JSON Web Token

JWT stands for JSON Web Token. In simple words, it is an effective way of transmitting information between parties on internet.

Let us first try to understand the purpose of tokens in general with an analogy of usage of tokens at some candy shop. You first pay for the items that you want to buy to the shop owner at the counter who then provides you some coin/rectangular token which you take to the vendor who verifies the token and only if the token is valid, gives you your items.

Few points to note in this example: –

1. The shop owner only has the duty of receiving the payment for user’s demanded items and then provide him a token which kind of authenticates him to purchase the said items.
2. The shop vendor has the duty to verify the token and provide the items. If you produce some token which he doesn’t recognize, he would deny your request.

Similarly, in most of today’s web application implementations, the authentication servers and the resource servers are maintained separately. The job of Authentication server is to authenticate a user once (May be using his username and password) and provide him with a token (serving as a delegated access permission), which can then be produced at the resource server to get access to said resources. Obviously, the token is first verified at the resource server, as to be granted by a legitimate authentication server.

JWT is one of the ways in which a server can generate the token. Between, as per OpenID connect specification, ID_Token can only be generated in JWT format.

Let’s dig a little deeper into the format of JWT: –

• It is string value with three parts, separated with ‘.’

Example:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.
eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.
SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

• Part 1: A Base64URL encoded value of a JSON payload specifying header, typically providing some metadata about the token.

Example:
{
“alg”: “RS256”,
“typ”: “JWT”
}

Here, alg (algorithm used to sign the JWT) and typ (denoting type of the token) are reserved keywords.

• Part 2: Base64URL encoded value of a json payload referred to as claims. It is used to provide user information.

Example:

{
“sub”: “1234567890”,”name”: “John Doe”,
“iat”: 1516239022
}

Note that the key words ‘sub’ (subject denoting the technical identifier of the user) and ‘iat’ (issued at denoting the time of issue of the token) are reserved claim names. One can also use private/public claims as far as they are not contradicting with reserved claims.

• Part 3: It is Message Authentication code (MAC) also called digital signature. It is created using some algorithm over the header and payload information to generate a string. Two of the famous algorithms are-
  1. HS256: On high level it combines the header, the payload and a secret private key (specific to the client) and hashes them together using some cryptographic hashing function (Ex- SHA-256) to produce an irreversible (contrary to encryption, which can be decrypted) string value. The resource server also is in possession of the same private secret key, which it can use to regenerate the signature to verify the authenticity of the token.
  2. RS256: One drawback of above algorithm is that both the authentication and resource server need to maintain the client private secret keys for verification purpose – which is an overhead and a bit risky from the point of safeguarding the secrets at multiple places. RS256 signatures use a particular type of keys, called RSA Keys. RSA is the name of an encryption/decryption algorithm that takes one key to encrypt and a second key to decrypt.

Note that RSA is not a Hashing function, because by definition the output of encryption can be reversed, and we can get back the initial result. In RS256 – we take the Header and the Payload, and encrypt it using RSA with the private key.

However, the process of RSA encryption is little slow and because of this at times, where the size of payload is considerable large, we might want to use some hybrid algorithm like RSA-SHA256, which combined the benefits of SHA256 hashing function to hash header and payload first (which reduces the size of the data) and then encrypting it using the RSA private key. Typical decoded JWT token’s signature part would look something like:

RSASHA256(
base64UrlEncode(header) + “.” +
base64UrlEncode(payload),
—–BEGIN PUBLIC KEY—–
Some valid public RSA key
—–END PUBLIC KEY—–
)