Disable "at least one output" restriction for Cardano, warn instead

common/tests/fixtures/cardano/sign_tx.json

@@ -330,6 +330,37 @@
         "serialized_tx": "83a500818258203b40265111d8bb3c3c608d95b3a0bf83461ace32d79336579a1939b3aad1c0b700018182583901eb0baa5e570cffbe2934db29df0b6a3d7c0430ee65d4c3a7ab2fefb91bc428e4720702ebd5dab4fb175324c192dc9bb76cc5da956e3c8dff0102182a030a048182008200581c122a946b9ad3d2ddf029d3a828f0468aece76895f15c9efbd69b4277a100818258205d010cf16fdeff40955633d6c565f3844a288a24967cf6b76acbeb271b4f13c15840a938b16bd81aea8d3aaf11e4d460dad1f36d34bf34ad066d0f5ce5d4137654145d998c3482aa823ff1acf021c6e2cd2774fff00361cbb9e72b98632307ee4000f6"
       }
     },
+    {
+      "description": "transaction with stake registration certificate (no outputs)",
+      "parameters": {
+        "protocol_magic": 764824073,
+        "network_id": 1,
+        "fee": 42,
+        "ttl": 10,
+        "certificates": [
+          {
+            "type": 0,
+            "path": "m/1852'/1815'/0'/2/0"
+          }
+        ],
+        "withdrawals": [],
+        "metadata": "",
+        "input_flow": [["YES"], ["SWIPE", "YES"], ["SWIPE", "YES"]],
+        "inputs": [
+          {
+            "path": "m/1852'/1815'/0'/0/0",
+            "prev_hash": "3b40265111d8bb3c3c608d95b3a0bf83461ace32d79336579a1939b3aad1c0b7",
+            "prev_index": 0
+          }
+        ],
+        "outputs": [
+        ]
+      },
+      "result": {
+        "tx_hash": "03535791d04fc1b4457fada025f1c1f7778b5c2d7fa580bbac8abd53b85d3255",
+        "serialized_tx": "83a500818258203b40265111d8bb3c3c608d95b3a0bf83461ace32d79336579a1939b3aad1c0b700018002182a030a048182008200581c122a946b9ad3d2ddf029d3a828f0468aece76895f15c9efbd69b4277a100818258205d010cf16fdeff40955633d6c565f3844a288a24967cf6b76acbeb271b4f13c1584047e6e902e81bbba5596cfabaa4f9a70f36b367e28ee81181771ccd32d38b19c1d8ae9b0afb2a79057b87f8de7862e8d2317d86246909aaa66e54445d47aa990bf6"
+      }
+    },
     {
       "description": "transaction with stake registration and stake delegation certificates",
       "parameters": {

core/src/apps/cardano/layout.py

@@ -152,7 +152,13 @@ async def show_warning_tx_staking_key_hash(
 
 
 async def confirm_transaction(
-    ctx, amount: int, fee: int, protocol_magic: int, ttl: int, has_metadata: bool
+    ctx,
+    amount: int,
+    fee: int,
+    protocol_magic: int,
+    ttl: int,
+    has_metadata: bool,
+    is_network_id_verifiable: bool,
 ) -> None:
     pages = []
 
@@ -164,8 +170,9 @@ async def confirm_transaction(
     pages.append(page1)
 
     page2 = Text("Confirm transaction", ui.ICON_SEND, ui.GREEN)
-    page2.normal("Network:")
+    if is_network_id_verifiable:
-    page2.bold(protocol_magics.to_ui_string(protocol_magic))
+        page2.normal("Network:")
+        page2.bold(protocol_magics.to_ui_string(protocol_magic))
     page2.normal("Transaction TTL:")
     page2.bold(str(ttl))
     pages.append(page2)
@@ -421,6 +428,15 @@ async def show_warning_address_foreign_staking_key(
     )
 
 
+async def show_warning_tx_network_unverifiable(ctx: wire.Context) -> None:
+    page1 = Text("Warning", ui.ICON_SEND, ui.GREEN)
+    page1.normal("Transaction has no outputs, network cannot be verified.")
+    page1.br_half()
+    page1.normal("Continue?")
+
+    await require_confirm(ctx, page1)
+
+
 async def show_warning_address_pointer(
     ctx: wire.Context, pointer: CardanoBlockchainPointerType
 ) -> None:

core/src/apps/cardano/sign_tx.py

@@ -43,6 +43,7 @@ from .layout import (
     confirm_transaction_network_ttl,
     confirm_withdrawal,
     show_warning_tx_different_staking_account,
+    show_warning_tx_network_unverifiable,
     show_warning_tx_no_staking_info,
     show_warning_tx_pointer_address,
     show_warning_tx_staking_key_hash,
@@ -183,9 +184,6 @@ def _validate_outputs(
     protocol_magic: int,
     network_id: int,
 ) -> None:
-    if not outputs:
-        raise wire.ProcessError("Transaction has no outputs!")
-
     total_amount = 0
     for output in outputs:
         total_amount += output.amount
@@ -466,6 +464,10 @@ def _cborize_byron_witnesses(
 async def _show_standard_tx(
     ctx: wire.Context, keychain: seed.Keychain, msg: CardanoSignTx
 ) -> None:
+    is_network_id_verifiable = _is_network_id_verifiable(msg)
+
+    if not is_network_id_verifiable:
+        await show_warning_tx_network_unverifiable(ctx)
     total_amount = await _show_outputs(ctx, keychain, msg)
 
     for certificate in msg.certificates:
@@ -476,7 +478,13 @@ async def _show_standard_tx(
 
     has_metadata = bool(msg.metadata)
     await confirm_transaction(
-        ctx, total_amount, msg.fee, msg.protocol_magic, msg.ttl, has_metadata
+        ctx=ctx,
+        amount=total_amount,
+        fee=msg.fee,
+        protocol_magic=msg.protocol_magic,
+        ttl=msg.ttl,
+        has_metadata=has_metadata,
+        is_network_id_verifiable=is_network_id_verifiable,
     )
 
 
@@ -569,3 +577,16 @@ def _should_hide_output(output: List[int], inputs: List[CardanoTxInputType]) ->
         ):
             return False
     return True
+
+
+def _is_network_id_verifiable(msg: CardanoSignTx) -> bool:
+    """
+    checks whether there is at least one element that contains
+    information about network ID, otherwise Trezor cannot
+    guarantee that the tx is actually meant for the given network
+    """
+    return (
+        len(msg.outputs) != 0
+        or len(msg.withdrawals) != 0
+        or _has_stake_pool_registration(msg)
+    )