Understanding JWT for apps

There is a nicely presented copy of the specification. JSON Web Token (JWT) is a compact URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JavaScript Object Notation (JSON) object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or MACed and/or encrypted.

Structure of a JWT token

A JWT token looks like this:

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eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJleHAiOjEzODY4OTkxMzEsImlzcyI6ImppcmE6MTU0ODk1OTUiLCJxc2giOiI4MDYzZmY0Y2ExZTQxZGY3YmM5MGM4YWI2ZDBmNjIwN2Q0OTFjZjZkYWQ3YzY2ZWE3OTdiNDYxNGI3MTkyMmU5IiwiaWF0IjoxMzg2ODk4OTUxfQ.uKqU9dTB6gKwG6jQCuXYAiMNdfNRw98Hw_IWuA5MaMo

Once you understand the format, it's actually pretty simple:

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<base64-encoded header>.<base64-encoded claims>.<base64-encoded signature>

In other words:

• You create a header object, with the JSON format, then you encode it as a base64.
• You create a claims object, with the JSON format, then you encode it in base64.
• You create a signature for the URI (we'll get into that later), then you encode it in base64.
• You concatenate the three items, with the "." separator.

You shouldn't actually have to do this manually, as there are libraries available in most languages, as we describe in the JWT libraries section. However it is important you understand the fields in the JSON header and claims objects described in the next sections.

JWT header

The JWT header declares that the encoded object is a JSON Web Token (JWT) and the JWT is a JWS that is MACed using the HMAC SHA-256 algorithm. For example:

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

Claims

The JWT claims object contains security information about the message. For example:

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{
    'iss': 'com.example.app',
    'iat': 1433780963,
    'qsh': '88396352255a6c933def07620a3281c9e27d8c668e0f4d01a8ecdbb74ca52c97',
    'sub': 'connection:479',
    'exp': 1437380963
}

You should use a little leeway when processing time-based claims, as clocks may drift apart. The JWT specification suggests no more than a few minutes. Judicious use of the time-based claims allows for replays within a limited window. This can be useful when all or part of a page is refreshed or when it is valid for a user to repeatedly perform identical actions (e.g., clicking the same button).

JWT libraries

Most modern languages have JWT libraries available. We recommend you use one of these libraries (or other JWT-compatible libraries) before trying to hand-craft the JWT token.

The JWT decoder is a handy web-based decoder for Atlassian Connect JWT tokens.

Creating a JWT token

Here is an example of creating a JWT token, in Java, using atlassian-jwt and nimbus-jwt:

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import java.io.UnsupportedEncodingException;
import java.security.NoSuchAlgorithmException;
import java.util.HashMap;
import com.atlassian.jwt.*;
import com.atlassian.jwt.core.writer.*;
import com.atlassian.jwt.httpclient.CanonicalHttpUriRequest;
import com.atlassian.jwt.writer.JwtJsonBuilder;
import com.atlassian.jwt.writer.JwtWriterFactory;

public class JWTSample {

    public String createUriWithJwt()
            throws UnsupportedEncodingException, NoSuchAlgorithmException {
        long issuedAt = System.currentTimeMillis() / 1000L;
        long expiresAt = issuedAt + 180L;
        String key = "atlassian-connect-app"; //the key from the app descriptor
        String sharedSecret = "...";    //the sharedsecret key received
                                        //during the app installation handshake
        String method = "GET";
        String baseUrl = "http://localhost:2990/jira";
        String contextPath = "/jira";
        String apiPath = "/rest/api/latest/serverInfo";

        JwtJsonBuilder jwtBuilder = new JsonSmartJwtJsonBuilder()
                .issuedAt(issuedAt)
                .expirationTime(expiresAt)
                .issuer(key);

        CanonicalHttpUriRequest canonical = new CanonicalHttpUriRequest(method,
                apiPath, contextPath, new HashMap());
        JwtClaimsBuilder.appendHttpRequestClaims(jwtBuilder, canonical);

        JwtWriterFactory jwtWriterFactory = new NimbusJwtWriterFactory();
        String jwtbuilt = jwtBuilder.build();
        String jwtToken = jwtWriterFactory.macSigningWriter(SigningAlgorithm.HS256,
                sharedSecret).jsonToJwt(jwtbuilt);

        String apiUrl = baseUrl + apiPath + "?jwt=" + jwtToken;
        return apiUrl;
    }
}

Decoding and verifying a JWT token

Here is a minimal example of decoding and verifying a JWT token, in Java, using atlassian-jwt and nimbus-jwt. Note: This example does not include any error handling. See AbstractJwtAuthenticator from atlassian-jwt for recommendations of how to handle the different error cases.

