Add the ignorechars parameter in the Base64 decoder

[serhiy-storchaka]

Feature or enhancement

Earlier standards for Base64 (RFC 1421, RFC 1521) on which the base64 and binascii modules are based, specified that any non-alphabet characters should be ignored, and this is the default behavior of these modules. More modern standard (RFC 4648) considers this a vulnerability, and specify that they should be rejected until the upper lever document states otherwise. Thus, strict_mode/validate parameters were added. But they are "all-or-nothing" -- either all non-alphabet characters are rejected, or other errors are ignored. In some cases we need to ignore only the part of errors -- for example, ignore newlines and whitespaces, but not other invalid characters, or ignore padding errors.

a85decode() has parameter ignorechars which could also be used to control Base64 decoding.

Linked PRs

• gh-144001: Add ignorechars parameter to Base64 decoder #144009
• gh-144001: Support ignorechars in binascii.a2b_base64() and base64.b64decode() #144024
• gh-144001: Support ignoring the invalid pad character in Base64 decoding #144306
• gh-144001: Simplify Base64 decoding with altchars and ignorechars specified #144324

[moneebullah25]

Hi @serhiy-storchaka

I've implemented this feature in PR #144009.

This PR adds ignorechars parameter to binascii.a2b_base64() and base64.b64decode() with O(1) lookup table, tests, and NEWS entry.

If this aligns with you have in your mind, feel free to merge or let me know what to cherry-pick from it. Otherwise If you'd prefer a different approach or it needs significant changes, please let me know or feel free to create your own implementation, if not I am happy to close mine in that case.

[serhiy-storchaka]

Sorry, @moneebullah25, but I already had my code before opening this issue. I only needed to solve a pair of corner cases.