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A JSON Web Token is supposed to have the following format:

token = encodeBase64(header) + '.' + encodeBase64(payload) + '.' + encodeBase64(signature)

When I use the jsonwebtoken node.js function to create a token:

jsonwebtoken.sign({username : "admin", password : "admin"}, publicKey, { algorithm:'HS256' });

It produces the following output:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VybmFtZSI6ImFkbWluIiwicGFzc3dvcmQiOiJhZG1pbiIsImlhdCI6MTY2OTIxMDE0MH0.Cj2-vgNkw2xChXMe5YjIrH9UYH6-pL7ArSERBVJO-zE

When you decode the header, you get:

'{"alg":"HS256","typ":"JWT"}'

But decoding the payload gives a padding error because the padding has been omitted. Only when the payload is changed to (correct number of '=' added):

eyJ1c2VybmFtZSI6ImFkbWluIiwicGFzc3dvcmQiOiJhZG1pbiIsImlhdCI6MTY2OTIxMDE0MH0=

Does it decode correctly to:

'{"username":"admin","password":"admin","iat":1669210140}'

And the signature cannot be decoded because it contains multiple '-', which is an invalid base64 character.

My question is this:

If each section is meant to be base64 encoded, then why is the padding ommitted from the payload, and what is the strange syntax of the signature?

I tried finding information on the jsonwebtoken GitHub repository , but couldn't find an explanation.

EDIT: After doing some more research, I discovered that JWT uses Base64URL encoding, not Base64 encoding. Will update this post once I have more information.

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1 Answer 1

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JWTs are not encoded in plain Base64, but in Base64url. For this version, padding is optional, and it uses different charset: '-' instead of '+' and '_' instead of '/'.

Sources:

https://jwt.io/introduction https://en.wikipedia.org/wiki/Base64#Variants_summary_table

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