diff --git a/clipsync/main.py b/clipsync/main.py
index eaee4d4..e03013a 100644
--- a/clipsync/main.py
+++ b/clipsync/main.py
@@ -152,7 +152,8 @@ def start(self) -> None:
 
         self._start_syncthing_with_retry()
 
-        assert self.syncthing.client is not None
+        if self.syncthing.client is None:
+            raise RuntimeError("Syncthing started but REST client was not initialized")
         self.clipboard = ClipboardSync(self.settings)
         self.clipboard.start()
 
@@ -309,7 +310,9 @@ def _on_pending_device(self, device_id: str, info: dict[str, object]) -> None:
         )
 
     def _accept_device(self, device_id: str) -> None:
-        assert self.syncthing.client is not None
+        if self.syncthing.client is None:
+            log.error("Cannot accept device %s: Syncthing client not available", device_id)
+            return
         info: dict[str, object]
         with self._pending_lock:
             info = self._pending.pop(device_id, {})
diff --git a/clipsync/syncthing.py b/clipsync/syncthing.py
index 81da5e1..4580ce7 100644
--- a/clipsync/syncthing.py
+++ b/clipsync/syncthing.py
@@ -712,7 +712,8 @@ def start(self) -> None:
         self._monitor.start()
 
     def _spawn(self) -> None:
-        assert self._binary is not None
+        if self._binary is None:
+            raise RuntimeError("Cannot spawn Syncthing: binary path not set")
         with self._lock:
             old = self._proc
             if old is not None and old.poll() is None:
diff --git a/clipsync/ui.py b/clipsync/ui.py
index 835b5db..bcaab81 100644
--- a/clipsync/ui.py
+++ b/clipsync/ui.py
@@ -87,7 +87,8 @@ def open(self, window: str) -> None:
 
     def _read_events(self, proc: subprocess.Popen[str]) -> None:
         try:
-            assert proc.stdout is not None
+            if proc.stdout is None:
+                raise RuntimeError("UI subprocess has no stdout pipe")
             for line in proc.stdout:
                 line = line.strip()
                 if not line:
@@ -1020,7 +1021,8 @@ def _finish_update_check(self, info: update.UpdateInfo | None, error: str | None
         if error is not None:
             self._status.configure(text=error)
             return
-        assert info is not None
+        if info is None:
+            return
         if not info.update_available:
             self._status.configure(text=f"You're up to date (v{info.current_version}).")
             return
diff --git a/tests/test_crypto.py b/tests/test_crypto.py
new file mode 100644
index 0000000..cf681d5
--- /dev/null
+++ b/tests/test_crypto.py
@@ -0,0 +1,131 @@
+"""Tests for clipboard encryption helpers (crypto.py)."""
+
+from __future__ import annotations
+
+import pytest
+
+from clipsync import crypto
+from clipsync.crypto import _ENC_MAGIC_V0, _ENC_MAGIC_V1, _LEGACY_SALT, _derive_key
+
+
+# ---------------------------------------------------------------------------
+# Encrypt / decrypt round-trip
+# ---------------------------------------------------------------------------
+
+
+def test_roundtrip_text() -> None:
+    plaintext = b"hello world"
+    assert crypto.decrypt(crypto.encrypt(plaintext, "secret"), "secret") == plaintext
+
+
+def test_roundtrip_binary() -> None:
+    data = bytes(range(256))
+    assert crypto.decrypt(crypto.encrypt(data, "pass"), "pass") == data
+
+
+def test_roundtrip_empty_bytes() -> None:
+    assert crypto.decrypt(crypto.encrypt(b"", "pw"), "pw") == b""
+
+
+def test_roundtrip_unicode_passphrase() -> None:
+    plaintext = b"data"
+    passphrase = "pässwörd\U0001f511"
+    assert crypto.decrypt(crypto.encrypt(plaintext, passphrase), passphrase) == plaintext
+
+
+# ---------------------------------------------------------------------------
+# Wrong passphrase
+# ---------------------------------------------------------------------------
+
+
+def test_wrong_passphrase_returns_none() -> None:
+    ciphertext = crypto.encrypt(b"secret data", "correct")
+    assert crypto.decrypt(ciphertext, "wrong") is None
+
+
+def test_empty_passphrase_wrong_returns_none() -> None:
+    ciphertext = crypto.encrypt(b"data", "notempty")
+    assert crypto.decrypt(ciphertext, "") is None
+
+
+# ---------------------------------------------------------------------------
+# V1 format properties
+# ---------------------------------------------------------------------------
+
+
+def test_encrypt_produces_v1_magic() -> None:
+    ct = crypto.encrypt(b"x", "pw")
+    assert ct.startswith(_ENC_MAGIC_V1)
+
+
+def test_v1_uses_random_salt_per_call() -> None:
+    ct1 = crypto.encrypt(b"same", "pw")
+    ct2 = crypto.encrypt(b"same", "pw")
+    assert ct1 != ct2
+
+
+# ---------------------------------------------------------------------------
+# V0 legacy format (backward compatibility)
+# ---------------------------------------------------------------------------
+
+
+def _make_v0_payload(plaintext: bytes, passphrase: str) -> bytes:
+    from cryptography.fernet import Fernet
+
+    key = _derive_key(passphrase, _LEGACY_SALT)
+    token = Fernet(key).encrypt(plaintext)
+    return _ENC_MAGIC_V0 + token
+
+
+def test_v0_legacy_decrypt() -> None:
+    payload = _make_v0_payload(b"legacy data", "oldpass")
+    assert crypto.decrypt(payload, "oldpass") == b"legacy data"
+
+
+def test_v0_wrong_passphrase_returns_none() -> None:
+    payload = _make_v0_payload(b"data", "correct")
+    assert crypto.decrypt(payload, "wrong") is None
+
+
+# ---------------------------------------------------------------------------
+# Truncated / malformed input
+# ---------------------------------------------------------------------------
+
+
+def test_truncated_v1_header_returns_none() -> None:
+    # V1 magic + salt that is too short (no token)
+    short = _ENC_MAGIC_V1 + b"\x00" * 5
+    assert crypto.decrypt(short, "pw") is None
+
+
+def test_garbage_returns_none() -> None:
+    assert crypto.decrypt(b"not encrypted at all", "pw") is None
+
+
+def test_empty_bytes_returns_none() -> None:
+    assert crypto.decrypt(b"", "pw") is None
+
+
+def test_partial_magic_returns_none() -> None:
+    assert crypto.decrypt(b"CSEN", "pw") is None
+
+
+# ---------------------------------------------------------------------------
+# is_encrypted
+# ---------------------------------------------------------------------------
+
+
+def test_is_encrypted_v1() -> None:
+    assert crypto.is_encrypted(crypto.encrypt(b"x", "pw")) is True
+
+
+def test_is_encrypted_v0() -> None:
+    assert crypto.is_encrypted(_make_v0_payload(b"x", "pw")) is True
+
+
+def test_is_encrypted_plaintext() -> None:
+    assert crypto.is_encrypted(b"plain text clipboard") is False
+
+
+def test_is_encrypted_empty() -> None:
+    assert crypto.is_encrypted(b"") is False