binascii.a2b_base64 strict mode accepts invalid base64 input

[zhangyoufu]

Bug report

Bug description:

>>> import binascii
>>> f = lambda s: binascii.a2b_base64(s, strict_mode=True)
>>> f('AAAA')
b'\x00\x00\x00'
>>> f('AAAA=')
b'\x00\x00\x00'
>>> f('AAAA==')
b'\x00\x00\x00'
>>> f('AAAA===')
b'\x00\x00\x00'
>>> f('AAAA====')
b'\x00\x00\x00'
>>> f('AAAA=====')
b'\x00\x00\x00'

Doc/library/binascii.rst defines that a valid base64 "Contains no excess data after padding (including excess padding, newlines, etc.)". However, current implement does not handle the edge case of excess padding after no padding.

CPython versions tested on:

3.12

Operating systems tested on:

macOS

Linked PRs

• gh-118314: Fix padding edge case in binascii.a2b_base64 strict mode #118320
• [3.12] gh-118314: Fix padding edge case in binascii.a2b_base64 strict mode (GH-118320) #118691