core/slip39: Disallow creating multiple member shares with member threshold 1. Add tests for invalid parameters to generate_mnemonics().

core/src/trezor/crypto/slip39.py

@@ -472,6 +472,14 @@ def generate_mnemonics(
             )
         )
 
+    if any(
+        member_threshold == 1 and member_count > 1
+        for member_threshold, member_count in groups
+    ):
+        raise ValueError(
+            "Creating multiple member shares with member threshold 1 is not allowed. Use 1-of-1 member sharing instead."
+        )
+
     encrypted_master_secret = _encrypt(
         master_secret, passphrase, iteration_exponent, identifier
     )

core/tests/test_trezor.crypto.slip39.py

@@ -64,6 +64,24 @@ class TestCryptoSlip39(unittest.TestCase):
             slip39.combine_mnemonics(mnemonics[0][1:4])
 
 
+    def test_invalid_sharing(self):
+        # Short master secret.
+        with self.assertRaises(ValueError):
+            slip39.generate_mnemonics(1, [(2, 3)], self.MS[:14])
+
+        # Odd length master secret.
+        with self.assertRaises(ValueError):
+            slip39.generate_mnemonics(1, [(2, 3)], self.MS + b"X")
+
+        # Group threshold exceeds number of groups.
+        with self.assertRaises(ValueError):
+            slip39.generate_mnemonics(3, [(3, 5), (2, 5)], self.MS)
+
+        # Group with multiple members and threshold 1.
+        with self.assertRaises(ValueError):
+            slip39.generate_mnemonics(2, [(3, 5), (1, 3), (2, 5)], self.MS)
+
+
     def test_vectors(self):
         for mnemonics, secret in vectors:
             if secret: