How to decrypt Authentication Cookie From Javascript

Question

Say I have some Form's authentication Cookie:

FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(1,
  "TESTTEST",
  DateTime.Now,
  DateTime.Now.AddMinutes(30),
  false,
  String.Empty,
  FormsAuthentication.FormsCookiePath);

Normally in C# we can decrypt this like so:

var authToken = FormsAuthentication.Decrypt(authCookie.Value);

I'm creating a tool for testing an I want to decrypt the cookie on the client-side.

If my machine key is: GHFDK45sDFGSKj234 How can I decrypt the Authentication from Javascript?

First you need to know the algorithm that the forms authentication is using. Then you need to find a javascript library that can decrypt that algorithm. — Callback Kid
– Callback Kid, Commented May 10, 2017 at 18:48
Thanks, I was trying to find that, so you can specify the encryption, and it defaults to SHA1 — johnny 5
– johnny 5, Commented May 10, 2017 at 18:50
@CallbackKid if you post that as an answer I'll mark it correct — johnny 5
– johnny 5, Commented May 10, 2017 at 18:50
@VinodSrivastav yeah that was my mistake the SHA-1 is for tampervalidation, I think the encryption is AES — johnny 5
– johnny 5, Commented May 10, 2017 at 19:08

Community · Accepted Answer · 2017-05-23 12:18:11Z

1

First you need to know the algorithm that the forms authentication is using. Then you need to find a javascript library that can decrypt that algorithm

edited May 23, 2017 at 12:18

CommunityBot

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answered May 10, 2017 at 19:09

Callback Kid

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2 Comments

Mad Myche Over a year ago

Is this cookie even javascript accessible? Do we really want to have the machine keys available to a web client?

Callback Kid Over a year ago

@MadMyche He says he's creating a testing tool. I doubt this will be in his production environment

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