Fix: Reject multisig policies with m=0 or m>n and guard against ZeroDivisionError in fee calculation and (#845)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a52da09..6ff4765 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,7 +1,9 @@
 # Changelog 26.03.1 - March 2025
 
 ### Security Fixes
-Reject PSBT inputs with non-standard sighash types before signing
+- Reject PSBT inputs with non-standard sighash types before signing
+- Fix ZeroDivisionError in fee calculation for zero-value output PSBTs
+- Validate multisig quorum: reject m=0 and m>n in key-value wallet files
 
 # Changelog 26.03.0 - March 2025
 
diff --git a/mkdocs.yml b/mkdocs.yml
index 088fe2c..5c9eab2 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -52,7 +52,7 @@ edit_uri: edit/main/docs
 docs_dir: docs
 site_dir: public
 extra:
-  latest_krux: krux-v26.03.0
+  latest_krux: krux-v26.03.1
   latest_installer: v0.0.21
   latest_installer_rpm: krux_installer-0.0.21-1.x86_64.rpm
   latest_installer_deb: krux_installer_0.0.21_amd64.deb
diff --git a/pyproject.toml b/pyproject.toml
index 2612728..ee439f4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -22,7 +22,7 @@
 
 [tool.poetry]
 name = "krux"
-version = "26.03.0"
+version = "26.03.1"
 description = "Open-source signing device firmware for Bitcoin"
 authors = ["Jeff S <jeffreesun@protonmail.com>"]
 readme = "README.md"
diff --git a/src/krux/metadata.py b/src/krux/metadata.py
index d4ebc55..7c367e6 100644
--- a/src/krux/metadata.py
+++ b/src/krux/metadata.py
@@ -19,5 +19,5 @@
 # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 # THE SOFTWARE.
-VERSION = "26.03.0"
+VERSION = "26.03.1"
 SIGNER_PUBKEY = "03339e883157e45891e61ca9df4cd3bb895ef32d475b8e793559ea10a36766689b"
diff --git a/src/krux/psbt.py b/src/krux/psbt.py
index 7dbf6f1..430614a 100644
--- a/src/krux/psbt.py
+++ b/src/krux/psbt.py
@@ -283,10 +283,13 @@ class PSBTSigner:
         fee = inp_amount - out_amount
 
         # fee percent with 1 decimal precision using math.ceil (minimum of 0.1)
-        fee_percent = max(
-            0.1,
-            (((fee * 10000 // (out_amount)) + 9) // 10) / 10,
-        )
+        if out_amount > 0:
+            fee_percent = max(
+                0.1,
+                (((fee * 10000 // out_amount) + 9) // 10) / 10,
+            )
+        else:
+            fee_percent = 100.0
 
         resume_fee_str = (
             t("Fee:")
diff --git a/src/krux/wallet.py b/src/krux/wallet.py
index df516e2..732c323 100644
--- a/src/krux/wallet.py
+++ b/src/krux/wallet.py
@@ -250,6 +250,10 @@ class Wallet:
         elif self.descriptor.is_basic_multisig:
             m = int(str(self.descriptor.miniscript.args[0]))
             n = len(self.descriptor.keys)
+            if m < 1:
+                raise ValueError("multisig quorum m must be at least 1, got %d" % m)
+            if m > n:
+                raise ValueError("multisig quorum m (%d) exceeds n (%d)" % (m, n))
             cosigners = [key.key.to_base58() for key in self.descriptor.keys]
             if self.descriptor.is_sorted:
                 cosigners = sorted(cosigners)
@@ -363,6 +367,11 @@ def parse_key_value_file(wallet_data):
         m = int(policy[: policy.index("of")].strip())
         n = int(policy[policy.index("of") + 2 :].strip())
 
+        if m < 1:
+            raise ValueError("multisig quorum m must be at least 1, got %d" % m)
+        if m > n:
+            raise ValueError("multisig quorum m (%d) exceeds n (%d)" % (m, n))
+
         keys = []
         for i in range(len(key_vals)):
             kv = key_vals[i]
@@ -462,6 +471,8 @@ def parse_wallet(wallet_data):
         descriptor, label = parse_key_value_file(wallet_data)
         if descriptor and label:
             return descriptor, label
+    except ValueError:
+        raise
     except:
         raise ValueError("invalid wallet format")
 
diff --git a/tests/test_psbt.py b/tests/test_psbt.py
index 56738f8..0c52a14 100644
--- a/tests/test_psbt.py
+++ b/tests/test_psbt.py
@@ -2257,3 +2257,21 @@ def test_sign_sats_vB(m5stickv):
     )
 
