PASS1-82: Delete input PSBT file after successful signing

ports/stm32/boards/Passport/modules/auth.py

@@ -684,7 +684,7 @@ def sign_transaction(psbt_len, flags=0x0, psbt_sha=None):
 
 def sign_psbt_file(filename):
     # sign a PSBT file found on a microSD card
-    from files import CardSlot, CardMissingError
+    from files import CardSlot, CardMissingError, securely_blank_file
     from common import dis, system
     # from sram4 import tmp_buf  -- the fd.readinto() below doesn't work for some odd reason, even though the fd.readinto() for firmware updates
     tmp_buf = bytearray(1024)
@@ -798,6 +798,8 @@ def sign_psbt_file(filename):
                                     # save transaction, in hex
                                     txid = psbt.finalize(fd)
 
+                        securely_blank_file(filename)
+
                     # success and done!
                     break
 

ports/stm32/boards/Passport/modules/files.py

@@ -246,4 +246,42 @@ class CardSlot:
 
         return fname, basename+ext
 
+def securely_blank_file(full_path):
+    # input PSBT file no longer required; so delete it
+    # - blank with zeros
+    # - rename to garbage (to hide filename after undelete)
+    # - delete 
+    # - ok if file missing already (card maybe have been swapped)
+    #
+    # NOTE: we know the FAT filesystem code is simple, see 
+    #       ../external/micropython/extmod/vfs_fat.[ch]
+
+    path, basename = full_path.rsplit('/', 1)
+
+    with CardSlot() as card:
+        try:
+            blk = bytes(64)
+
+            with open(full_path, 'r+b') as fd:
+                size = fd.seek(0, 2)
+                fd.seek(0)
+
+                # blank it
+                for i in range((size // len(blk)) + 1):
+                    fd.write(blk)
+
+                assert fd.seek(0, 1) >= size
+
+            # probably pointless, but why not:
+            os.sync()
+
+        except OSError as exc:
+            # missing file is okay
+            if exc.args[0] == ENOENT: return
+            raise
+
+        # rename it and delete
+        new_name = path + '/' + ('x'*len(basename))
+        os.rename(full_path, new_name)
+        os.remove(new_name)
 # EOF