Python：base64解码时忽略“错误填充”错误

I have some data that is base64 encoded that I want to convert back to binary even if there is a padding error in it. If I use

base64.decodestring(b64_string)

it raises an ‘Incorrect padding’ error. Is there another way?

UPDATE: Thanks for all the feedback. To be honest, all the methods mentioned sounded a bit hit and miss so I decided to try openssl. The following command worked a treat:

openssl enc -d -base64 -in b64string -out binary_data

As said in other responses, there are various ways in which base64 data could be corrupted.

However, as Wikipedia says, removing the padding (the ‘=’ characters at the end of base64 encoded data) is “lossless”:

From a theoretical point of view, the padding character is not needed, since the number of missing bytes can be calculated from the number of Base64 digits.

So if this is really the only thing “wrong” with your base64 data, the padding can just be added back. I came up with this to be able to parse “data” URLs in WeasyPrint, some of which were base64 without padding:

import base64
import re

def decode_base64(data, altchars=b'+/'):
    """Decode base64, padding being optional.

    :param data: Base64 data as an ASCII byte string
    :returns: The decoded byte string.

    """
    data = re.sub(rb'[^a-zA-Z0-9%s]+' % altchars, b'', data)  # normalize
    missing_padding = len(data) % 4
    if missing_padding:
        data += b'='* (4 - missing_padding)
    return base64.b64decode(data, altchars)

Tests for this function: weasyprint/tests/test_css.py#L68

Just add padding as required. Heed Michael’s warning, however.

b64_string += "=" * ((4 - len(b64_string) % 4) % 4) #ugh

It seems you just need to add padding to your bytes before decoding. There are many other answers on this question, but I want to point out that (at least in Python 3.x) base64.b64decode will truncate any extra padding, provided there is enough in the first place.

So, something like: b'abc=' works just as well as b'abc==' (as does b'abc=====').

What this means is that you can just add the maximum number of padding characters that you would ever need—which is three (b'===')—and base64 will truncate any unnecessary ones.

This lets you write:

base64.b64decode(s + b'===')

which is simpler than:

base64.b64decode(s + b'=' * (-len(s) % 4))

“Incorrect padding” can mean not only “missing padding” but also (believe it or not) “incorrect padding”.

If suggested “adding padding” methods don’t work, try removing some trailing bytes:

lens = len(strg)
lenx = lens - (lens % 4 if lens % 4 else 4)
try:
    result = base64.decodestring(strg[:lenx])
except etc

Update: Any fiddling around adding padding or removing possibly bad bytes from the end should be done AFTER removing any whitespace, otherwise length calculations will be upset.

It would be a good idea if you showed us a (short) sample of the data that you need to recover. Edit your question and copy/paste the result of print repr(sample).

Update 2: It is possible that the encoding has been done in an url-safe manner. If this is the case, you will be able to see minus and underscore characters in your data, and you should be able to decode it by using base64.b64decode(strg, '-_')

If you can’t see minus and underscore characters in your data, but can see plus and slash characters, then you have some other problem, and may need the add-padding or remove-cruft tricks.

If you can see none of minus, underscore, plus and slash in your data, then you need to determine the two alternate characters; they’ll be the ones that aren’t in [A-Za-z0-9]. Then you’ll need to experiment to see which order they need to be used in the 2nd arg of base64.b64decode()

Update 3: If your data is “company confidential”:
(a) you should say so up front
(b) we can explore other avenues in understanding the problem, which is highly likely to be related to what characters are used instead of + and / in the encoding alphabet, or by other formatting or extraneous characters.

One such avenue would be to examine what non-“standard” characters are in your data, e.g.

from collections import defaultdict
d = defaultdict(int)
import string
s = set(string.ascii_letters + string.digits)
for c in your_data:
   if c not in s:
      d[c] += 1
print d

Use

string += '=' * (-len(string) % 4)  # restore stripped '='s

Credit goes to a comment somewhere here.

>>> import base64

>>> enc = base64.b64encode('1')

>>> enc
>>> 'MQ=='

>>> base64.b64decode(enc)
>>> '1'

>>> enc = enc.rstrip('=')

>>> enc
>>> 'MQ'

>>> base64.b64decode(enc)
...
TypeError: Incorrect padding

>>> base64.b64decode(enc + '=' * (-len(enc) % 4))
>>> '1'

>>> 

If there’s a padding error it probably means your string is corrupted; base64-encoded strings should have a multiple of four length. You can try adding the padding character (=) yourself to make the string a multiple of four, but it should already have that unless something is wrong

Check the documentation of the data source you’re trying to decode. Is it possible that you meant to use base64.urlsafe_b64decode(s) instead of base64.b64decode(s)? That’s one reason you might have seen this error message.

Decode string s using a URL-safe alphabet, which substitutes – instead of + and _ instead of / in the standard Base64 alphabet.

This is for example the case for various Google APIs, like Google’s Identity Toolkit and Gmail payloads.

Adding the padding is rather… fiddly. Here’s the function I wrote with the help of the comments in this thread as well as the wiki page for base64 (it’s surprisingly helpful) https://en.wikipedia.org/wiki/Base64#Padding.

import logging
import base64
def base64_decode(s):
    """Add missing padding to string and return the decoded base64 string."""
    log = logging.getLogger()
    s = str(s).strip()
    try:
        return base64.b64decode(s)
    except TypeError:
        padding = len(s) % 4
        if padding == 1:
            log.error("Invalid base64 string: {}".format(s))
            return ''
        elif padding == 2:
            s += b'=='
        elif padding == 3:
            s += b'='
        return base64.b64decode(s)

You can simply use base64.urlsafe_b64decode(data) if you are trying to decode a web image. It will automatically take care of the padding.

