Incorrect padding while decoding a Base64 string

Question

I am trying to decode a Base64 encoded byte string to a valid HTTP URL. I have tried appending necessary padding (=). But it still does not seem to work.

I have tried the following code.

import base64
encoded = b"aHR0cHM6Ly9mb3Jtcy5nbGUvWU5ZXQ0d2NRWHVLNnNwdjU="
decoded = base64.b64decode(encoded)
print(decoded)

The string encoded has a missing character as a part of noise. Is there a way to detect that missing character and then perform the decode operation?

possible duplicate of https://stackoverflow.com/questions/3302946/how-to-decode-base64-url-in-python — INDRAJITH EKANAYAKE, Jan 15 '21 at 05:09
This means the base64 is corrupted somehow. If it's created by your function, check the algorithm once again. The only way to do this bruteforce the string as demonstrated by @SakibAlamin — tbhaxor, Jan 16 '21 at 06:05

Sakib Md Al Amin · Answer 1 · 2021-01-16T07:15:19.983

So, you have this aHR0cHM6Ly9mb3Jtcy5nbGUvWU5ZXQ0d2NRWHVLNnNwdjU= base64 encoding of an URL with exactly one character missing.

For the missing character, you've 64 choices: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789+/ (for base64) and 48 possible positions to put the missing character in -a-H-R-0-c-H-M-6-L-y-9-m-b-3-J-t-c-y-5-n-b-G-U-v-W-U-5-Z-X-Q-0-d-2-N-R-W-H-V-L-N-n-N-w-d-j-U-=- (- indicates the possible positions)

So, you've 64 * 48 = 3072 possible encoded strings. Either you can try to generate them by your hand or write some code to do the same.

Once you generate them, you can decode the string to get the URL using some built-in libraries & check whether this URL is valid or not. If you also need to know whether this URL exists or not, you can make an HTTP request to the URL & check the response StatusCode.

Code:

package main

import (
    "encoding/base64"
    "fmt"
    "net/http"
)

func main() {
    encodedURL := "aHR0cHM6Ly9mb3Jtcy5nbGUvWU5ZXQ0d2NRWHVLNnNwdjU="
    options := "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789+/"

    length := len(encodedURL)

    for i := 0; i <= length; i++ {
        for idx := 0; idx < 64; idx++ {
            tempEncoded := encodedURL[:i] + options[idx:idx+1] + encodedURL[i:]
            decodedURL, _ := base64.URLEncoding.DecodeString(tempEncoded)

            resp, err := http.Get(string(decodedURL))
            if err == nil && resp.StatusCode == http.StatusOK {
                fmt.Println("this URL is valid & exists: ", string(decodedURL))
            }
        }
    }
}

Do you realise the question is tagged as Python? – MisterMiyagi Jan 15 '21 at 14:56 — MisterMiyagi, Jan 15 '21 at 14:56
Oops! Didn't realize it & I hope he gets the idea though! – Sakib Md Al Amin Jan 15 '21 at 15:02 — Sakib Md Al Amin, Jan 15 '21 at 15:02

score 0 · Answer 2 · answered Jan 15 '21 at 05:12

0

when the length of the unencoded input is not a multiple of three, the encoded output must have padding added so that its length is a multiple of four.

len(encoded) is 47, it should be 48, So append another =

encoded = b"aHR0cHM6Ly9mb3Jtcy5nbGUvWU5ZXQ0d2NRWHVLNnNwdjU=="
print(decoded)
b'https://forms.gle/YNY]\r\x1d\xd8\xd4V\x1dR\xcd\x9c\xdc\x1d\x8d'

answered Jan 15 '21 at 05:12

Sam Daniel

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That I tried. However, the URL generated is still invalid. – Saimon Jan 15 '21 at 05:13
Is there a way to detect the missing character from the original string? – Saimon Jan 15 '21 at 05:15
encode the original string to base64 and compare with your `encoded` – Sam Daniel Jan 15 '21 at 05:17

Incorrect padding while decoding a Base64 string

2 Answers2