tools: add deserialize_tx.py to decode contents of hex-encoded signed BTC transaction.

This code will probably come in handy if/when we implement a microwallet.

tools/deserialize_tx.py

@@ -0,0 +1,85 @@
+#!/usr/bin/env python3
+
+import binascii
+import os
+import sys
+
+try:
+    import construct as c
+except ImportError:
+    sys.stderr.write("This tool requires Construct. Install it with 'pip install Construct'.\n")
+    sys.exit(1)
+
+from construct import this, len_
+
+if os.isatty(sys.stdin.fileno()):
+    tx_hex = input("Enter transaction in hex format: ")
+else:
+    tx_hex = sys.stdin.read().strip()
+
+tx_bin = binascii.unhexlify(tx_hex)
+
+
+CompactUintStruct = c.Struct(
+    "base" / c.Int8ul,
+    "ext" / c.Switch(this.base, {0xfd: c.Int16ul, 0xfe: c.Int32ul, 0xff: c.Int64ul}),
+)
+
+
+class CompactUintAdapter(c.Adapter):
+    def _encode(self, obj, context, path):
+        if obj < 0xfd:
+            return {"base": obj}
+        if obj < 2 ** 16:
+            return {"base": 0xfd, "ext": obj}
+        if obj < 2 ** 32:
+            return {"base": 0xfe, "ext": obj}
+        if obj < 2 ** 64:
+            return {"base": 0xff, "ext": obj}
+        raise ValueError("Value too big for compact uint")
+
+    def _decode(self, obj, context, path):
+        return obj["ext"] or obj["base"]
+
+
+class ConstFlag(c.Adapter):
+    def __init__(self, const):
+        self.const = const
+        super().__init__(c.Optional(c.Const(const)))
+
+    def _encode(self, obj, context, path):
+        return self.const if obj else None
+
+    def _decode(self, obj, context, path):
+        return obj is not None
+
+
+CompactUint = CompactUintAdapter(CompactUintStruct)
+
+TxInput = c.Struct(
+    "tx" / c.Bytes(32),
+    "index" / c.Int32ul,
+    # TODO coinbase tx
+    "script" / c.Prefixed(CompactUint, c.GreedyBytes),
+    "sequence" / c.Int32ul,
+)
+
+TxOutput = c.Struct(
+    "value" / c.Int64ul,
+    "pk_script" / c.Prefixed(CompactUint, c.GreedyBytes),
+)
+
+StackItem = c.Prefixed(CompactUint, c.GreedyBytes)
+TxInputWitness = c.PrefixedArray(CompactUint, StackItem)
+
+Transaction = c.Struct(
+    "version" / c.Int32ul,
+    "segwit" / ConstFlag(b"\x00\x01"),
+    "inputs" / c.PrefixedArray(CompactUint, TxInput),
+    "outputs" / c.PrefixedArray(CompactUint, TxOutput),
+    "witness" / c.If(this.segwit, TxInputWitness[len_(this.inputs)]),
+    "lock_time" / c.Int32ul,
+    c.Terminated,
+)
+
+print(Transaction.parse(tx_bin))