Check if a string is encoded in base64 using Python

Question

Is there a good way to check if a string is encoded in base64 using Python?

Answer 1

I know I'm almost 8 years late but you can use a regex expression thus you can verify if a given input is BASE64.

import re

encoding_type = 'Encoding type: '
base64_encoding = 'Base64'


def is_base64():
    element = input("Enter encoded element: ")
    expression = "^([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{2}==)?$"

    matches = re.match(expression, element)

    if matches:
        print(f"{encoding_type + base64_encoding}")
    else:
        print("Unknown encoding type.")


is_base64()

Answer 2

@geoffspear is correct in that this is not 100% possible but you can get pretty close by checking the string header to see if it matches that of a base64 encoded string (re: How to check whether a string is base64 encoded or not).

# check if a string is base64 encoded.
def isBase64Encoded(s):
    pattern = re.compile("^([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{4}|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{2}==)$")
    if not s or len(s) < 1:
        return False
    else:
        return pattern.match(s)

Also not that in my case I wanted to return false if the string is empty to avoid decoding as there's no use in decoding nothing.

Answer 3

import base64
import binascii

try:
    base64.decodestring("foo")
except binascii.Error:
    print "no correct base64"

Answer 4

I was looking for a solution to the same problem, then a very simple one just struck me in the head. All you need to do is decode, then re-encode. If the re-encoded string is equal to the encoded string, then it is base64 encoded.
Here is the code:

import base64

def isBase64(s):
    try:
        return base64.b64encode(base64.b64decode(s)) == s
    except Exception:
        return False

That's it!

Edit: Here's a version of the function that works with both the string and bytes objects in Python 3:

import base64

def isBase64(sb):
        try:
                if isinstance(sb, str):
                        # If there's any unicode here, an exception will be thrown and the function will return false
                        sb_bytes = bytes(sb, 'ascii')
                elif isinstance(sb, bytes):
                        sb_bytes = sb
                else:
                        raise ValueError("Argument must be string or bytes")
                return base64.b64encode(base64.b64decode(sb_bytes)) == sb_bytes
        except Exception:
                return False