Keep server data in an encrypted database, and harden every part

Storage. The server keeps its data in SQLite (a file in the data
directory, the default) or PostgreSQL (database, KEEPORG_DATABASE_URL or
-database). Every row, the KDBX versions included, is sealed with
AES-256-GCM under a key derived from the master key and named by an HMAC.
The master key is required to start and a different one is refused. A data
directory from 0.10 or earlier is imported on first start. Several servers
may share one PostgreSQL database. Go goes to 1.26 for the pure-Go
drivers (modernc.org/sqlite, jackc/pgx).

Hardening, after an audit of the server, both clients and the deployment:
- Single sign-on approves a device only after an explicit Approve in the
  browser that started it, bound by a cookie and PKCE; identities are never
  linked by username, by verified email only when allowed, never to admins.
- Sign-in attempts are limited per address and per account, passwords are
  checked outside the store lock with uniform timing, one-time codes work
  once, and starting two-factor needs the password.
- Merge vaults bound KDF parameters, payload size and XML shape, check
  pushes before storing them, and run at most two merges at once. A version
  is written in the same transaction as its metadata.
- Tokens in URLs only on the event stream, which ends on revocation; CSP,
  frame-ancestors, no-store and HSTS; sign-in CSRF; 404 for databases
  without access; new trusted_proxies.
- Clients open only the server's approval page, keep case-variant and
  retired file names apart, never overwrite a save made during a pull,
  require https except locally, send the stream token in a header, bound
  their reads and escape notifications and terminal output.
- The .deb creates the master key, systemd is sandboxed further, nginx sets
  modern TLS and HSTS, and CI deploys from main only.

Server 0.12.0, desktop 0.6.0, Android 0.4.0 (versionCode 4).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/secret/keyring.go

@@ -3,7 +3,10 @@ package secret
 import (
 	"crypto/aes"
 	"crypto/cipher"
+	"crypto/hmac"
+	"crypto/sha256"
 	"encoding/base64"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"os"
@@ -21,6 +24,7 @@ import (
 // both holds the vaults.
 type Keyring struct {
 	aead cipher.AEAD
+	key  []byte
 }
 
 // Environment variables that supply the master key. Both are read before
@@ -131,7 +135,54 @@ func NewKeyring(key []byte) (*Keyring, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &Keyring{aead: aead}, nil
+	return &Keyring{aead: aead, key: append([]byte(nil), key...)}, nil
+}
+
+// Derive returns a keyring for one purpose, keyed by HMAC-SHA256 of the
+// master key and the purpose's name. Each use of the master key gets its own
+// key this way: a ciphertext made for one purpose does not open under
+// another, and no subkey reveals the master key.
+func (k *Keyring) Derive(purpose string) *Keyring {
+	m := hmac.New(sha256.New, k.key)
+	m.Write([]byte("keeporg/" + purpose))
+	sub, err := NewKeyring(m.Sum(nil))
+	if err != nil {
+		panic("secret: derive: " + err.Error()) // a SHA-256 sum is always 32 bytes
+	}
+	return sub
+}
+
+// Seal encrypts data with AES-256-GCM under a fresh nonce, authenticating
+// aad with it. The output is the nonce followed by the ciphertext.
+func (k *Keyring) Seal(aad, data []byte) []byte {
+	nonce := randBytes(k.aead.NonceSize())
+	return k.aead.Seal(nonce, nonce, data, aad)
+}
+
+// ErrSealed means sealed data did not open: a different key, different
+// associated data, or tampering.
+var ErrSealed = errors.New("secret: sealed data does not open with this key")
+
+// Open decrypts what Seal produced with the same aad.
+func (k *Keyring) Open(aad, sealed []byte) ([]byte, error) {
+	n := k.aead.NonceSize()
+	if len(sealed) < n {
+		return nil, ErrSealed
+	}
+	plain, err := k.aead.Open(nil, sealed[:n], sealed[n:], aad)
+	if err != nil {
+		return nil, ErrSealed
+	}
+	return plain, nil
+}
+
+// MAC is a keyed, deterministic digest of value, in hex. It names a row by a
+// value without storing the value: equal inputs give equal names, and the
+// name says nothing about the input to anyone without the key.
+func (k *Keyring) MAC(value string) string {
+	m := hmac.New(sha256.New, k.key)
+	m.Write([]byte(value))
+	return hex.EncodeToString(m.Sum(nil))
 }
 
 // Wrap encrypts a vault secret. The vault's own id is authenticated with it,