Inside Bitcoin

1#!/usr/bin/env python3
2# Copyright (c) 2017 Pieter Wuille
3# Distributed under the MIT software license, see the accompanying
4# file COPYING or http://www.opensource.org/licenses/mit-license.php.
5"""Reference implementation for Bech32/Bech32m and segwit addresses."""
6import unittest
7from enum import Enum
8
9CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l"
10BECH32_CONST = 1
11BECH32M_CONST = 0x2bc830a3
12
13class Encoding(Enum):
14    """Enumeration type to list the various supported encodings."""
15    BECH32 = 1
16    BECH32M = 2
17
18
19def bech32_polymod(values):
20    """Internal function that computes the Bech32 checksum."""
21    generator = [0x3b6a57b2, 0x26508e6d, 0x1ea119fa, 0x3d4233dd, 0x2a1462b3]
22    chk = 1
23    for value in values:
24        top = chk >> 25
25        chk = (chk & 0x1ffffff) << 5 ^ value
26        for i in range(5):
27            chk ^= generator[i] if ((top >> i) & 1) else 0
28    return chk
29
30
31def bech32_hrp_expand(hrp):
32    """Expand the HRP into values for checksum computation."""
33    return [ord(x) >> 5 for x in hrp] + [0] + [ord(x) & 31 for x in hrp]
34
35
36def bech32_verify_checksum(hrp, data):
37    """Verify a checksum given HRP and converted data characters."""
38    check = bech32_polymod(bech32_hrp_expand(hrp) + data)
39    if check == BECH32_CONST:
40        return Encoding.BECH32
41    elif check == BECH32M_CONST:
42        return Encoding.BECH32M
43    else:
44        return None
45
46def bech32_create_checksum(encoding, hrp, data):
47    """Compute the checksum values given HRP and data."""
48    values = bech32_hrp_expand(hrp) + data
49    const = BECH32M_CONST if encoding == Encoding.BECH32M else BECH32_CONST
50    polymod = bech32_polymod(values + [0, 0, 0, 0, 0, 0]) ^ const
51    return [(polymod >> 5 * (5 - i)) & 31 for i in range(6)]
52
53
54def bech32_encode(encoding, hrp, data):
55    """Compute a Bech32 or Bech32m string given HRP and data values."""
56    combined = data + bech32_create_checksum(encoding, hrp, data)
57    return hrp + '1' + ''.join([CHARSET[d] for d in combined])
58
59
60def bech32_decode(bech):
61    """Validate a Bech32/Bech32m string, and determine HRP and data."""
62    if ((any(ord(x) < 33 or ord(x) > 126 for x in bech)) or
63            (bech.lower() != bech and bech.upper() != bech)):
64        return (None, None, None)
65    bech = bech.lower()
66    pos = bech.rfind('1')
67    if pos < 1 or pos + 7 > len(bech) or len(bech) > 90:
68        return (None, None, None)
69    if not all(x in CHARSET for x in bech[pos+1:]):
70        return (None, None, None)
71    hrp = bech[:pos]
72    data = [CHARSET.find(x) for x in bech[pos+1:]]
73    encoding = bech32_verify_checksum(hrp, data)
74    if encoding is None:
75        return (None, None, None)
76    return (encoding, hrp, data[:-6])
77
78
79def convertbits(data, frombits, tobits, pad=True):
80    """General power-of-2 base conversion."""
81    acc = 0
82    bits = 0
83    ret = []
84    maxv = (1 << tobits) - 1
85    max_acc = (1 << (frombits + tobits - 1)) - 1
86    for value in data:
87        if value < 0 or (value >> frombits):
88            return None
89        acc = ((acc << frombits) | value) & max_acc
90        bits += frombits
91        while bits >= tobits:
92            bits -= tobits
93            ret.append((acc >> bits) & maxv)
94    if pad:
95        if bits:
96            ret.append((acc << (tobits - bits)) & maxv)
97    elif bits >= frombits or ((acc << (tobits - bits)) & maxv):
98        return None
99    return ret
100
101
102def decode_segwit_address(hrp, addr):
103    """Decode a segwit address."""
104    encoding, hrpgot, data = bech32_decode(addr)
105    if hrpgot != hrp:
106        return (None, None)
107    decoded = convertbits(data[1:], 5, 8, False)
108    if decoded is None or len(decoded) < 2 or len(decoded) > 40:
109        return (None, None)
110    if data[0] > 16:
111        return (None, None)
112    if data[0] == 0 and len(decoded) != 20 and len(decoded) != 32:
113        return (None, None)
114    if (data[0] == 0 and encoding != Encoding.BECH32) or (data[0] != 0 and encoding != Encoding.BECH32M):
115        return (None, None)
116    return (data[0], decoded)
117
118
119def encode_segwit_address(hrp, witver, witprog):
120    """Encode a segwit address."""
121    encoding = Encoding.BECH32 if witver == 0 else Encoding.BECH32M
122    ret = bech32_encode(encoding, hrp, [witver] + convertbits(witprog, 8, 5))
123    if decode_segwit_address(hrp, ret) == (None, None):
124        return None
125    return ret
126
127class TestFrameworkScript(unittest.TestCase):
128    def test_segwit_encode_decode(self):
129        def test_python_bech32(addr):
130            hrp = addr[:4]
131            self.assertEqual(hrp, "bcrt")
132            (witver, witprog) = decode_segwit_address(hrp, addr)
133            self.assertEqual(encode_segwit_address(hrp, witver, witprog), addr)
134
135        # P2WPKH
136        test_python_bech32('bcrt1qthmht0k2qnh3wy7336z05lu2km7emzfpm3wg46')
137        # P2WSH
138        test_python_bech32('bcrt1qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq3xueyj')
139        test_python_bech32('bcrt1qft5p2uhsdcdc3l2ua4ap5qqfg4pjaqlp250x7us7a8qqhrxrxfsqseac85')
140        # P2TR
141        test_python_bech32('bcrt1p0xlxvlhemja6c4dqv22uapctqupfhlxm9h8z3k2e72q4k9hcz7vqc8gma6')
142