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#JWT basics

The JWT is sent as part of the authorisation header of every API request. Typically it looks like this—

// Auth header format
Authorization: Bearer <JWT_TOKEN>
// Sample JWT encoded
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJhdWQiOiI4ZjQ3ZTQxYy00Njg0LTRhZjEtYjFiOC0yNDZjODMxMTIwMzMiLCJpYXQiOjE1NzI0OTMzNTYsImp0aSI6ImM1MzQ0N2UzLTk5M2EtNDRhZS05Yzc4LTMxZjQ1YzNjMDgyZSJ9.xfYii9lfWMHtMBgjK5U6NFN_6_Q9jw8UMQu8Jnvftbs

The JWT in itself has three parts—A header, payload, and a signature. Setu uses the following format—

// JWT format
urlsafe_base64(<JWT_HEADER>).urlsafe_base64(<JWT_PAYLOAD>).urlsafe_base64(<SIGNATURE>)
// Sample JWT decoded
{
    "alg" : "HS256",
    "typ" : "JWT"
},
{
    "aud" : "8f47e41c-4684-4af1-b1b8-246c83112033", // This is the schemeID
    "iat" : 1572493356,
    "jti" : "c53447e3-993a-44ae-9c78-31f45c3c082e"
}

This means that—

header    : eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9
payload   : eyJhdWQiOiI4ZjQ3ZTQxYy00Njg0LTRhZjEtYjFiOC0yNDZjODMxMTIwMzMiLCJpYXQiOjE1NzI0OTMzNTYsImp0aSI6ImM1MzQ0N2UzLTk5M2EtNDRhZS05Yzc4LTMxZjQ1YzNjMDgyZSJ9
signature : xfYii9lfWMHtMBgjK5U6NFN_6_Q9jw8UMQu8Jnvftbs
Header

This is a base64 encoded value. It contains details about the algorithm used to calculate the signature. This could have values such as HMAC SHA256 or RSA. For example—

// Sample JWT header
{
  "alg" : "HS256",
  "typ" : "JWT"
}
// is encoded to
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9
Payload

This is also a base64 encoded value. This is the main part of the JWT, and contains details about the entity, and the request made (also called “claims”). For example—

// Sample JWT payload
{
    "aud" : "8f47e41c-4684-4af1-b1b8-246c83112033", // This is the schemeID
    "iat" : 1572493356,
    "jti" : "c53447e3-993a-44ae-9c78-31f45c3c082e"
}
// is encoded to
eyJhdWQiOiI4ZjQ3ZTQxYy00Njg0LTRhZjEtYjFiOC0yNDZjODMxMTIwMzMiLCJpYXQiOjE1NzI0OTMzNTYsImp0aSI6ImM1MzQ0N2UzLTk5M2EtNDRhZS05Yzc4LTMxZjQ1YzNjMDgyZSJ9
  • aud is the scheme_id in JWT. The entity sending the API response shares this with the entity making the API requests. Setu would provide this value under credentials for making calls to Setu APIs. You should set this value and share it with Setu to enable Setu to make calls to your APIs.
  • iat  is the epoch timestamp in seconds, at which the request was issued. Requests older than 30 seconds are considered stale, and hence will be rejected.
  • jti is a unique ID for every request that can be used to identify the request for logging, debugging and tracking purpose. Since this is unique for every request, the same JWT token should not be reused or repeated for different requests.
Signature

Using the algorithm specified in the header, along with the encoded header, encoded payload and secret, the signature is constructed in the following way—

HMACSHA256(
    base64UrlEncode(header) + "." +
    base64UrlEncode(payload),
    secret
)

The secret is a private key shared between Setu and you, used to sign the token to verify the sender of the JWT, ie, if the claims are coming from the aud that they claim to be coming from. The request is authenticated based on both the validity of the signature, and also the verification of each claim individually. For example—

