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 and segwit addresses."""
6
7
8CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l"
9
10
11def bech32_polymod(values):
12    """Internal function that computes the Bech32 checksum."""
13    generator = [0x3b6a57b2, 0x26508e6d, 0x1ea119fa, 0x3d4233dd, 0x2a1462b3]
14    chk = 1
15    for value in values:
16        top = chk >> 25
17        chk = (chk & 0x1ffffff) << 5 ^ value
18        for i in range(5):
19            chk ^= generator[i] if ((top >> i) & 1) else 0
20    return chk
21
22
23def bech32_hrp_expand(hrp):
24    """Expand the HRP into values for checksum computation."""
25    return [ord(x) >> 5 for x in hrp] + [0] + [ord(x) & 31 for x in hrp]
26
27
28def bech32_verify_checksum(hrp, data):
29    """Verify a checksum given HRP and converted data characters."""
30    return bech32_polymod(bech32_hrp_expand(hrp) + data) == 1
31
32
33def bech32_create_checksum(hrp, data):
34    """Compute the checksum values given HRP and data."""
35    values = bech32_hrp_expand(hrp) + data
36    polymod = bech32_polymod(values + [0, 0, 0, 0, 0, 0]) ^ 1
37    return [(polymod >> 5 * (5 - i)) & 31 for i in range(6)]
38
39
40def bech32_encode(hrp, data):
41    """Compute a Bech32 string given HRP and data values."""
42    combined = data + bech32_create_checksum(hrp, data)
43    return hrp + '1' + ''.join([CHARSET[d] for d in combined])
44
45
46def bech32_decode(bech):
47    """Validate a Bech32 string, and determine HRP and data."""
48    if ((any(ord(x) < 33 or ord(x) > 126 for x in bech)) or
49            (bech.lower() != bech and bech.upper() != bech)):
50        return (None, None)
51    bech = bech.lower()
52    pos = bech.rfind('1')
53    if pos < 1 or pos + 7 > len(bech) or len(bech) > 90:
54        return (None, None)
55    if not all(x in CHARSET for x in bech[pos+1:]):
56        return (None, None)
57    hrp = bech[:pos]
58    data = [CHARSET.find(x) for x in bech[pos+1:]]
59    if not bech32_verify_checksum(hrp, data):
60        return (None, None)
61    return (hrp, data[:-6])
62
63
64def convertbits(data, frombits, tobits, pad=True):
65    """General power-of-2 base conversion."""
66    acc = 0
67    bits = 0
68    ret = []
69    maxv = (1 << tobits) - 1
70    max_acc = (1 << (frombits + tobits - 1)) - 1
71    for value in data:
72        if value < 0 or (value >> frombits):
73            return None
74        acc = ((acc << frombits) | value) & max_acc
75        bits += frombits
76        while bits >= tobits:
77            bits -= tobits
78            ret.append((acc >> bits) & maxv)
79    if pad:
80        if bits:
81            ret.append((acc << (tobits - bits)) & maxv)
82    elif bits >= frombits or ((acc << (tobits - bits)) & maxv):
83        return None
84    return ret
85
86
87def decode(hrp, addr):
88    """Decode a segwit address."""
89    hrpgot, data = bech32_decode(addr)
90    if hrpgot != hrp:
91        return (None, None)
92    decoded = convertbits(data[1:], 5, 8, False)
93    if decoded is None or len(decoded) < 2 or len(decoded) > 40:
94        return (None, None)
95    if data[0] > 16:
96        return (None, None)
97    if data[0] == 0 and len(decoded) != 20 and len(decoded) != 32:
98        return (None, None)
99    return (data[0], decoded)
100
101
102def encode(hrp, witver, witprog):
103    """Encode a segwit address."""
104    ret = bech32_encode(hrp, [witver] + convertbits(witprog, 8, 5))
105    if decode(hrp, ret) == (None, None):
106        return None
107    return ret
108