Add unit tests and update docs

src/requirements.txt

@@ -7,4 +7,6 @@ monstr @ git+https://github.com/monty888/monstr.git@master#egg=monstr
 mnemonic
 aiohttp
 bcrypt
-bip85
+bip85
+pytest>=7.0
+

src/tests/test_fingerprint_encryption.py

@@ -0,0 +1,38 @@
+import hashlib
+import sys
+from pathlib import Path
+from tempfile import TemporaryDirectory
+
+from cryptography.fernet import Fernet
+
+sys.path.append(str(Path(__file__).resolve().parents[1]))
+
+from utils.fingerprint import generate_fingerprint
+from password_manager.encryption import EncryptionManager
+
+
+def test_generate_fingerprint_deterministic():
+    seed = "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about"
+    expected = (
+        hashlib.sha256(seed.strip().lower().encode("utf-8")).hexdigest()[:16].upper()
+    )
+    fp1 = generate_fingerprint(seed)
+    fp2 = generate_fingerprint(seed.upper())
+    assert fp1 == expected
+    assert fp1 == fp2
+
+
+def test_encryption_round_trip():
+    with TemporaryDirectory() as tmpdir:
+        key = Fernet.generate_key()
+        manager = EncryptionManager(key, Path(tmpdir))
+        data = b"secret data"
+        rel_path = Path("testfile.enc")
+        manager.encrypt_and_save_file(data, rel_path)
+        decrypted = manager.decrypt_file(rel_path)
+        assert decrypted == data
+
+        # parent seed round trip
+        seed = "correct horse battery staple"
+        manager.encrypt_parent_seed(seed)
+        assert manager.decrypt_parent_seed() == seed