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In addition to the above, this PR as is causes a bunch of test suite failures that need to be addressed.

@gpshead While writing the tests for my PR I noticed something:
binascii.b2a_base64 encodes a given payload and adds a newline at the end (no idea what's the reason behind it).
When trying to decode ab==\n, the Excess data after padding ... error raises, which makes the relation between b2a and a2b broken.

I thought about ignoring newlines (or any other character in table_a2b_base64 that's mapped to -1) after the padding,
but the same security problem raises, since it's still excess data.

So, I'm opening this to discussion - should the b2a method be re-implemented too?

b2a_base64 has always added a newline by default. (digging into why would require code history spelunking probably back to the 1990s; I could guess but it doesn't really matter). The ability to disable adding the newline was added in 3.6 via https://bugs.python.org/issue25357 as the base64 module does not want the newline.

Decoder wise it is a good idea for a2b_base64 to accept what b2a_base64 produces by default so I recommend we continue to accept a single trailing \n if present. We're still much better off from a security perspective being strict and only having a mere two possible variants of input ascii data that lead to the same bytes output instead of today's infinite possibilities by ignoring all trailing data after padding = signs.

I suggest adding an additional keyword only parameter as a flag when implementing this:

# Ignores a single newline at the end, regardless of `=`.
a2b_base64(ascii, ignore_trailing_newline=True)  # Make this the default.
# and
a2b_base64(ascii, ignore_trailing_newline=False)   # No extra data is allowed ever (raises an Error).

There will be a minority of applications out there that want the extra data ignoring behavior. This allows them to retain that without implementing their function wrapper to discard it first. If I'm understanding the current behavior correctly, people who want their code to have today's existing behavior of ignoring all trailing data can use and be compatible with the library before and after this fix:

binary = a2b_base64(ascii.split('=', 1)[0])

Adding such a ignore_trailing_newline=True parameter to allow disabling the newline behavior is probably best done in a followup PR for this issue as the new parameter can't be backported to stable releases.

The reason I recommend that default behavior at all is to maintain compatibility with the expectations of existing code including the round trip from b2a -> a2b working.

If we ever want to change the default newline behavior in the future, it should be done on both b2a and a2b with our usual "couple of release cycles" deprecation period. I'm not convinced the mere newline behavior default warrants changing.

It is also plausible that other users of a2b_base64 in the stdlib may want to ignore more padding and should be updated to use the split trick or similar. For example https://tools.ietf.org/html/rfc4648#section-3.3 suggests MIME and thus the email package is likely one that prefers leniency. Code inspection to find out would be required if no tests explicitly cover these situations.

@gpshead Finally had the time to come back to this subject.
I thought about it for a few more days and here are some more insights:

In the current implementation, every character that isn't one of the following is ignored:

''.join([chr(_) if (table_a2b_base64[_] == -1) else '' for _ in range(128)])
# '\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !"#$%&\'()*,-.:;<>?@[\\]^_`{|}~\x7f

Which can lead to the same problem that we talked about earlier.
For example:

a2b_base64(b'ab==') # b'i'
a2b_base64(b'a:(){:|:&};:b==') # b'i'

So, if we combine your solution with this "bigger" problem, the solution is much simpler,
and it is to raise an exception when:

if (strict_mode && table_a2b_base64[ascii_data[i]] == -1) {
    binascii_state *state = PyModule_GetState(module);
    if (state) {
        PyErr_SetString(state->Error, "Only base64 data is allowed when using strict mode");
    }
    ...
}

So the parameter would be strict_mode, which by default will be False.
(Strict Mode - non-base64 data in not allowed)
What's your opinion on this matter?

Forgot to mention that the strict_mode flag controls the excess data behavior too,
so when it's set to True and there's excess data (even newlines) - an exception will raise.

The counterpart function (b2a) will have the same flag to respond to the new update, ofc.
Edit: b2a already has the newline flag, so strict_mode is not necessary there.

Figured that implementing it will save some time later if the idea is approved, so I did it.

Some examples:

>>> from binascii import a2b_base64
>>> a2b_base64("Yb==\n")
b'a'
>>> a2b_base64("Yb==")
b'a'
>>> a2b_base64("Yb :(){:|:&};: ==")
b'a'
>>> a2b_base64("Yb :(){:|:&};: ==", strict_mode=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
binascii.Error: Only base64 data is allowed when using strict mode
>>> a2b_base64("Y b==", strict_mode=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
binascii.Error: Only base64 data is allowed when using strict mode
>>> a2b_base64("Yb==\n", strict_mode=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
binascii.Error: Excess data after padding is not allowed when using strict mode
>>> a2b_base64("Y\x00b==", strict_mode=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
binascii.Error: Only base64 data is allowed when using strict mode
>>> a2b_base64("Y\nb==", strict_mode=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
binascii.Error: Only base64 data is allowed when using strict mode

Documentation:

a2b_base64(data, /, *, strict_mode=False)
    Decode a line of base64 data.
    
    strict_mode
      When set to True, bytes that are not part of the base64 standard are not allowed.
      The same applies to excess data after padding (= / ==).

I haven't had a chance to look this over yet, but I like where you've headed with this and the addition of strict_mode.

Awesome!
Let me know how we proceed with this issue when you have the time 🙂

@gpshead can you re-review my changes?

FWIW I believe my change of the error message from 'Malformed padding' to 'Leading padding' in this PR was half incorrect. I overlooked the label and goto that caused that message to be used in multiple circumstances. The later goto malformed_padding should probably just be turned into one raising its own unique error message.

not a huge deal, but a potentially misleading error message in the = in the middle of data circumstance.