core/bitcoin: Support multiple change-outputs.

core/CHANGELOG.md

@@ -14,6 +14,7 @@ _Most likely to be released on July 1st._
 - Dedicated `initialized` field in storage.
 - Support EXTERNAL transaction inputs with a SLIP-0019 proof of ownership.  [#1052]
 - Support pre-signed EXTERNAL transaction inputs.
+- Support multiple change-outputs.  [#1098]
 
 ### Changed
 - `Features.pin_cached` renamed to `unlocked`.
@@ -168,3 +169,4 @@ Version 2.0.5 [Mar 2018]
 [#1056]: https://github.com/trezor/trezor-firmware/issues/1056
 [#1067]: https://github.com/trezor/trezor-firmware/issues/1067
 [#1074]: https://github.com/trezor/trezor-firmware/issues/1074
+[#1098]: https://github.com/trezor/trezor-firmware/issues/1098

core/src/apps/bitcoin/sign_tx/__init__.py

@@ -67,6 +67,13 @@ async def sign_tx(
             res = await layout.confirm_feeoverthreshold(ctx, req.fee, req.coin)
             utils.unimport_end(mods)
             progress.report_init()
+        elif isinstance(req, helpers.UiConfirmChangeCountOverThreshold):
+            mods = utils.unimport_begin()
+            res = await layout.confirm_change_count_over_threshold(
+                ctx, req.change_count
+            )
+            utils.unimport_end(mods)
+            progress.report_init()
         elif isinstance(req, helpers.UiConfirmNonDefaultLocktime):
             mods = utils.unimport_begin()
             res = await layout.confirm_nondefault_locktime(ctx, req.lock_time)

core/src/apps/bitcoin/sign_tx/bitcoin.py

@@ -38,6 +38,9 @@ _BIP32_MAX_LAST_ELEMENT = const(1000000)
 # the number of bytes to preallocate for serialized transaction chunks
 _MAX_SERIALIZED_CHUNK_SIZE = const(2048)
 
+# the maximum number of change-outputs allowed without user confirmation
+_MAX_SILENT_CHANGE_COUNT = const(2)
+
 
 class Bitcoin:
     async def signer(self) -> None:
@@ -92,6 +95,7 @@ class Bitcoin:
         self.external_in = 0  # sum of external input amounts
         self.total_out = 0  # sum of output amounts
         self.change_out = 0  # change output amount
+        self.change_count = 0  # the number of change-outputs
         self.weight = tx_weight.TxWeightCalculator(tx.inputs_count, tx.outputs_count)
 
         # transaction and signature serialization
@@ -154,6 +158,8 @@ class Bitcoin:
         # fee > (coin.maxfee per byte * tx size)
         if fee > (self.coin.maxfee_kb / 1000) * (self.weight.get_total() / 4):
             await helpers.confirm_feeoverthreshold(fee, self.coin)
+        if self.change_count > _MAX_SILENT_CHANGE_COUNT:
+            await helpers.confirm_change_count_over_threshold(self.change_count)
         if self.tx.lock_time > 0:
             await helpers.confirm_nondefault_locktime(self.tx.lock_time)
         if not self.external:
@@ -268,9 +274,10 @@ class Bitcoin:
         self.external_in += txi.amount
 
     async def confirm_output(self, txo: TxOutputType, script_pubkey: bytes) -> None:
-        if self.change_out == 0 and self.output_is_change(txo):
+        if self.output_is_change(txo):
             # output is change and does not need confirmation
-            self.change_out = txo.amount
+            self.change_out += txo.amount
+            self.change_count += 1
         else:
             await helpers.confirm_output(txo, self.coin)
 

core/src/apps/bitcoin/sign_tx/helpers.py

@@ -63,6 +63,13 @@ class UiConfirmFeeOverThreshold:
     __eq__ = utils.obj_eq
 
 
+class UiConfirmChangeCountOverThreshold:
+    def __init__(self, change_count: int):
+        self.change_count = change_count
+
+    __eq__ = utils.obj_eq
+
+
 class UiConfirmForeignAddress:
     def __init__(self, address_n: list):
         self.address_n = address_n
@@ -93,6 +100,10 @@ def confirm_feeoverthreshold(fee: int, coin: CoinInfo) -> Awaitable[Any]:  # typ
     return (yield UiConfirmFeeOverThreshold(fee, coin))
 
 
+def confirm_change_count_over_threshold(change_count: int) -> Awaitable[Any]:  # type: ignore
+    return (yield UiConfirmChangeCountOverThreshold(change_count))
+
+
 def confirm_foreign_address(address_n: list) -> Awaitable[Any]:  # type: ignore
     return (yield UiConfirmForeignAddress(address_n))
 

core/src/apps/bitcoin/sign_tx/layout.py

@@ -99,6 +99,20 @@ async def confirm_feeoverthreshold(
     await require_confirm(ctx, text, ButtonRequestType.FeeOverThreshold)
 
 
+async def confirm_change_count_over_threshold(
+    ctx: wire.Context, change_count: int
+) -> None:
+    from trezor.ui.text import Text
+    from apps.common.confirm import require_confirm
+
+    text = Text("Warning", ui.ICON_SEND, ui.GREEN)
+    text.normal("There are {}".format(change_count))
+    text.normal("change-outputs.")
+    text.br_half()
+    text.normal("Continue?")
+    await require_confirm(ctx, text, ButtonRequestType.SignTx)
+
+
 async def confirm_nondefault_locktime(ctx: wire.Context, lock_time: int) -> None:
     from trezor.ui.text import Text
     from apps.common.confirm import require_confirm