diff --git a/src/trezor/wire/wire_codec.py b/src/trezor/wire/wire_codec.py
index 0be68816..364778af 100644
--- a/src/trezor/wire/wire_codec.py
+++ b/src/trezor/wire/wire_codec.py
@@ -35,7 +35,6 @@ _MSG_FOOTER_LEN = ustruct.calcsize(_MSG_FOOTER)
 
 def parse_report(data):
     marker, session_id = ustruct.unpack(_REP_HEADER, data)
-    # TODO: handle v1 protocol
     return marker, session_id, data[_REP_HEADER_LEN:]
 
 
@@ -77,7 +76,7 @@ Sends (msg_type, data_len) to target, followed by data chunks.
 Throws EOFError after last data chunk, in case of valid checksum.
 Throws MessageChecksumError to target if data doesn't match the checksum.
 '''
-    message = yield  # read first report
+    message = memoryview((yield))  # read first report
     msg_type, data_len, data_tail = parse_message(message)
 
     target = genfunc(msg_type, data_len, session_id, *args)
@@ -86,11 +85,9 @@ Throws MessageChecksumError to target if data doesn't match the checksum.
     checksum = 0  # crc32
     nreports = 1
 
-    compute_checksum = hasattr(ubinascii, 'crc32')
-
     while data_len > 0:
         if nreports > 1:
-            data_tail = yield  # read next report
+            data_tail = memoryview((yield))  # read next report
         nreports += 1
 
         data_chunk = data_tail[:data_len]  # slice off the garbage at the end
@@ -98,46 +95,35 @@ Throws MessageChecksumError to target if data doesn't match the checksum.
         data_len -= len(data_chunk)
         target.send(data_chunk)
 
-        if compute_checksum:
-            checksum = ubinascii.crc32(data_chunk, checksum) & 0xffffffff
+        checksum = ubinascii.crc32(data_chunk, checksum)
 
     msg_footer = data_tail[:_MSG_FOOTER_LEN]
     if len(msg_footer) < _MSG_FOOTER_LEN:
         data_tail = yield  # read report with the rest of checksum
         msg_footer += data_tail[:_MSG_FOOTER_LEN - len(msg_footer)]
 
-    if compute_checksum:
-        data_checksum, = parse_message_footer(msg_footer)
-    else:
-        data_checksum = checksum
+    data_checksum, = parse_message_footer(msg_footer)
     if data_checksum != checksum:
-        target.throw(MessageChecksumError(
-            'Message checksum mismatch, expected %d, received %d' % (checksum, data_checksum)))
+        target.throw(MessageChecksumError((checksum, data_checksum)))
     else:
         target.throw(EOFError())
 
 
 def encode_wire_message(msg_type, msg_data, session_id, target):
-    report = bytearray(_REP_LEN)
+    report = memoryview(bytearray(_REP_LEN))
     serialize_report_header(report, REP_MARKER_HEADER, session_id)
     serialize_message_header(report, msg_type, len(msg_data))
 
-    msg_data = memoryview(msg_data)
-    report = memoryview(report)
-
-    source_data = msg_data
+    source_data = memoryview(msg_data)
     target_data = report[_REP_HEADER_LEN + _MSG_HEADER_LEN:]
 
-    compute_checksum = hasattr(ubinascii, 'crc32')
-
-    if compute_checksum:
-        checksum = ubinascii.crc32(msg_data) & 0xffffffff
-    else:
-        checksum = 0
+    checksum = ubinascii.crc32(msg_data)
 
     msg_footer = bytearray(_MSG_FOOTER_LEN)
     serialize_message_footer(msg_footer, checksum)
 
+    first = True
+
     while True:
         # move as much as possible from source to target
         n = min(len(target_data), len(source_data))
@@ -157,8 +143,10 @@ def encode_wire_message(msg_type, msg_data, session_id, target):
             break
 
         # reset to skip the magic and session ID
-        serialize_report_header(report, REP_MARKER_DATA, session_id)
-        target_data = report[_REP_HEADER_LEN:]
+        if first:
+            serialize_report_header(report, REP_MARKER_DATA, session_id)
+            target_data = report[_REP_HEADER_LEN:]
+            first = False
 
 
 def encode_session_open_message(session_id, target):