mnemonic: tighten seed_type check for old "2fa" type seeds

Seeds in the set difference could already not be restored: they raised
an exception in the wizard; now these are not recognised as valid seeds
anymore (so e.g. OK button in wizard will be disabled).
Also see comments in code.

electrum/mnemonic.py

@@ -241,13 +241,17 @@ def is_old_seed(seed: str) -> bool:
 
 
 def seed_type(x: str) -> str:
+    num_words = len(x.split())
     if is_old_seed(x):
         return 'old'
-    elif is_new_seed(x):
+    elif is_new_seed(x, version.SEED_PREFIX):
         return 'standard'
     elif is_new_seed(x, version.SEED_PREFIX_SW):
         return 'segwit'
-    elif is_new_seed(x, version.SEED_PREFIX_2FA):
+    elif is_new_seed(x, version.SEED_PREFIX_2FA) and (num_words == 12 or num_words >= 20):
+        # Note: in Electrum 2.7, there was a breaking change in key derivation
+        #       for this seed type. Unfortunately the seed version/prefix was reused,
+        #       and now we can only distinguish them based on number of words. :(
         return '2fa'
     elif is_new_seed(x, version.SEED_PREFIX_2FA_SW):
         return '2fa_segwit'

electrum/plugins/trustedcoin/trustedcoin.py

@@ -575,8 +575,8 @@ class TrustedCoinPlugin(BasePlugin):
             raise Exception(f'unexpected seed type: {t}')
         words = seed.split()
         n = len(words)
-        # old version use long seed phrases
-        if n >= 20:
+        if t == '2fa':
+            if n >= 20:  # old scheme
                 # note: pre-2.7 2fa seeds were typically 24-25 words, however they
                 # could probabilistically be arbitrarily shorter due to a bug. (see #3611)
                 # the probability of it being < 20 words is about 2^(-(256+12-19*11)) = 2^(-59)
@@ -584,11 +584,16 @@ class TrustedCoinPlugin(BasePlugin):
                     raise Exception('old 2fa seed cannot have passphrase')
                 xprv1, xpub1 = self.get_xkeys(' '.join(words[0:12]), t, '', "m/")
                 xprv2, xpub2 = self.get_xkeys(' '.join(words[12:]), t, '', "m/")
-        elif not t == '2fa' or n == 12:
+            elif n == 12:  # new scheme
                 xprv1, xpub1 = self.get_xkeys(seed, t, passphrase, "m/0'/")
                 xprv2, xpub2 = self.get_xkeys(seed, t, passphrase, "m/1'/")
             else:
-            raise Exception('unrecognized seed length: {} words'.format(n))
+                raise Exception(f'unrecognized seed length for "2fa" seed: {n}')
+        elif t == '2fa_segwit':
+            xprv1, xpub1 = self.get_xkeys(seed, t, passphrase, "m/0'/")
+            xprv2, xpub2 = self.get_xkeys(seed, t, passphrase, "m/1'/")
+        else:
+            raise Exception(f'unexpected seed type: {t}')
         return xprv1, xpub1, xprv2, xpub2
 
     def create_keystore(self, wizard, seed, passphrase):