core/ripple: increase the maximum amount allowed to be sent

The maximum amount is defined in [Ripple's docs](https://xrpl.org/basic-data-types.html#currencies) but it's most likely in XRP and not in drops ("satoshi" equivalent). This commit increase the value to the drops equivalent.

closes #327

core/src/apps/ripple/helpers.py

@@ -17,6 +17,9 @@ DIVISIBILITY = const(6)  # 1000000 drops equal 1 XRP
 MIN_FEE = const(10)
 # max is not defined officially but we check to make sure
 MAX_FEE = const(1000000)  # equals 1 XRP
+# https://xrpl.org/basic-data-types.html#specifying-currency-amounts
+# the value in docs is in XRP, we declare it here in drops
+MAX_ALLOWED_AMOUNT = const(100000000000000000)
 
 FLAG_FULLY_CANONICAL = 0x80000000
 

core/src/apps/ripple/serialize.py

@@ -8,8 +8,6 @@
 # the actual data follow. This currently only supports the Payment
 # transaction type and the fields that are required for it.
 
-from micropython import const
-
 from trezor.messages.RippleSignTx import RippleSignTx
 
 from . import helpers
@@ -82,11 +80,9 @@ def write_type(w: bytearray, field: dict):
 
 
 def serialize_amount(value: int) -> bytearray:
-    MAX_ALLOWED_AMOUNT = const(100000000000)
-
     if value < 0:
         raise ValueError("Only non-negative integers are supported")
-    if value > MAX_ALLOWED_AMOUNT:
+    if value > helpers.MAX_ALLOWED_AMOUNT:
         raise ValueError("Value is too large")
 
     b = bytearray(value.to_bytes(8, "big"))