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c#jwtbase64signatureverify

verify signature of JWT Token c#

I have some problems to verify the signature of a JWT token I get. The token is signed with HS256. The code where I try to create a signature to proof the received one is:

JwtSecurityToken token = tokenHandler.ReadJwtToken(tokenString);

byte[] keyBytes = Encoding.UTF8.GetBytes("secret");

HMACSHA256 hmac = new HMACSHA256(keyBytes);
byte[] signatureBytes = hmac.ComputeHash(Encoding.UTF8.GetBytes(token.RawHeader + "." + token.RawPayload));
string signature = Convert.ToBase64String(signatureBytes);

The signature I get from the received token is for example:

pYscLlinuNhO-sFyEIRRLZP7yrl8GopGJ3I6QSxg2tU

But the signature I get from my algorithm is in this case:

pYscLlinuNhO+sFyEIRRLZP7yrl8GopGJ3I6QSxg2tU=

So the signatures are close, but not equal. I don't get what I'm doing wrong at the verification of the signature. Letters and numbers seems to be correct every time but special characters are mostly different and there is always a '=' at the end of the signature. Maybe someone knows what I'm doing wrong.

Solution

  • The three parts of a JWT are Base64Url encoded:

    A JWT is represented as a sequence of URL-safe parts separated by period ('.') characters. Each part contains a base64url-encoded value.

    But you used Base64 encoding. Base64Url uses '-' and '_' instead of '+' and '/' and also omits the padding '=' on the end.

    Here is an example how to convert the base64 to bas64url encoding in C#