Python decode base64 incorrect padding

[Solved] Error “incorrect padding” in decoding of Base64 module in Python

Problem description
decodes the field information encoded by Base64, which is normally decoded in the base64 encoding and decoding tool, but b64decode and A2B in the modules Base64 and binascii under python_ Decoding error in Base64 and other methods
the error information is as follows

--------------------------------------------------------------------------- Error Traceback (most recent call last) in get_proxies(urls) 14 try: ---> 15 raw = base64.b64decode(response) 16 except Exception as r: c:\program files\python3\lib\base64.py in b64decode(s, altchars, validate) 86 raise binascii.Error('Non-base64 digit found') ---> 87 return binascii.a2b_base64(s) 88 Error: Incorrect padding 

Base64 in Python is read in 4, so the field to be decoded should be a multiple of 4, not enough to add ‘=’

# The field a to be decoded is judged, and if it is a multiple of 4, it is unchanged, and if not, how much is missing, how much is made up a = a + '=' * (4 - len(a) % 4) if len(a) % 4 != 0 else a 

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Python: Ignore ‘Incorrect Padding’ Error When Base64 Decoding

You have at least one string in your CSV file that is either not a Base64 string, is a corrupted (damaged) Base64 string, or is a string that is missing the required = padding. Your example value, ABHPdSaxrhjAWA= , is short one = or is missing another data character.

Base64 strings, properly padded, have a length that is a multiple of 4, so you can easily re-add the padding:

value = csvlines[0]
if len(value) % 4: 
# not a multiple of 4, add padding: 
value += '=' * (4 - len(value) % 4) 
csvlines[0] = value.decode("base64").encode("hex")

If the value then still fails to decode, then your input was corrupted or not valid Base64 to begin with.

For the example error, ABHPdSaxrhjAWA= , the above adds one = to make it decodable:

>>> value = 'ABHPdSaxrhjAWA='
>>> if len(value) % 4:
. # not a multiple of 4, add padding:
. value += '=' * (4 - len(value) % 4)
. 
>>> value
'ABHPdSaxrhjAWA=='
>>> value.decode('base64')
'\x00\x11\xcfu&\xb1\xae\x18\xc0X'
>>> value.decode('base64').encode('hex')
'0011cf7526b1ae18c058'

I need to emphasise that your data may simply be corrupted. Your console output includes one value that worked, and one that failed. The one that worked is one character longer, and that’s the only difference:

ABHvPdSaxrhjAWA=
ABHPdSaxrhjAWA=

Note the v in the 4th place; this is missing from the second example. This could indicate that something happened to your CSV data that caused that character to be dropped from the second example. Adding in padding can make the second value decodable again, but the result would be wrong. We can’t tell you which of those two options is the cause here.

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

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.

binascii.Error: Incorrect padding, even when string length is multiple of 4

by checking your link, your string has 200000 bytes all right, but it contains the header:

strOne = b"data:image/png;base64,iVBORw0KGgoAAAANSU.

This is part of MIME message or something. You have to strip this first.

pad = len(strOne)%4
strOne += b» we believe in team work» 🙂

python b64decode incorrect padding

Your description of what you are doing sounds OK. Choice of the input piece size affects only the efficiency. Padding bytes are minimised if the length of each input piece (except of course the last) is a multiple of 3.

You need to show us both your server code and your client code. Alternatively: on the server, log the input and the pieces transmitted. On the client, log the pieces received. Compare.

Curiosity: Why don’t you just b64encode the whole string, split the encoded result however you like, transmit the pieces, at the client reassemble the pieces using b».join(pieces) and b64decode that?

Further curiosity: I thought the contents of a UDP packet could be any old binary bunch of bytes; why are you doing base64 encoding at all?

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base64 throws ‘incorrect padding’ exception when the issue is NOT with the padding #77951

base64 throws ‘incorrect padding’ exception when the issue is NOT with the padding #77951

3.7 (EOL) end of life 3.8 only security fixes extension-modules C modules in the Modules dir type-bug An unexpected behavior, bug, or error

Comments

Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

assignee = None closed_at = Date 2018-06-11.05:31:13.374> created_at = Date 2018-06-04.19:53:31.414> labels = ['extension-modules', '3.8', 'type-bug', '3.7'] title = "base64 throws 'incorrect padding' exception when the issue is NOT with the padding" updated_at = Date 2018-06-11.05:31:13.373> user = 'https://github.com/dniq'

activity = Date 2018-06-11.05:31:13.373> actor = 'ned.deily' assignee = 'none' closed = True closed_date = Date 2018-06-11.05:31:13.374> closer = 'ned.deily' components = ['Extension Modules'] creation = Date 2018-06-04.19:53:31.414> creator = 'dniq' dependencies = [] files = [] hgrepos = [] issue_num = 33770 keywords = ['patch'] message_count = 23.0 messages = ['318691', '318697', '318701', '318704', '318716', '318721', '318727', '318728', '318729', '318733', '318752', '318755', '318762', '318789', '319093', '319094', '319101', '319102', '319104', '319202', '319248', '319249', '319272'] nosy_count = 7.0 nosy_names = ['terry.reedy', 'mark.dickinson', 'taleinat', 'ned.deily', 'r.david.murray', 'serhiy.storchaka', 'dniq'] pr_nums = ['7416', '7602'] priority = 'normal' resolution = 'fixed' stage = 'resolved' status = 'closed' superseder = None type = 'behavior' url = 'https://bugs.python.org/issue33770' versions = ['Python 3.7', 'Python 3.8']

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