[test] Add functional test to test early key response behaviour in BIP 324

- A node initiates a v2 connection by sending 64 bytes ellswift
- In BIP 324 "The responder waits until one byte is received which does not match the
  V1_PREFIX (16 bytes consisting of the network magic followed by "version\x00\x00\x00\x00\x00".)"
- It's possible that the 64 bytes ellswift sent by an initiator starts with a prefix of V1_PREFIX
- Example form of 64 bytes ellswift could be:
	4 bytes network magic + 60 bytes which aren't prefixed with remaining V1_PREFIX
- We test this behaviour:
	- when responder receives 4 byte network magic -> no response received by initiator
	- when first mismatch happens -> response received by initiator

2 files changed, 86 insertions, 0 deletions

diff --git a/test/functional/p2p_v2_earlykeyresponse.py b/test/functional/p2p_v2_earlykeyresponse.py
new file mode 100755
index 0000000000..5f6c978661
--- /dev/null
+++ b/test/functional/p2p_v2_earlykeyresponse.py
@@ -0,0 +1,85 @@
+#!/usr/bin/env python3
+# Copyright (c) 2022 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+import random
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.crypto.ellswift import ellswift_create
+from test_framework.p2p import P2PInterface
+from test_framework.v2_p2p import EncryptedP2PState
+
+
+class TestEncryptedP2PState(EncryptedP2PState):
+    """ Modify v2 P2P protocol functions for testing that "The responder waits until one byte is received which does
+    not match the 16 bytes consisting of the network magic followed by "version\x00\x00\x00\x00\x00"." (see BIP 324)
+
+    - if `send_net_magic` is True, send first 4 bytes of ellswift (match network magic) else send remaining 60 bytes
+    - `can_data_be_received` is a variable used to assert if data is received on recvbuf.
+            - v2 TestNode shouldn't respond back if we send V1_PREFIX and data shouldn't be received on recvbuf.
+              This state is represented using `can_data_be_received` = False.
+            - v2 TestNode responds back when mismatch from V1_PREFIX happens and data can be received on recvbuf.
+              This state is represented using `can_data_be_received` = True.
+    """
+
+    def __init__(self):
+        super().__init__(initiating=True, net='regtest')
+        self.send_net_magic = True
+        self.can_data_be_received = False
+
+    def initiate_v2_handshake(self, garbage_len=random.randrange(4096)):
+        """Initiator begins the v2 handshake by sending its ellswift bytes and garbage.
+        Here, the 64 bytes ellswift is assumed to have it's 4 bytes match network magic bytes. It is sent in 2 phases:
+            1. when `send_network_magic` = True, send first 4 bytes of ellswift (matches network magic bytes)
+            2. when `send_network_magic` = False, send remaining 60 bytes of ellswift
+        """
+        if self.send_net_magic:
+            self.privkey_ours, self.ellswift_ours = ellswift_create()
+            self.sent_garbage = random.randbytes(garbage_len)
+            self.send_net_magic = False
+            return b"\xfa\xbf\xb5\xda"
+        else:
+            self.can_data_be_received = True
+            return self.ellswift_ours[4:] + self.sent_garbage
+
+
+class PeerEarlyKey(P2PInterface):
+    """Custom implementation of P2PInterface which uses modified v2 P2P protocol functions for testing purposes."""
+    def __init__(self):
+        super().__init__()
+        self.v2_state = None
+
+    def connection_made(self, transport):
+        """64 bytes ellswift is sent in 2 parts during `initial_v2_handshake()`"""
+        self.v2_state = TestEncryptedP2PState()
+        super().connection_made(transport)
+
+    def data_received(self, t):
+        # check that data can be received on recvbuf only when mismatch from V1_PREFIX happens (send_net_magic = False)
+        assert self.v2_state.can_data_be_received and not self.v2_state.send_net_magic
+
+
+class P2PEarlyKey(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 1
+        self.extra_args = [["-v2transport=1", "-peertimeout=3"]]
+
+    def run_test(self):
+        self.log.info('Sending ellswift bytes in parts to ensure that response from responder is received only when')
+        self.log.info('ellswift bytes have a mismatch from the 16 bytes(network magic followed by "version\\x00\\x00\\x00\\x00\\x00")')
+        node0 = self.nodes[0]
+        self.log.info('Sending first 4 bytes of ellswift which match network magic')
+        self.log.info('If a response is received, assertion failure would happen in our custom data_received() function')
+        # send happens in `initiate_v2_handshake()` in `connection_made()`
+        peer1 = node0.add_p2p_connection(PeerEarlyKey(), wait_for_verack=False, send_version=False, supports_v2_p2p=True)
+        self.log.info('Sending remaining ellswift and garbage which are different from V1_PREFIX. Since a response is')
+        self.log.info('expected now, our custom data_received() function wouldn\'t result in assertion failure')
+        ellswift_and_garbage_data = peer1.v2_state.initiate_v2_handshake()
+        peer1.send_raw_message(ellswift_and_garbage_data)
+        peer1.wait_for_disconnect(timeout=5)
+        self.log.info('successful disconnection when MITM happens in the key exchange phase')
+
+
+if __name__ == '__main__':
+    P2PEarlyKey().main()
diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
index 8c2cdfb70b..b88f96172a 100755
--- a/test/functional/test_runner.py
+++ b/test/functional/test_runner.py
@@ -260,6 +260,7 @@ BASE_SCRIPTS = [
     'p2p_invalid_tx.py --v2transport',
     'p2p_v2_transport.py',
     'p2p_v2_encrypted.py',
+    'p2p_v2_earlykeyresponse.py',
     'example_test.py',
     'wallet_txn_doublespend.py --legacy-wallet',
     'wallet_multisig_descriptor_psbt.py --descriptors',