Convert.FromBase64string is throwing invalid Base-64 exception when decoding an encoded JSON string from a jwt payload with integer values

Question

I am writing a function to manually verify and get the header and payload pairs of a jwt access_token i am testing with the sample token available at https://jwt.io/ My code is very simple at this point

string[] tokenSegments = Token.Split('.');
string tokenHeaders = Encoding.UTF8.GetString(Convert.FromBase64String(tokenSegments[0]));
string tokenPayload = Encoding.UTF8.GetString(Convert.FromBase64String(tokenSegments[1]));

when it reaches the tokenPayloadLine im getting the following exception

System.FormatException: The input is not a valid Base-64 string as it contains >a non-base 64 character, more than two padding characters, or an illegal >character among the padding characters. at System.Convert.FromBase64CharPtr(Char* inputPtr, Int32 inputLength) at System.Convert.FromBase64String(String s)

and the exception is being thrown at this level

Convert.FromBase64String(tokenSegments[1])

please note that i can get the header values without any exception

i have tried and edited the Token to change the integer value available at the payload to make it a string and then i was successful at getting the payload

just a comparison the encoded string that is throwing the exception is

eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ

and the one that is working fine is

eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoiMTUxNjIzOTAyMiJ9

if you decode both of them you can find that the only difference is the integer value causing the exception

am i doing something wrong? is this not an intended use for the Convert Function? any thought or suggestion would be helpful also please note that i need to do this manually as i am fully aware of the jwt libraries available online.

I would have expected an invalid length exception, not this one. — bommelding, Jan 16 '19 at 11:30
JWT doesn't use plain Base64, it uses something called [base64url encoding](https://brockallen.com/2014/10/17/base64url-encoding/). The `Convert` methods can't handle that (but the page I've linked to helpfully includes some code, and links to library methods). — Jeroen Mostert, Jan 16 '19 at 11:39
Possible duplicate of [TextEncodings.Base64Url.Decode vs Convert.FromBase64String](https://stackoverflow.com/questions/41685931/textencodings-base64url-decode-vs-convert-frombase64string) — Jeroen Mostert, Jan 16 '19 at 11:59

score 17 · Accepted Answer · edited Jun 27 '19 at 11:36

17

Base64 needs padding with = at the end, not every tool adds those.

string  s = // some base64 witout padding;
s = s.PadRight(s.Length + (s.Length * 3) % 4, '=');  // add padding
var data = Convert.FromBase64String(s);

edited Jun 27 '19 at 11:36

GTInsight

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answered Jan 16 '19 at 11:45

bommelding

2,969
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Thanks, this worked for me. Can anyone add some explanation to this answer? I don't fully understand what this is doing. – JakeD Jul 01 '22 at 15:38

Convert.FromBase64string is throwing invalid Base-64 exception when decoding an encoded JSON string from a jwt payload with integer values

1 Answers1