     # TODO: Add a multisig with descriptor so change can be deteted
+
+
+def test_fee_percent_zero_out_amount(mocker, m5stickv, tdata):
+    """Test that fee calculation handles zero out_amount (OP_RETURN only PSBTs)
+    without raising ZeroDivisionError (audit finding C7)"""
+    from embit.networks import NETWORKS
+    from krux.psbt import PSBTSigner
+    from krux.key import Key, TYPE_SINGLESIG
+    from krux.wallet import Wallet
+    from krux.qr import FORMAT_NONE
+
+    wallet = Wallet(Key(tdata.TEST_MNEMONIC, TYPE_SINGLESIG, NETWORKS["test"]))
+    signer = PSBTSigner(wallet, tdata.P2WPKH_PSBT, FORMAT_NONE)
+
+    # Call _get_resume_fee with out_amount=0 (simulates OP_RETURN only PSBT)
+    resume_fee_str, fee_percent = signer._get_resume_fee(1000, 0, {})
+    assert fee_percent == 100.0
+    assert "100.0%" in resume_fee_str
diff --git a/tests/test_wallet.py b/tests/test_wallet.py
index f113d0c..19a445d 100644
--- a/tests/test_wallet.py
+++ b/tests/test_wallet.py
@@ -1875,3 +1875,110 @@ def test_parse_wallet_via_ur_account(mocker, m5stickv):
         descriptor, label = parse_wallet(wallet_data)
         assert str(descriptor) == DESCRIPTORS[i]
         print(DESCRIPTORS[i])
+
+
+def test_parse_key_value_file_rejects_m_zero(m5stickv):
+    """Audit C8: m=0 in multisig policy means zero signatures required"""
+    from krux.wallet import parse_key_value_file
+
+    wallet_data = """
+    Name: Bad Wallet
+    Policy: 0 of 3
+    Derivation: m/45'
+    Format: P2SH
+
+    d3a80c8b:
+    xpub6ApMSMmLpzeYTv72drQnbXkzg1nedyDQVyvtZVUWCS3B1pSSp24Y1tDtzcnD1ovCBhb3QuhYtN5rE8ayoZ3c2fNBYYmPN9VQVSNTKKFe8SH
+
+    55f8fc5d:
+    xpub6Ac49WroT3nhb4uicbE5EUD7WiH2Xooubauvqw5fJYTLbmWFHnGjXRXwkPnFcTgK47KzzKTJNcjua2PisceZfwdoCUXYmX5Ju2v4RU2C7ps
+
+    3e15470d:
+    xpub6AFDaW88dK7HvhWpHPGwGRym7h2pk8BZYjNUQ5GSkqeoGcMXC9cai7zBCwVeRFrD4wGeWkS3wSqU2jh9nEs2f5SHiM8Fa96ffce7maiKVtm
+    """
+
+    with pytest.raises(ValueError, match="m must be at least 1"):
+        parse_key_value_file(wallet_data)
+
+
+def test_parse_key_value_file_rejects_m_greater_than_n(m5stickv):
+    """Audit C8: m > n means more signatures required than keys available"""
+    from krux.wallet import parse_key_value_file
+
+    wallet_data = """
+    Name: Bad Wallet
+    Policy: 4 of 3
+    Derivation: m/45'
+    Format: P2SH
+
+    d3a80c8b:
+    xpub6ApMSMmLpzeYTv72drQnbXkzg1nedyDQVyvtZVUWCS3B1pSSp24Y1tDtzcnD1ovCBhb3QuhYtN5rE8ayoZ3c2fNBYYmPN9VQVSNTKKFe8SH
+
+    55f8fc5d:
+    xpub6Ac49WroT3nhb4uicbE5EUD7WiH2Xooubauvqw5fJYTLbmWFHnGjXRXwkPnFcTgK47KzzKTJNcjua2PisceZfwdoCUXYmX5Ju2v4RU2C7ps
+
+    3e15470d:
+    xpub6AFDaW88dK7HvhWpHPGwGRym7h2pk8BZYjNUQ5GSkqeoGcMXC9cai7zBCwVeRFrD4wGeWkS3wSqU2jh9nEs2f5SHiM8Fa96ffce7maiKVtm
+    """
