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How to check if JWT Token is Tampered ?

JWT Tokens has three parts:
  1. Header
  2. Pay Load
  3. Signature

Header includes the encryption algorithm and token type.
e.g:
{
   "alg" : "HS256",
   "typ" : "JWT"
}

 Signature is created using base64url encoding the header and payload and concatenating them with a period(.) as a seperator.
Take the signature of the token and decode it from base64, take the encryption algorithm from the header and generate the signature for the base64 encoded header + '.' + base64 encoded payload.
If the signature received and calculated are matching then nobody has tampered the JWT.


- Mayank Gupta

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