binascii.a2b_base64 ignores padding inside a string #94703

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1. DemiMarie

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Jul 9, 2022
2. yourlefthandman commented on Jul 9, 2022

   

   yourlefthandman

   on Jul 9, 2022

   Last edited by yourlefthandman

   Contributor

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   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
3. DemiMarie commented on Jul 10, 2022

   

   DemiMarie

   on Jul 10, 2022

   Author

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   Does it reject this input?

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

   ?
4. yourlefthandman commented on Jul 10, 2022

   

   yourlefthandman

   on Jul 10, 2022

   Contributor

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   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
   
5. Mohitduklan commented on Jul 24, 2022

   

   Mohitduklan

   on Jul 24, 2022

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   Is it because python 3.10.5 uses REF 4648. https://docs.python.org/3/library/base64.html
6. idan22moral commented on Nov 4, 2022

   

   idan22moral

   on Nov 4, 2022

   Last edited by idan22moral

   Contributor

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   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
7. DemiMarie

   closed this as completedon Nov 4, 2022