Input string   : eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJhdWQiOiI4ZjQ3ZTQxYy00Njg0LTRhZjEtYjFiOC0yNDZjODMxMTIwMzMiLCJpYXQiOjE1NzI0OTMzNTYsImp0aSI6ImM1MzQ0N2UzLTk5M2EtNDRhZS05Yzc4LTMxZjQ1YzNjMDgyZSJ9
Secret         : 7c2e036c-908f-48ba-abe3-cd8745a6fa99
HA256 Signaure : xfYii9lfWMHtMBgjK5U6NFN_6_Q9jw8UMQu8Jnvftbs

You can try this on your command line by running—

echo -n "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJhdWQiOiI4ZjQ3ZTQxYy00Njg0LTRhZjEtYjFiOC0yNDZjODMxMTIwMzMiLCJpYXQiOjE1NzI0OTMzNTYsImp0aSI6ImM1MzQ0N2UzLTk5M2EtNDRhZS05Yzc4LTMxZjQ1YzNjMDgyZSJ9.xfYii9lfWMHtMBgjK5U6NFN_6_Q9jw8UMQu8Jnvftbs" | openssl dgst -sha256 -hmac BBzKUWAZeAS2tBsk0FtJT4ep -binary|openssl base64 -e -A | sed s/+/-/ | sed -E s/=+$//

Read more about how we arrived at this command here ↗

#JWT authentication for Setu APIs

The JSON Web Token mechanism is used for securely communicating with Setu. It is an open standard for representing claims securely between two parties.

The JWT website ↗ covers the basics of how it works and the concepts involved. This guide provides a simple description of how JWT is implemented at Setu.

For Setu products, JWT keys have a one-to-one mapping with individual product configurations. Essentially, each product configuration comes with its own JWT key that a merchant can use to authorise API requests.

While we support JWT auth, we recommend using the more secure OAuth keys, which come with features like the ability to access multiple products with the same key, or regenerate and delete keys as needed and so on.

#Calling Setu-hosted APIs

A merchant will call Setu APIs for creating payment links, checking link status, for fetching reports etc and can use JWT to authenticate such requests with Setu.

If you are an Admin for your Bridge account, you should be able to see the API keys card under “Org settings” in the left panel. Click the JWT keys card to view keys for all your product configs. Read more ↗

#Sample code

Practically, you never need to worry about the encoding and decoding a JWT.

A lot of third party libraries exist that can do this for you easily —you can take a look at all the available libraries in most of the programming languages at Libraries for Token Signing/Verification section here ↗.

Setu / partner can accept a request as legitimate when the JWT format is recognised, ie, the payload is verified and the signature is valid. Requests can be rejected if—

  1. The time since generation of the token is more than 2 minutes.
  2. The aud claim is not verified.
  3. The signature is invalid.

Below here are some samples in popular languages.

