModal));
 
     // Re-auth modal
     $('#reauthForm').addEventListener('submit', e => { e.preventDefault(); closeReauth(true); });
@@ -3211,6 +3363,10 @@ async function init() {
                 closeConfirm(false);
                 return;
             }
+            if (!$('#changeMasterModal').classList.contains('is-hidden')) {
+                closeChangeMasterModal();
+                return;
+            }
             closePalette();
             closeSlideOver();
             closeEntryModal();