From 7604359f650c506dced008ac9a4fc5abebc9b668 Mon Sep 17 00:00:00 2001
From: Sasha Mitchell <sash.t.mitchell@gmail.com>
Date: Sat, 3 Oct 2026 21:32:56 +0700
Subject: [PATCH] Decode a Base58 string of ones as that many zero bytes

The digit 1 is zero, and each leading 1 is already a zero byte. Packing the leftover 0 added one extra byte, so '1' decoded as two zero bytes.
---
 NEWS                       |  6 ++++++
 stdnum/bitcoin.py          |  5 +++--
 tests/test_bitcoin.doctest | 13 +++++++++++++
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/NEWS b/NEWS
index 8eb60b3a..d5ca41b7 100644
--- a/NEWS
+++ b/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
 -----------------------
 
diff --git a/stdnum/bitcoin.py b/stdnum/bitcoin.py
index 4cf2bd90..6eac7470 100644
--- a/stdnum/bitcoin.py
+++ b/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
 
 
diff --git a/tests/test_bitcoin.doctest b/tests/test_bitcoin.doctest
index 022bd846..1038619c 100644
--- a/tests/test_bitcoin.doctest
+++ b/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).