Javascript atob(str) equivilent in c#

Question

I did this:

 byte[] data = Convert.FromBase64String(str);
 string decodedString = Encoding.UTF8.GetString(data);
 Console.WriteLine(decodedString);

but got Unhandled Exception: System.FormatException: Invalid length for a Base-64 char array or string.

In javascript using atob(str) gives me the right decoded str.

javascript console:

atob("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")
"{"iss":"https://identity-staging.ascend.xyz","aud":"https://identity-staging.ascend.xyz/resources","client_id":"6994A4A8-0E65-4FED-A82B-C684A0DD1758","scope":["openid","profile","sub.read","data.write","data.read","alg.execute"],"sub":"377c095b-783b-47e7-97b1-5aed98c038fc","amr":"external","auth_time":1407615507,"idp":"https://sts.windows.net/0840c760-6f7b-4556-b337-8c090e2d458d/","name":"[email protected]","exp":1407836711,"nbf":1407833111}"

I will update the question to leave that out. yes its a array. the parts[1] is the same string i tested with in javascript that works — Poul K. Sørensen
– Poul K. Sørensen, Commented Aug 12, 2014 at 9:13
how is the string defined, are you sure the string is properly escaped? — Gary.S
– Gary.S, Commented Aug 12, 2014 at 9:16
I just copy pasted the same string that i tested in javascript into c# code. str = "my_jwt_token"; Will update the question again to show code — Poul K. Sørensen
– Poul K. Sørensen, Commented Aug 12, 2014 at 9:22
Reference: developer.mozilla.org/en-US/docs/Web/API/Window/atob — Mark Schultheiss
– Mark Schultheiss, Commented Sep 5 at 16:47

Poul K. Sørensen · Accepted Answer · 2014-08-12 09:27:22Z

16

    var str = "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";
    int mod4 = str.Length % 4;
    if (mod4 > 0)
    {
        str += new string('=', 4 - mod4);
    }

solved it in c#

answered Aug 12, 2014 at 9:27

Poul K. Sørensen

17.7k24 gold badges141 silver badges296 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Paul Alexander Over a year ago

would I need to use this on both encoding and decoding sides? or just decoding?

Poul K. Sørensen Over a year ago

just for decoding, its due to that base64 string can omit the == at the end, but c# expect the length to be divisible by 4 (hence the often == at the end).

Junior Mayhe Over a year ago

This fixes a broken encoded input string like fC4wMDI0MDIyNYMyNTczRIrXALxhHLMzMx and convert it to a valid encoded fC4wMDI0MDIyNYMyNTczRIrXALxhHLMzMx==. Now I am able to use System.Text.Encoding.UTF8.GetString(System.Convert.FromBase64String(encoedString))

vibs2006 · Accepted Answer · 2025-09-05 13:31:39Z

12

Use javascript's window.btoa function to encode the string in Base 64 Format in Javascript Frontend UI. To decode back same string in C# (the equivalent of javascript's window.atob function) please see the following code.

Sample usage of javascripts btoa method as shown as below
var encodedBase64String = window.btoa("base64 encoded string");

(Most probably you are trying to post back data that might contain special characters or line breaks [e.g input from a rich text editor etc] back to the controller or you might creating custom basic basic authentication filter on server side)

C# Code Below

string base64Encoded = "YmFzZTY0IGVuY29kZWQgc3RyaW5n";
string base64Decoded;
byte[] data = System.Convert.FromBase64String(base64Encoded);
base64Decoded = System.Text.UTF8Encoding.Default.GetString(data);
Console.WriteLine(base64Decoded)

You can see its working sample at https://dotnetfiddle.net/abxwSw

edited Sep 5 at 13:31

answered Jun 9, 2018 at 17:37

vibs2006

6,6524 gold badges42 silver badges44 bronze badges

1 Comment

user3497034 Over a year ago

Thank you ! its working like a charm...Thanks

Collectives™ on Stack Overflow

Javascript atob(str) equivilent in c#

2 Answers 2

3 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related