Implement BIP39 mnemonic tests

Add tests for mnemonic functions to validate BIP39 compliance.

Author: Pymmdrza

tests/test_mnemonic.py

@@ -0,0 +1,50 @@
+"""Mnemonic helpers should follow BIP39 reference vectors."""
+from __future__ import annotations
+
+import pytest
+
+from libcrypto.mnemonic import (
+    InvalidMnemonicError,
+    generate_mnemonic,
+    mnemonic_to_entropy,
+    mnemonic_to_seed,
+    validate_mnemonic,
+)
+
+
+REFERENCE_MNEMONIC = "abandon " * 11 + "about"
+REFERENCE_MNEMONIC = REFERENCE_MNEMONIC.strip()
+REFERENCE_SEED = (
+    "c55257c360c07c72029aebc1b53c05ed0362ada38ead3e3e9efa3708e5349553"
+    "1f09a6987599d18264c1e1c92f2cf141630c7a3c4ab7c81b2f001698e7463b04"
+)
+
+
+def test_mnemonic_to_seed_matches_bip39_vector() -> None:
+    """Mnemonic to seed must reproduce the official BIP39 test vector."""
+    seed = mnemonic_to_seed(REFERENCE_MNEMONIC, passphrase="TREZOR")
+
+    assert seed.hex() == REFERENCE_SEED
+
+
+def test_validate_mnemonic_detects_invalid_word() -> None:
+    """An unknown word should result in validation failure."""
+    invalid_phrase = REFERENCE_MNEMONIC.replace("about", "ab0ut")
+
+    assert validate_mnemonic(invalid_phrase) is False
+
+
+def test_mnemonic_to_entropy_invalid_checksum() -> None:
+    """Changing the final word should trigger a checksum error."""
+    altered_phrase = REFERENCE_MNEMONIC.replace("about", "absorb")
+
+    with pytest.raises(InvalidMnemonicError):
+        mnemonic_to_entropy(altered_phrase)
+
+
+def test_generate_mnemonic_default_length() -> None:
+    """Generated mnemonics should produce 12-word phrases by default."""
+    mnemonic = generate_mnemonic()
+
+    assert len(mnemonic.split()) == 12
+    assert validate_mnemonic(mnemonic) is True