`binascii.a2b_base64` ignores padding inside a string

[DemiMarie]

Bug report

The following code should raise binascii.Error but does not:

import binascii
binascii.a2b_base64(b'=aa==')

It appears (from looking at the source code) that this is also the case in 3.11, even in strict mode.

Your environment

• CPython versions tested on: 3.10
• Operating system and architecture: Fedora 36, Linux 5.15.52, x86_64

[yourlefthandman]

When running the code from main, the behaviour is as you expect it to be :)

>>> binascii.a2b_base64(b'=aa==', strict_mode=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
binascii.Error: Leading padding not allowed

The following code handles it-

if (strict_mode && ascii_len > 0 && ascii_data[0] == '=') {
    state = get_binascii_state(module);
    if (state) {
        PyErr_SetString(state->Error, "Leading padding not allowed");
    }
    goto error_end;
}

Looking at 3.11, i see the code is there too - link

[DemiMarie]

Does it reject this input?

import binascii
binascii.a2b_base64(b'aaaa=aa==', strict_mode=True)

?

[yourlefthandman]

Yup :)

>>> import binascii
>>> binascii.a2b_base64(b'aaaa=aa==', strict_mode=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
binascii.Error: Discontinuous padding not allowed

[Mohitduklan]

Is it because python 3.10.5 uses REF 4648. https://docs.python.org/3/library/base64.html

[idan22moral]

Hi @DemiMarie,
This behavior is normal in 3.10, since the strict_mode flag was not introduced yet.
In 3.11.0 it behaves as expected:

Python 3.11.0 (main, Oct 24 2022, 18:26:48) [MSC v.1933 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import binascii
>>> binascii.a2b_base64(b'=aa==')
b'i'
>>> binascii.a2b_base64(b'=aa==', strict_mode=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
binascii.Error: Leading padding not allowed
>>> binascii.a2b_base64(b'aaaa=aa==')
b'i\xa6\x9ai'
>>> binascii.a2b_base64(b'aaaa=aa==', strict_mode=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
binascii.Error: Discontinuous padding not allowed

For further information, please read:

• https://docs.python.org/3/library/binascii.html?highlight=strict_mode#binascii.a2b_base64
• bpo-43086: Added handling for excess data in binascii.a2b_base64 #24402