+
+    with pytest.raises(ValueError, match="m .* exceeds n"):
+        parse_key_value_file(wallet_data)
+
+
+def test_descriptor_rejects_m_zero(m5stickv):
+    """Audit C8: embit already rejects sortedmulti(0,...) at descriptor level"""
+    from embit.descriptor import Descriptor
+
+    with pytest.raises(Exception, match="multi.*0"):
+        Descriptor.from_string(
+            "wsh(sortedmulti(0,"
+            "[55f8fc5d/48h/0h/0h/2h]xpub6EKmKYGYc1WY6t9d3d9SksR8keSaPZbFa6tqsGiH4xVxx8d2YyxSX7WG6yXEX3CmG54dPCxaapDw1XsjwCmfoqP7tbsAeqMVfKvqSAu4ndy/0/*,"
+            "[3e15470d/48h/0h/0h/2h]xpub6F2P6Pz5KLPgCc6pTBd2xxCunaSYWc8CdkL28W5z15pJrN3aCYY7mCUAkCMtqrgT2wdhAGgRnJxAkCCUpGKoXKxQ57yffEGmPwtYA3DEXwu/0/*,"
+            "[d3a80c8b/48h/0h/0h/2h]xpub6FKYY6y3oVi7ihSCszFKRSeZj5SzrfSsUFXhKqjMV4iigrLhxwMX3mrjioNyLTZ5iD3u4wU9S3tyzpJGxhd5geaXoQ68jGz2M6dfh2zJrUv/0/*))"
+        )
+
+
+def test_wallet_load_rejects_descriptor_m_zero(mocker, m5stickv):
+    """Audit C8: defense-in-depth in Wallet.load rejects m=0 from descriptor"""
+    from unittest.mock import MagicMock
+    from krux.wallet import Wallet
+
+    mock_descriptor = MagicMock()
+    mock_descriptor.key = None
+    mock_descriptor.is_basic_multisig = True
+    mock_descriptor.miniscript.args.__getitem__ = lambda self, i: 0  # m=0
+    mock_descriptor.keys = [MagicMock(), MagicMock(), MagicMock()]  # n=3
+
+    mocker.patch("krux.wallet.parse_wallet", return_value=(mock_descriptor, "Test"))
+    mocker.patch("krux.wallet.to_unambiguous_descriptor", return_value=mock_descriptor)
+    mocker.patch(
+        "krux.wallet.version_to_network_versiontype",
+        return_value=("main", "xpub"),
+    )
+
+    wallet = Wallet(None)
+    with pytest.raises(ValueError, match="m must be at least 1"):
+        wallet.load("fake_data", 0)
+
+
+def test_wallet_load_rejects_descriptor_m_greater_than_n(mocker, m5stickv):
+    """Audit C8: defense-in-depth in Wallet.load rejects m>n from descriptor"""
+    from unittest.mock import MagicMock
+    from krux.wallet import Wallet
+
+    mock_descriptor = MagicMock()
+    mock_descriptor.key = None
+    mock_descriptor.is_basic_multisig = True
+    mock_descriptor.miniscript.args.__getitem__ = lambda self, i: 4  # m=4
+    mock_descriptor.keys = [MagicMock(), MagicMock(), MagicMock()]  # n=3
+
+    mocker.patch("krux.wallet.parse_wallet", return_value=(mock_descriptor, "Test"))
+    mocker.patch("krux.wallet.to_unambiguous_descriptor", return_value=mock_descriptor)
+    mocker.patch(
+        "krux.wallet.version_to_network_versiontype",
+        return_value=("main", "xpub"),
+    )
+
+    wallet = Wallet(None)
+    with pytest.raises(ValueError, match="m .* exceeds n"):
+        wallet.load("fake_data", 0)