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

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Jul 24, 2018
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bpo-34164: Fix handling of incorrect padding in base64.b32decode(). #8351

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bpo-34164: Fix handling of incorrect padding in base64.b32decode().
serhiy-storchaka Jul 20, 2018
feec40c
Address comments.
serhiy-storchaka Jul 24, 2018
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16 changes: 4 additions & 12 deletions Lib/base64.py
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Original file line number Diff line number Diff line change
Expand Up @@ -231,23 +231,15 @@ def b32decode(s, casefold=False, map01=None):
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')
decoded[-5:] = last[:-((padchars * 5 + 4) // 8)]
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I would suggest to put the complex operation a a separated line. Example:

n = ((padchars * 5 + 4) // 8)
decoded[-5:] = last[:-n]

Oh, GitHub lost my comment. I had to write it again.

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leftover = (43 - 5 * padchars) // 8  # 1: 4, 3: 3, 4: 2, 6: 1

or

leftover = (5, 4, 4, 3, 2, 2, 1, 1, 0)[padchars]

or

leftover = (..., 4, ..., 3, 2, ..., 1)[padchars]

What is better?

return bytes(decoded)



# RFC 3548, Base 16 Alphabet specifies uppercase, but hexlify() returns
# lowercase. The RFC also recommends against accepting input case
# insensitively.
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19 changes: 14 additions & 5 deletions Lib/test/test_base64.py
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Original file line number Diff line number Diff line change
Expand Up @@ -343,11 +343,20 @@ def test_b32decode_casefold(self):
self.assertRaises(binascii.Error, base64.b32decode, data_str)

def test_b32decode_error(self):
for data in [b'abc', b'ABCDEF==', b'==ABCDEF']:
with self.assertRaises(binascii.Error):
base64.b32decode(data)
with self.assertRaises(binascii.Error):
base64.b32decode(data.decode('ascii'))
tests = [b'abc', b'ABCDEF==', b'==ABCDEF']
prefixes = [b'M', b'ME', b'MFRA', b'MFRGG', b'MFRGGZA', b'MFRGGZDF']
for i in range(0, 17):
if i:
tests.append(b'='*i)
for prefix in prefixes:
if len(prefix) + i != 8:
tests.append(prefix + b'='*i)
for data in tests:
with self.subTest(data=data):
with self.assertRaises(binascii.Error):
base64.b32decode(data)
with self.assertRaises(binascii.Error):
base64.b32decode(data.decode('ascii'))

def test_b16encode(self):
eq = self.assertEqual
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2018-07-20-18-06-00.bpo-34164.fNfT-q.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
:func:`base64.b32decode` could raise UnboundLocalError or OverflowError for
incorrect padding. Now it always raises :exc:`base64.Error` in these cases.