From 8481bf42da71c9ba0a96d6cc3ca46b6d9eeb3000 Mon Sep 17 00:00:00 2001
From: Peter Todd <pete@petertodd.org>
Date: Tue, 21 Nov 2023 01:33:08 -0500
Subject: [PATCH] Add (insecure!) Bitcoin headers over DNS support

Disabled by default.

Use this with Matt Corallo's https://bitcoinheaders.net/ service.
---
 otsclient/args.py        | 10 ++++-----
 otsclient/cmds.py        | 17 +++++++++------
 otsclient/dns_headers.py | 46 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 62 insertions(+), 11 deletions(-)
 create mode 100644 otsclient/dns_headers.py

diff --git a/otsclient/args.py b/otsclient/args.py
index e84b07e..f42c723 100644
--- a/otsclient/args.py
+++ b/otsclient/args.py
@@ -78,6 +78,10 @@ def make_common_options_arg_parser():
                         help="Bitcoin node URL to connect to (defaults to local "
                              "configuration)")
 
+    parser.add_argument('--bitcoin-dns', metavar='DOMAIN', action='store', type=str,
+                         default=None,
+                         help='Fetch Bitcoin headers over DNS if local node is unavailable')
+
     return parser
 
 def handle_common_options(args, parser):
@@ -142,11 +146,7 @@ def setup_bitcoin():
         else:
             assert False
 
-        try:
-            return bitcoin.rpc.Proxy(service_url=args.bitcoin_node)
-        except Exception as exp:
-            logging.error("Could not connect to Bitcoin node: %s" % exp)
-            sys.exit(1)
+        return bitcoin.rpc.Proxy(service_url=args.bitcoin_node)
 
     args.setup_bitcoin = setup_bitcoin
 
diff --git a/otsclient/cmds.py b/otsclient/cmds.py
index d18c185..9c1a9ed 100644
--- a/otsclient/cmds.py
+++ b/otsclient/cmds.py
@@ -38,6 +38,7 @@
 import opentimestamps.calendar
 
 import otsclient
+import otsclient.dns_headers
 
 def remote_calendar(calendar_uri):
     """Create a remote calendar with User-Agent set appropriately"""
@@ -406,19 +407,23 @@ def attestation_key(item):
                                 (attestation.height, b2lx(msg)))
                 continue
 
-            proxy = args.setup_bitcoin()
 
             try:
+                proxy = args.setup_bitcoin()
                 block_count = proxy.getblockcount()
                 blockhash = proxy.getblockhash(attestation.height)
+                block_header = proxy.getblockheader(blockhash)
             except IndexError:
                 logging.error("Bitcoin block height %d not found; %d is highest known block" % (attestation.height, block_count))
                 continue
-            except ConnectionError as exp:
-                logging.error("Could not connect to local Bitcoin node: %s" % exp)
-                continue
-
-            block_header = proxy.getblockheader(blockhash)
+            except Exception as exp:
+                if args.bitcoin_dns is None:
+                    logging.error("Could not connect to local Bitcoin node: %s" % exp)
+                    continue
+                else:
+                    logging.warning("WARNING: Falling back to insecure Bitcoin headers over DNS")
+                    block_header = otsclient.dns_headers.get_header_from_dns(args.bitcoin_dns, attestation.height)
+                    blockhash = block_header.GetHash()
 
             logging.debug("Attestation block hash: %s" % b2lx(blockhash))
 
diff --git a/otsclient/dns_headers.py b/otsclient/dns_headers.py
new file mode 100644
index 0000000..e12f8f4
--- /dev/null
+++ b/otsclient/dns_headers.py
@@ -0,0 +1,46 @@
+# Copyright (C) The OpenTimestamps developers
+#
+# This file is part of the OpenTimestamps Client.
+#
+# It is subject to the license terms in the LICENSE file found in the top-level
+# directory of this distribution.
+#
+# No part of the OpenTimestamps Client, including this file, may be copied,
+# modified, propagated, or distributed except according to the terms contained
+# in the LICENSE file.
+
+import logging
+import socket
+
+from ipaddress import IPv6Address
+
+from bitcoin.core import b2x, x, CBlockHeader
+
+def get_header_from_dns(domain, n):
+    domain = '%d.%d.%s' % (n, n / 10000, domain)
+
+    logging.debug("Getting block header %d from %s" % (n, domain))
+
+    nibble_chunks = []
+    for (_family, _type, _port, _name, (addr, _, _, _)) in \
+                socket.getaddrinfo(domain, None, family=socket.AF_INET6, type=socket.SocketKind.SOCK_DGRAM):
+
+        addr = IPv6Address(addr)
+
+        addr_bytes = addr.packed
+
+        if addr_bytes[0:2] != b'\x20\x01':
+            continue
+
+        idx = addr_bytes[2] >> 4
+
+        nibble_chunks.append((idx, b2x(addr_bytes[2:])[1:]))
+
+    header_nibbles = ''
+    for (_n, chunk) in sorted(nibble_chunks):
+        header_nibbles += chunk
+
+    header_bytes = x(header_nibbles)
+    assert header_bytes[0] == 0
+
+    return CBlockHeader.deserialize(header_bytes[1:])