Decode a Base58 string of ones as that many zero bytes

NEWS

@@ -1,3 +1,9 @@
+changes from 2.2
+----------------
+
+* Fix Base58 decoding of a Bitcoin address made only of the digit 1. That digit is a zero byte, so a string of them was one byte too long.
+
+
 changes from 2.1 to 2.2
 -----------------------
 

stdnum/bitcoin.py

@@ -68,10 +68,11 @@ def b58decode(s: str) -> bytes:
     """Decode a Base58 encoded string to a bytestring."""
     value = reduce(lambda a, c: a * 58 + _base58_alphabet.index(c), s, 0)
     result = b''
-    while value >= 256:
+    # A zero value has no payload bytes. Each leading 1 is already a zero
+    # byte, so packing the leftover 0 here would add one extra.
+    while value:
         value, mod = divmod(value, 256)
         result = struct.pack('B', mod) + result
-    result = struct.pack('B', value) + result
     return struct.pack('B', 0) * (len(s) - len(s.lstrip('1'))) + result
 
 

tests/test_bitcoin.doctest

@@ -23,6 +23,19 @@ useful as module documentation.
 >>> from stdnum import bitcoin
 
 
+The digit 1 is zero. A string of them is that many zero bytes, and an empty
+string is empty. A leftover zero must not add another byte.
+
+>>> bitcoin.b58decode('1')
+b'\x00'
+>>> bitcoin.b58decode('11')
+b'\x00\x00'
+>>> bitcoin.b58decode('2')
+b'\x01'
+>>> bitcoin.b58decode('')
+b''
+
+
 These are found and constructed P2PKH addresses (P2SH addresses are basically
 the same because they follow the same validation, except that the first digit
 is a 3).