jwt token decoding even when wrong token is provided for some letters

[flyingboy007]

I was just trying out jwt tokens on a rails app. Using this jwt library.

JWT.encode({sss: "333"}, 'SECRET_KEY')

 #returned below token
"eyJhbGciOiJIUzI1NiJ9.eyJzc3MiOiIzMzMifQ.CwX_1FztYHVpyx_G27u938SceilsVc5AB5Akwqlo2HA"

Then I decoded using the above token

JWT.decode("eyJhbGciOiJIUzI1NiJ9.eyJzc3MiOiIzMzMifQ.CwX_1FztYHVpyx_G27u938SceilsVc5AB5Akwqlo2HA", 'SECRET_KEY')

#returns below response correctly
[{"sss"=>"333"}, {"alg"=>"HS256"}]

But if I try to change the last letter of the token to B instead of current A it is still returning the same response which is weird

 JWT.decode("eyJhbGciOiJIUzI1NiJ9.eyJzc3MiOiIzMzMifQ.CwX_1FztYHVpyx_G27u938SceilsVc5AB5Akwqlo2HB", 'SECRET_KEY')

#Getting this response even though the token I provided is wrong
[{"sss"=>"333"}, {"alg"=>"HS256"}]

Actually I am getting the same response for all characters upto D

If I use F and others above then its showing error as expected

>> JWT.decode("eyJhbGciOiJIUzI1NiJ9.eyJzc3MiOiIzMzMifQ.CwX_1FztYHVpyx_G27u938SceilsVc5AB5Akwqlo2HF", 'SECRET_KEY')
JWT::VerificationError (Signature verification raised)
	from (irb):34

What could be the reason for this? Is it a bug or am I doing something wrong here?

[flyingboy007]

The reason is the base64url encoding. The three parts of a JWT are all base64url encoded. Base64 encoding transforms the input data to a 6-Bit representation, mapped to a set of 64 ASCII characters. If you have 3 bytes source data, the base64 encoded result is 4 characters long, each character representing a 6 bit value, so 4 * 6 bits = 24 bits.

In your case, the encoded signature has 43 characters, which means 43 * 6 = 258 bits.
So you could theoretically encode 258 bits, but the signature is only 256 bits (32 byte) long, which means there are 2 insignificant bits on the end.

A look on the base64 encoding table shows that 'A' to 'D' represent the 6 bit values 0 (000000) to 4 (000011), so the first four bits, which are still significant, are all identical, and only the last two, insignificant bits are changing. But the character 'E' stands for 5 (000100) and would change the last bit of the 256 bit value.

Conclusion: it's all fine, nothing wrong here, it works as expected.

Credits: Thank you JPS for giving the answer in SO.

[jamb1975]

Base64.Decoder decoder = Base64.getUrlDecoder();
log.info("Payload Json-> {}", decoder.decode(token.split("\.")[0]) );