There are two ways to correct the input data described here, or, more specifically and in line with the OP, to make Python module base64’s b64decode method able to process the input data to something without raising an un-caught exception:

1. Append == to the end of the input data and call base64.b64decode(…)

2. If that raises an exception, then

   i. Catch it via try/except,

   ii. (R?)Strip any = characters from the input data (N.B. this may not be necessary),

   iii. Append A== to the input data (A== through P== will work),

   iv. Call base64.b64decode(…) with those A==-appended input data

The result from Item 1. or Item 2. above will yield the desired result.

Caveats

This does not guarantee the decoded result will be what was originally encoded, but it will (sometimes?) give the OP enough to work with:

Even with corruption I want to get back to the binary because I can still get some useful info from the ASN.1 stream”).

See What we know and Assumptions below.

TL;DR

From some quick tests of base64.b64decode(…)

3. it appears that it ignores non-[A-Za-z0-9+/] characters; that includes ignoring =s unless they are the last character(s) in a parsed group of four, in which case the =s terminate the decoding (a=b=c=d= gives the same result as abc=, and a==b==c== gives the same result as ab==).

4. It also appears that all characters appended are ignored after the point where base64.b64decode(…) terminates decoding e.g. from an = as the fourth in a group.

As noted in several comments above, there are either zero, or one, or two, =s of padding required at the end of input data for when the [number of parsed characters to that point modulo 4] value is 0, or 3, or 2, respectively. So, from items 3. and 4. above, appending two or more =s to the input data will correct any [Incorrect padding] problems in those cases.

HOWEVER, decoding cannot handle the case where the [total number of parsed characters modulo 4] is 1, because it takes a least two encoded characters to represent the first decoded byte in a group of three decoded bytes. In uncorrupted encoded input data, this [N modulo 4]=1 case never happens, but as the OP stated that characters may be missing, it could happen here. That is why simply appending =s will not always work, and why appending A== will work when appending == does not. N.B. Using [A] is all but arbitrary: it adds only cleared (zero) bits to the decoded, which may or not be correct, but then the object here is not correctness but completion by base64.b64decode(…) sans exceptions.

What we know from the OP and especially subsequent comments is

• It is suspected that there are missing data (characters) in the Base64-encoded input data
• The Base64 encoding uses the standard 64 place-values plus padding: A-Z; a-z; 0-9; +; /; = is padding. This is confirmed, or at least suggested, by the fact that openssl enc ... works.

Assumptions

• The input data contain only 7-bit ASCII data
• The only kind of corruption is missing encoded input data
• The OP does not care about decoded output data at any point after that corresponding to any missing encoded input data

Github

Here is a wrapper to implement this solution:

https://github.com/drbitboy/missing_b64

Incorrect padding error is caused because sometimes, metadata is also present in the encoded string If your string looks something like: ‘data:image/png;base64,…base 64 stuff….’ then you need to remove the first part before decoding it.

Say if you have image base64 encoded string, then try below snippet..

from PIL import Image
from io import BytesIO
from base64 import b64decode
imagestr = 'data:image/png;base64,...base 64 stuff....'
im = Image.open(BytesIO(b64decode(imagestr.split(',')[1])))
im.save("image.png")

Simply add additional characters like “=” or any other and make it a multiple of 4 before you try decoding the target string value. Something like;

if len(value) % 4 != 0: #check if multiple of 4
    while len(value) % 4 != 0:
        value = value + "="
    req_str = base64.b64decode(value)
else:
    req_str = base64.b64decode(value)

In case this error came from a web server: Try url encoding your post value. I was POSTing via “curl” and discovered I wasn’t url-encoding my base64 value so characters like “+” were not escaped so the web server url-decode logic automatically ran url-decode and converted + to spaces.

“+” is a valid base64 character and perhaps the only character which gets mangled by an unexpected url-decode.

In my case I faced that error while parsing an email. I got the attachment as base64 string and extract it via re.search. Eventually there was a strange additional substring at the end.

dHJhaWxlcgo8PCAvU2l6ZSAxNSAvUm9vdCAxIDAgUiAvSW5mbyAyIDAgUgovSUQgWyhcMDAyXDMz
MHtPcFwyNTZbezU/VzheXDM0MXFcMzExKShcMDAyXDMzMHtPcFwyNTZbezU/VzheXDM0MXFcMzEx
KV0KPj4Kc3RhcnR4cmVmCjY3MDEKJSVFT0YK

--_=ic0008m4wtZ4TqBFd+sXC8--

When I deleted --_=ic0008m4wtZ4TqBFd+sXC8-- and strip the string then parsing was fixed up.

So my advise is make sure that you are decoding a correct base64 string.

You should use

base64.b64decode(b64_string, ' /')

By default, the altchars are '+/'.

I ran into this problem as well and nothing worked. I finally managed to find the solution which works for me. I had zipped content in base64 and this happened to 1 out of a million records…

This is a version of the solution suggested by Simon Sapin.

In case the padding is missing 3 then I remove the last 3 characters.

Instead of “0gA1RD5L/9AUGtH9MzAwAAA==”

We get “0gA1RD5L/9AUGtH9MzAwAA”

        missing_padding = len(data) % 4
        if missing_padding == 3:
            data = data[0:-3]
        elif missing_padding != 0:
            print ("Missing padding : " + str(missing_padding))
            data += '=' * (4 - missing_padding)
        data_decoded = base64.b64decode(data)   

According to this answer Trailing As in base64 the reason is nulls. But I still have no idea why the encoder messes this up…