fix: PBKDF2 key derivation (#5), HMAC salted recovery codes (#4), HSTS (#8)

- SQLCipher key: SHA256 -> PBKDF2-HMAC-SHA256 with 100k iterations
- Recovery code hashing: SHA256 -> HMAC-SHA256 with salt
- Add Strict-Transport-Security, X-Content-Type-Options, X-Frame-Options, CSP headers

app.py

@@ -68,7 +68,10 @@ def require_api_key(f):
 
 
 def _derive_sqlcipher_key(bcrypt_hash: str) -> str:
-    return hashlib.sha256(bcrypt_hash.encode()).hexdigest()[:64]
+    """Derive SQLCipher key from bcrypt hash using PBKDF2 (#5)."""
+    salt = b"pinvault-sqlcipher-v1"  # Fixed salt - bcrypt hash provides entropy
+    derived = hashlib.pbkdf2_hmac("sha256", bcrypt_hash.encode(), salt, 100_000)
+    return derived.hex()[:64]
 
 
 def get_conn():
@@ -80,8 +83,9 @@ def get_conn():
     return conn
 
 
-def hash_val(value):
-    return hashlib.sha256(value.encode()).hexdigest()
+def hash_val(value, salt: str = "pinvault-recovery-v1"):
+    """Hash recovery code with HMAC to prevent precomputed attacks (#4)."""
+    return hmac.new(salt.encode(), value.encode(), hashlib.sha256).hexdigest()
 
 
 def generate_recovery_code():