Python
## pyjwt==1.7.1
import jwt
import datetime
import uuid
scheme_id = "8f47e41c-4684-4af1-b1b8-246c83112033"
secret    = "7c2e036c-908f-48ba-abe3-cd8745a6fa99"
payload   = {
    "aud" : scheme_id,
    "iat" : datetime.datetime.utcnow(),
    "jti" : str(uuid.uuid1())
}
// generate a token like this
token = jwt.encode(payload, secret, algorithm="HS256")
// And to decode the token and verify the aud claim—
try:
    # verified claim
    jwt.decode(token, secret, audience=scheme_id)
    print("Verified token")
except jwt.PyJWTError:
    # unverified claim
    print("Token error")
PHP
<?php
require __DIR__.'/vendor/autoload.php';
use FirebaseJWTJWT;
$key = "7c2e036c-908f-48ba-abe3-cd8745a6fa99";
$aud = "8f47e41c-4684-4af1-b1b8-246c83112033";
// create token
$token = array(
    "aud" => $aud,
    "iat" => 1572493356,
    "jti" => "c53447e3-993a-44ae-9c78-31f45c3c082e"
);
$jwt = JWT::encode($token, $key);
print_r($jwt);
echo '<br/>';
// Decode Logic
/**
 * You can add a leeway to account for when there is a clock skew times between
 * the signing and verifying servers. It is recommended that this leeway should
 * not be bigger than a few minutes.
 *
 * Source: http://self-issued.info/docs/draft-ietf-oauth-json-web-token.html#nbfDef
 */
JWT::$leeway = 60; // $leeway in seconds
$decoded = JWT::decode($jwt, $key, array('HS256'));
$audClaimInToken = $decoded -> aud;
print_r($audClaimInToken);
echo '<br/>';
if($audClaimInToken == $aud){
  echo "Success";
}
else{
  echo "Fail";
}
?>
C#
using System;
using System.IdentityModel.Tokens.Jwt;
using System.Security.Claims;
using System.Text;
using Microsoft.IdentityModel.Tokens;
public class JwtAuthManager
{
    static string SecretKey = "ReallyLongSecret"; // this should come from your configuration file
    static string audience = "YourAudClaim";// this should come from your configuration file
    public string GenerateJWTToken()
    {
        byte[] symmetricKey = Encoding.ASCII.GetBytes(JwtAuthManager.SecretKey);
        var token_Handler = new JwtSecurityTokenHandler();
        var now = DateTime.UtcNow;
        var securitytokenDescriptor = new SecurityTokenDescriptor
        {
            Audience = JwtAuthManager.audience,
            IssuedAt = now,
            SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(symmetricKey), SecurityAlgorithms.HmacSha256)
        };
        var stoken = token_Handler.CreateToken(securitytokenDescriptor);
        var token = token_Handler.WriteToken(stoken);
        return token;
    }
    public bool ValidateToken(string token)
    {
        var simplePrinciple = JwtAuthManager.GetPrincipal(token);
        Console.WriteLine(simplePrinciple);
        if (simplePrinciple == null)
            return false;
        // You can implement more validation to check whether username exists in your DB or not or something else. 
        return true;
    }
    public static ClaimsPrincipal GetPrincipal(string token)
    {
        try
        {
            var jwtTokenHandler = new JwtSecurityTokenHandler();
            var jwtToken = jwtTokenHandler.ReadToken(token) as JwtSecurityToken;
            if (jwtToken == null)
                return null;
            byte[] symmetricKey = Encoding.ASCII.GetBytes(JwtAuthManager.SecretKey);
            var validationParameters = new TokenValidationParameters()
            {
                ValidateAudience = true,
                RequireExpirationTime = false,
                ValidateIssuer = false,
                ValidAudience = JwtAuthManager.audience,
                IssuerSigningKey = new SymmetricSecurityKey(symmetricKey)
            };
            SecurityToken securityToken;
            var principal = jwtTokenHandler.ValidateToken(token, validationParameters, out securityToken);
            Console.WriteLine("Principle");
            Console.WriteLine(principal);
            return principal;
        }
        catch (Exception e)
        {
            Console.WriteLine(e);
            return null;
        }
    }
}
Java
package com.setu.helpers;
import com.auth0.jwt.JWT;
import com.auth0.jwt.JWTVerifier;
import com.auth0.jwt.algorithms.Algorithm;
import com.auth0.jwt.exceptions.JWTVerificationException;
import com.auth0.jwt.interfaces.DecodedJWT;
import java.util.Date;
import java.util.UUID;
public class SetuJwtHelper {
    private String schemedId;
    private String secret;
    public SetuJwtHelper(String secret, String schemedId) {
        this.schemedId = schemedId;
        this.secret = secret;
    }
    public void setSchemedId(String schemedId) {
        this.schemedId = schemedId;
    }
    public void setSecret(String secret) {
        this.secret = secret;
    }
    public String yieldBearerToken() {
        Algorithm algorithm = Algorithm.HMAC256(this.secret);
        String token = JWT.create()
                .withAudience(this.schemedId)
                .withIssuedAt(new Date())
                .withClaim("jti",  UUID.randomUUID().toString())
                .sign(algorithm);
        return "Bearer " + token;
    }
    public void verifyBearerToken (String bearerToken) throws JWTVerificationException {
        bearerToken = bearerToken.replace("Bearer ", "");
        Algorithm algorithm = Algorithm.HMAC256(this.secret);
        JWTVerifier verifier = JWT.require(algorithm)
                .acceptLeeway(10)
                .withAudience(this.schemedId)
                .build(); //Reusable verifier instance
        DecodedJWT jwt = verifier.verify(bearerToken);
    }
}

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