How to avoid "Incorrect padding" error while Base64 Decoding this string in Python

Question

I have the following string and I want to use Python to Base64 decode it.

eyJqa3UiOiJodHRwczovL2U5N2I4YTlkNjcyZTRjZTQ4NDVlYzY5NDdjZDY2ZWY2LXNiLmJhYXMubmludGVuZG8uY29tLzEuMC4wL2NlcnRpZmljYXRlcyIsImtpZCI6ImZlOWRiYmZmLTQ3MGItNDZjOC04YmFmLTFiNzY5OGRlZTViZSIsImFsZyI6IlJTMjU2In0.eyJpc3MiOiJodHRwczovL2U5N2I4YTlkNjcyZTRjZTQ4NDVlYzY5NDdjZDY2ZWY2LXNiLmJhYXMubmludGVuZG8uY29tIiwiZXhwIjoxNTQ1MTg1NDk2LCJ0eXAiOiJpZF90b2tlbiIsImF1ZCI6IjhkOTc1NTllNjNlY2NkNTYiLCJiczpkaWQiOiI2NjJhZTQwOWYwNTQyYTBjIiwic3ViIjoiOTNkYmYwNDdiYTI3NzQ5NSIsImp0aSI6IjY1NDg4ZjJmLTI1NzAtNDBkYy04ODQ3LTMzODNlZWIxMGJiYiIsIm5pbnRlbmRvIjp7ImFpIjoiMDEwMGY4MDAwMDQ5MjAwMCIsImF2IjoiMDAwMCIsImVkaSI6ImJjNTdiYmM3MTZlMDA1MGFmOWRhN2NkYTIzMWRjZDgyIiwiYXQiOjE1NDUxNzQ2OTZ9LCJpYXQiOjE1NDUxNzQ2OTZ9.ZMUIt3wYrbfhXnnDh4WraGlKrZy0YuL5prluY70sU_-0W5XvWIB-xmTrLz7LJWHEGwTskcWf81_HBq_mSb75rMfTAEBwBmOJ4ITmhdnXksz8w7EDOWuPPSEft5XLMNOMD16ztEOYe5ddU_iqNEbT56L7fcAJEXv0FWy6H_OutxOglYpDaNkcj6CWJ7dpA0JbqerR9dEszaLwyn1ZBDPVD0YeAIm5bEr61imeedzMb0amxlTl4R87mqK6epsFUnRy6p6Klr27_DlTLQ-gej09W7NeNzONCj4thHgCr9szAiaN28krfTc2fobz3qFCoC_eQghiIIZBe_-Lksng3Eg6tw

When I go to this https://www.base64decode.org/ and plop it in, I can see that the first portion of the decoded string is readable plain-text:

However when I try to use Python to decode it, it fails:

>>> import base64
>>> x = "eyJ......"
>>> base64.b64decode(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/base64.py", line 76, in b64decode
    raise TypeError(msg)
TypeError: Incorrect padding

I tried this remedy. But it didn't change the resulting error. How do I get around this problem? Why won't it decode like that website does it?

I have checked your base64 string it has some incorrect padding also some kind special characters are there. That might be the reason Python gives this error. Please try on JavaScript function btoa — Ankur Shah, Dec 19 '18 at 00:44

score 14 · Accepted Answer · edited Sep 23 '20 at 09:33

Well, it looks like the message is split by . into three parts. The first two parts are base64 encoded, while the last one is not:

import base64

res = "eyJqa3UiOiJodHRwczovL2U5N2I4YTlkNjcyZTRjZTQ4NDVlYzY5NDdjZDY2ZWY2LXNiLmJhYXMubmludGVuZG8uY29tLzEuMC4wL2NlcnRpZmljYXRlcyIsImtpZCI6ImZlOWRiYmZmLTQ3MGItNDZjOC04YmFmLTFiNzY5OGRlZTViZSIsImFsZyI6IlJTMjU2In0.eyJpc3MiOiJodHRwczovL2U5N2I4YTlkNjcyZTRjZTQ4NDVlYzY5NDdjZDY2ZWY2LXNiLmJhYXMubmludGVuZG8uY29tIiwiZXhwIjoxNTQ1MTg1NDk2LCJ0eXAiOiJpZF90b2tlbiIsImF1ZCI6IjhkOTc1NTllNjNlY2NkNTYiLCJiczpkaWQiOiI2NjJhZTQwOWYwNTQyYTBjIiwic3ViIjoiOTNkYmYwNDdiYTI3NzQ5NSIsImp0aSI6IjY1NDg4ZjJmLTI1NzAtNDBkYy04ODQ3LTMzODNlZWIxMGJiYiIsIm5pbnRlbmRvIjp7ImFpIjoiMDEwMGY4MDAwMDQ5MjAwMCIsImF2IjoiMDAwMCIsImVkaSI6ImJjNTdiYmM3MTZlMDA1MGFmOWRhN2NkYTIzMWRjZDgyIiwiYXQiOjE1NDUxNzQ2OTZ9LCJpYXQiOjE1NDUxNzQ2OTZ9.ZMUIt3wYrbfhXnnDh4WraGlKrZy0YuL5prluY70sU_-0W5XvWIB-xmTrLz7LJWHEGwTskcWf81_HBq_mSb75rMfTAEBwBmOJ4ITmhdnXksz8w7EDOWuPPSEft5XLMNOMD16ztEOYe5ddU_iqNEbT56L7fcAJEXv0FWy6H_OutxOglYpDaNkcj6CWJ7dpA0JbqerR9dEszaLwyn1ZBDPVD0YeAIm5bEr61imeedzMb0amxlTl4R87mqK6epsFUnRy6p6Klr27_DlTLQ-gej09W7NeNzONCj4thHgCr9szAiaN28krfTc2fobz3qFCoC_eQghiIIZBe_-Lksng3Eg6tw"

for i in res.split("."):
    print(base64.b64decode(i + '=' * (-len(i) % 4)))

I guess the last one is a signature, which is used to validate the first two parts. Do you get this string from a cookie? Or from a submitted form?

Edit

So for anyone sees this answer, the given string is a JWT string.

How to avoid "Incorrect padding" error while Base64 Decoding this string in Python

1 Answers1

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