commit 0b2716918345b2bff0bb5c36086d5de368125536
author Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com> Tue Jul 24 03:53:39 2018 -0700
committer Serhiy Storchaka <storchaka@gmail.com> Tue Jul 24 13:53:39 2018 +0300
tree 0b5d8563abf3437c5da53b57e8a42ed71524027b
parent 220afffb683af1ba9e04c37535cfaa41348e9ebb

bpo-34164: Fix handling of incorrect padding in base64.b32decode(). (GH-8351) (GH-8435)

Now base64.Error is always raised instead of UnboundLocalError or
OverflowError.
(cherry picked from commit ac0b3c2f4d86fc056b833a4e6b9a380741244a63)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/base64.py

diff --git a/Lib/base64.py b/Lib/base64.py
index eb8f258..2be9c39 100755
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -231,23 +231,16 @@
             raise binascii.Error('Non-base32 digit found') from None
         decoded += acc.to_bytes(5, 'big')
     # Process the last, partial quanta
-    if padchars:
+    if l % 8 or padchars not in {0, 1, 3, 4, 6}:
+        raise binascii.Error('Incorrect padding')
+    if padchars and decoded:
         acc <<= 5 * padchars
         last = acc.to_bytes(5, 'big')
-        if padchars == 1:
-            decoded[-5:] = last[:-1]
-        elif padchars == 3:
-            decoded[-5:] = last[:-2]
-        elif padchars == 4:
-            decoded[-5:] = last[:-3]
-        elif padchars == 6:
-            decoded[-5:] = last[:-4]
-        else:
-            raise binascii.Error('Incorrect padding')
+        leftover = (43 - 5 * padchars) // 8  # 1: 4, 3: 3, 4: 2, 6: 1
+        decoded[-5:] = last[:leftover]
     return bytes(decoded)
 
 
-
 # RFC 3548, Base 16 Alphabet specifies uppercase, but hexlify() returns
 # lowercase.  The RFC also recommends against accepting input case
 # insensitively.