apps.wallet: refactor message digest method into shared module

naming would need some further thoughts, though

src/apps/common/signverify.py

@@ -0,0 +1,13 @@
+from trezor.crypto.hashlib import sha256
+
+from .signtx import HashWriter, write_varint
+
+def message_digest(coin, message):
+
+    h = HashWriter(sha256)
+    write_varint(h, len(coin.signed_message_header))
+    h.extend(coin.signed_message_header)
+    write_varint(h, len(message))
+    h.extend(message)
+
+    return sha256(h.getvalue()).digest()

src/apps/wallet/layout_sign_message.py

@@ -5,11 +5,11 @@ from trezor.utils import unimport
 @unimport
 async def layout_sign_message(msg, session_id):
     from trezor.messages.MessageSignature import MessageSignature
-    from trezor.crypto.hashlib import sha256
     from trezor.crypto.curve import secp256k1
-    from ..common.signtx import node_derive, HashWriter, write_varint
+    from ..common.signtx import node_derive
     from ..common.seed import get_root_node
     from ..common import coins
+    from ..common.signverify import message_digest
 
     ui.display.clear()
     ui.display.text(10, 30, 'Signing message',
@@ -26,13 +26,7 @@ async def layout_sign_message(msg, session_id):
     seckey = node.private_key()
     address = node.address(coin.address_type)
 
-    h = HashWriter(sha256)
-    write_varint(h, len(coin.signed_message_header))
-    h.extend(coin.signed_message_header)
-    write_varint(h, len(message))
-    h.extend(message)
-
-    digest = sha256(h.getvalue()).digest()
+    digest = message_digest(coin, message)
 
     signature = secp256k1.sign(seckey, digest)
 

src/apps/wallet/layout_verify_message.py

@@ -9,7 +9,7 @@ async def layout_verify_message(msg, session_id):
     from trezor.crypto import base58
     from ..common import address_type
     from ..common import coins
-    from ..common.signtx import node_derive, HashWriter, write_varint
+    from ..common.signverify import message_digest
 
     address = msg.address
     message = msg.message
@@ -23,13 +23,7 @@ async def layout_verify_message(msg, session_id):
     ui.display.text(10, 60, message, ui.MONO, ui.WHITE, ui.BLACK)
     ui.display.text(10, 80, address, ui.MONO, ui.WHITE, ui.BLACK)
 
-    h = HashWriter(sha256)
-    write_varint(h, len(coin.signed_message_header))
-    h.extend(coin.signed_message_header)
-    write_varint(h, len(message))
-    h.extend(message)
-
-    digest = sha256(h.getvalue()).digest()
+    digest = message_digest(coin, message)
     pubkey = secp256k1.verify_recover(signature, digest)
 
     if not pubkey: