partial impl. of dipunm's feedback in issue SeedSigner#43

* Not even offered to SeedXOR against itself.
* Cannot SeedXOR against a seed's inversion.
* Cannot SeedXOR against a seed w/ passphrase.

src/seedsigner/models/seed.py

@@ -134,10 +134,14 @@ def reverse_bytes(self):
         self._generate_seed()
 
     def seed_xor(self, other):
-        if len(self.mnemonic_list) != len(other.mnemonic_list):
+        self_entropy = bip39.mnemonic_to_bytes(self.mnemonic_str)
+        other_entropy = bip39.mnemonic_to_bytes(other.mnemonic_str)
+        if len(self_entropy) != len(other_entropy):
             raise Exception("XOR requires seeds of similar mnemonic length!")
-        if self.mnemonic_list == other.mnemonic_list:
+        if self_entropy == other_entropy:
             raise Exception("You may not XOR a seed with itself.")
+        if self_entropy == inv_bits(other_entropy):
+            raise Exception("You may not XOR a seed with its inversion.")
         new_entropy = xor_bytes(self.entropy, other.entropy)
         self._mnemonic = bip39.mnemonic_from_bytes(new_entropy).split()
         self._generate_seed()

src/seedsigner/views/seed_views.py

@@ -311,7 +311,11 @@ def run(self):
         REVERSE_BYTES = "Reverse bytes"
         SEED_XOR = "Seed XOR"
         button_data = [INVERT_BITS, REVERSE_BITS, REVERSE_BYTES]
-        if len([x for x in self.controller.storage.seeds if len(x.mnemonic_list) == len(self.seed.mnemonic_list)]):
+        # todo: in 3 places, make it just 1
+        if len([x for x in self.controller.storage.seeds
+                if len(x.mnemonic_list) == len(self.seed.mnemonic_list) \
+                and x.mnemonic_list != self.seed.mnemonic_list \
+                and len(x.passphrase) == 0]):
             button_data.append(SEED_XOR)
 
         ret = ButtonListScreen(
@@ -346,8 +350,12 @@ def run(self):
         text = "Select seed to XOR with"
         button_data = []
         for seed in seeds:
-            button_str = seed.get_fingerprint(self.settings.get_value(SettingsConstants.SETTING__NETWORK))
-            button_data.append((button_str, SeedSignerCustomIconConstants.FINGERPRINT, "blue"))
+            # todo: in 3 places, make it just 1
+            if len(seed.mnemonic_list) == len(self.seed.mnemonic_list) \
+            and seed.mnemonic_list != self.seed.mnemonic_list \
+            and len(seed.passphrase) == 0:
+                button_str = seed.get_fingerprint(self.settings.get_value(SettingsConstants.SETTING__NETWORK))
+                button_data.append((button_str, SeedSignerCustomIconConstants.FINGERPRINT, "blue"))
 
         selected_menu_num = seed_screens.SeedSelectSeedScreen(
             title=title,
@@ -368,7 +376,11 @@ class SeedXORApplyView(View):
     def __init__(self, seed_num: int = None):
         super().__init__()
         self.seed = self.controller.storage.get_pending_seed()
-        seeds = [x for x in self.controller.storage.seeds if len(x.mnemonic_list) == len(self.seed.mnemonic_list)]
+        # todo: in 3 places, make it just 1
+        seeds = [x for x in self.controller.storage.seeds
+                 if len(x.mnemonic_list) == len(self.seed.mnemonic_list) \
+                 and x.mnemonic_list != self.seed.mnemonic_list \
+                 and len(x.passphrase) == 0]
         if seed_num is not None:
             self.other = seeds[seed_num]