diff --git a/tests/test_transaction_signing.py b/tests/test_transaction_signing.py
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+++ b/tests/test_transaction_signing.py
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+"""
+tests/test_transaction_signing.py
+
+Unit tests for MiniChain transaction signing and verification.
+
+Covers:
+  1. Valid transaction — properly signed tx verifies successfully.
+  2. Modified transaction data — tampering after signing breaks verification.
+  3. Invalid public key — Transaction.sign() raises ValueError at signing time
+     when the signing key does not match the sender field; a forged sender
+     field set after signing causes verify() to return False.
+  4. Replay protection — duplicate nonce is rejected by state validation.
+"""
+
+import pytest
+from nacl.signing import SigningKey
+from nacl.encoding import HexEncoder
+
+from minichain import Transaction, State
+
+
+# ------------------------------------------------------------------
+# Fixtures
+# ------------------------------------------------------------------
+
+@pytest.fixture
+def alice():
+    sk = SigningKey.generate()
+    pk = sk.verify_key.encode(encoder=HexEncoder).decode()
+    return sk, pk
+
+
+@pytest.fixture
+def bob():
+    sk = SigningKey.generate()
+    pk = sk.verify_key.encode(encoder=HexEncoder).decode()
+    return sk, pk
+
+
+@pytest.fixture
+def funded_state(alice):
+    _, alice_pk = alice
+    state = State()
+    state.credit_mining_reward(alice_pk, 100)
+    return state
+
+
+# ------------------------------------------------------------------
+# 1. Valid transaction
+# ------------------------------------------------------------------
+
+def test_valid_signature_verifies(alice, bob):
+    """A properly signed transaction must pass signature verification."""
+    alice_sk, alice_pk = alice
+    _, bob_pk = bob
+
+    tx = Transaction(alice_pk, bob_pk, 10, nonce=0)
+    tx.sign(alice_sk)
+
+    assert tx.verify(), "A correctly signed transaction should verify successfully."
+
+
+# ------------------------------------------------------------------
+# 2. Modified transaction data
+# ------------------------------------------------------------------
+
+def test_tampered_amount_fails_verification(alice, bob):
+    """Changing `amount` after signing must invalidate the signature."""
+    alice_sk, alice_pk = alice
+    _, bob_pk = bob
+
+    tx = Transaction(alice_pk, bob_pk, 10, nonce=0)
+    tx.sign(alice_sk)
+    tx.amount = 9999  # tamper
+
+    assert not tx.verify(), "A transaction with a tampered amount must not verify."
+
+
+def test_tampered_receiver_fails_verification(alice, bob):
+    """Changing `receiver` after signing must invalidate the signature."""
+    alice_sk, alice_pk = alice
+    _, bob_pk = bob
+
+    tx = Transaction(alice_pk, bob_pk, 10, nonce=0)
+    tx.sign(alice_sk)
+
+    attacker_sk = SigningKey.generate()
+    tx.receiver = attacker_sk.verify_key.encode(encoder=HexEncoder).decode()  # tamper
+
+    assert not tx.verify(), "A transaction with a tampered receiver must not verify."
+
+
+def test_tampered_nonce_fails_verification(alice, bob):
+    """Changing `nonce` after signing must invalidate the signature."""
+    alice_sk, alice_pk = alice
+    _, bob_pk = bob
+
+    tx = Transaction(alice_pk, bob_pk, 10, nonce=0)
+    tx.sign(alice_sk)
+    tx.nonce = 99  # tamper
+
+    assert not tx.verify(), "A transaction with a tampered nonce must not verify."
+
+
+# ------------------------------------------------------------------
+# 3. Invalid public key
+# ------------------------------------------------------------------
+
+def test_wrong_sender_key_raises(alice, bob):
+    """Signing with a key that doesn't match sender must raise ValueError."""
+    _, alice_pk = alice
+    bob_sk, bob_pk = bob
+
+    tx = Transaction(alice_pk, bob_pk, 10, nonce=0)
+
+    with pytest.raises(ValueError, match="Signing key does not match sender"):
+        tx.sign(bob_sk)
+
+
+def test_forged_sender_field_fails_verification(alice, bob):
+    """Manually swapping `sender` after signing must fail verification."""
+    alice_sk, alice_pk = alice
+    _, bob_pk = bob
+
+    tx = Transaction(alice_pk, bob_pk, 10, nonce=0)
+    tx.sign(alice_sk)
+    tx.sender = bob_pk  # forge sender
+
+    assert not tx.verify(), "A transaction with a forged sender field must not verify."
+
+
+def test_unsigned_transaction_fails_verification(alice, bob):
+    """A transaction that was never signed must fail verification."""
+    _, alice_pk = alice
+    _, bob_pk = bob
+
+    tx = Transaction(alice_pk, bob_pk, 10, nonce=0)
+    # No call to tx.sign()
+
+    assert not tx.verify(), "An unsigned transaction must not verify."
+
+
+# ------------------------------------------------------------------
+# 4. Replay protection
+# ------------------------------------------------------------------
+
+def test_replay_attack_same_nonce_rejected(alice, bob, funded_state):
+    """Replaying the same transaction must be rejected the second time."""
+    alice_sk, alice_pk = alice
+    _, bob_pk = bob
+
+    tx = Transaction(alice_pk, bob_pk, 10, nonce=0)
+    tx.sign(alice_sk)
+
+    assert funded_state.apply_transaction(tx), "First submission must succeed."
+    assert not funded_state.apply_transaction(tx), "Replayed transaction must be rejected."
+    # Ensure the rejected replay did not mutate the ledger
+    assert funded_state.get_account(alice_pk)["balance"] == 90, \
+        "Alice's balance must not change after a rejected replay."
+    assert funded_state.get_account(alice_pk)["nonce"] == 1, \
+        "Alice's nonce must not advance after a rejected replay."
+
+
+def test_out_of_order_nonce_rejected(alice, bob, funded_state):
+    """A transaction with a skipped nonce must be rejected."""
+    alice_sk, alice_pk = alice
+    _, bob_pk = bob
+
+    tx = Transaction(alice_pk, bob_pk, 10, nonce=5)
+    tx.sign(alice_sk)
+
+    assert not funded_state.apply_transaction(tx), "A transaction with a skipped nonce must be rejected."
+    # Ensure the rejected transaction did not mutate the ledger
+    assert funded_state.get_account(alice_pk)["balance"] == 100, \
+        "Alice's balance must remain unchanged after a rejected transaction."
+    assert funded_state.get_account(alice_pk)["nonce"] == 0, \
+        "Alice's nonce must remain unchanged after a rejected transaction."
+
+
+def test_sequential_nonces_accepted(alice, bob, funded_state):
+    """Two transactions with consecutive nonces must both succeed."""
+    alice_sk, alice_pk = alice
+    _, bob_pk = bob
+
+    tx0 = Transaction(alice_pk, bob_pk, 10, nonce=0)
+    tx0.sign(alice_sk)
+    assert funded_state.apply_transaction(tx0)
+
+    tx1 = Transaction(alice_pk, bob_pk, 10, nonce=1)
+    tx1.sign(alice_sk)
+    assert funded_state.apply_transaction(tx1)
+
+    assert funded_state.get_account(alice_pk)["nonce"] == 2, \
+        "Alice's nonce should advance to 2 after two accepted transactions."
+    assert funded_state.get_account(alice_pk)["balance"] == 80, \
+        "Alice's balance should be 80 after two 10-coin transfers."
+    assert funded_state.get_account(bob_pk)["balance"] == 20, \
+        "Bob's balance should be 20 after receiving two transfers."
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