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import com.atlassian.jwt.*;
import com.atlassian.jwt.core.http.JavaxJwtRequestExtractor;
import com.atlassian.jwt.core.reader.*;
import com.atlassian.jwt.exception.*;
import com.atlassian.jwt.reader.*;
import javax.servlet.http.HttpServletRequest;
import java.io.UnsupportedEncodingException;
import java.security.NoSuchAlgorithmException;
import java.util.Map;

public class JWTVerificationSample {

    public Jwt verifyRequest(HttpServletRequest request,
                             JwtIssuerValidator issuerValidator,
                             JwtIssuerSharedSecretService issuerSharedSecretService)
            throws UnsupportedEncodingException, NoSuchAlgorithmException,
                   JwtVerificationException, JwtIssuerLacksSharedSecretException,
                   JwtUnknownIssuerException, JwtParseException {
        JwtReaderFactory jwtReaderFactory = new NimbusJwtReaderFactory(
                issuerValidator, issuerSharedSecretService);
        JavaxJwtRequestExtractor jwtRequestExtractor = new JavaxJwtRequestExtractor();
        CanonicalHttpRequest canonicalHttpRequest
                = jwtRequestExtractor.getCanonicalHttpRequest(request);
        Map<String, ? extends JwtClaimVerifier> requiredClaims = JwtClaimVerifiersBuilder.build(canonicalHttpRequest);
        String jwt = jwtRequestExtractor.extractJwt(request);
        return jwtReaderFactory.getReader(jwt).readAndVerify(jwt, requiredClaims);
    }
}

Decoding a JWT token

Decoding the JWT token reverses the steps followed during the creation of the token, to extract the header, claims and signature. Here is an example in Java:

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String jwtToken = ...;//e.g. extracted from the request
String[] base64EncodedSegments = jwtToken.split('.');
String base64EncodedHeader = base64EncodedSegments[0];
String base64EncodedClaims = base64EncodedSegments[1];
String signature = base64EncodedSegments[2];
String header = base64decode(base64EncodedHeader);
String claims = base64decode(base64EncodedClaims);

This gives us the following:

Header:

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

Claims:

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{
    "iss": "jira:15489595",
    "iat": 1386898951,
    "qsh": "8063ff4ca1e41df7bc90c8ab6d0f6207d491cf6dad7c66ea797b4614b71922e9",
    "exp": 1386899131
}

Signature:

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uKqU9dTB6gKwG6jQCuXYAiMNdfNRw98Hw_IWuA5MaMo

Verifying a JWT token

JWT libraries typically provide methods to be able to verify a received JWT token. Here is an example using nimbus-jose-jwt and json-smart:

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import com.nimbusds.jose.JOSEException;
import com.nimbusds.jose.JWSObject;
import com.nimbusds.jose.JWSVerifier;
import com.nimbusds.jwt.JWTClaimsSet;
import net.minidev.json.JSONObject;

public JWTClaimsSet read(String jwt, JWSVerifier verifier) throws ParseException, JOSEException
{
    JWSObject jwsObject = JWSObject.parse(jwt);

    if (!jwsObject.verify(verifier))
    {
        throw new IllegalArgumentException("Fraudulent JWT token: " + jwt);
    }

    JSONObject jsonPayload = jwsObject.getPayload().toJSONObject();
    return JWTClaimsSet.parse(jsonPayload);
}

Advanced: Creating a JWT token manually

More details on JWT tokens

The format of a JWT token is simple: <base64-encoded header>.<base64-encoded claims>.<signature>.

• Each section is separated from the others by a period character (.).
• Each section is base-64 encoded, so you will need to decode each one to make them human-readable.
• The header specifies a very small amount of information that the receiver needs in order to parse and verify the JWT token.
• All JWT token headers state that the type is "JWT".
• The algorithm used to sign the JWT token is needed so that the receiver can verify the signature.
• The claims are a list of assertions that the issuer is making: each says that "this named field" has "this value".
• Some, like the "iss" claim, which identifies the issuer of this JWT token, have standard names and uses.
• Others are custom claims. We limit our use of custom claims as much as possible, for ease of implementation.
• The signature is computed by using an algorithm such as HMAC SHA-256 plus the header and claims sections.
• The receiver verifies that the signature must have been computed using the genuine JWT header and claims sections, the indicated algorithm and a previously established secret.
• An attacker tampering with the header or claims will cause signature verification to fail.
• An attacker signing with a different secret will cause signature verification to fail.
• There are various algorithm choices legal in the JWT spec. In atlassian-connect version 1.0 we support HMAC SHA-256. Important: your implementation should discard any JWT tokens which specify alg: none as these are not subject to signature verification.

Steps to Follow

1. Create a header JSON object.

• Convert the header JSON object to a UTF-8 encoded string and base-64 encode it. That gives you encodedHeader.
• Create a claims JSON object, including a query string hash
• Convert the claims JSON object to a UTF-8 encoded string and base-64 encode it. That gives you encodedClaims.
• Concatenate the encoded header, a period character (.) and the encoded claims set. That gives you signingInput = encodedHeader + "." + encodedClaims.
• Compute the signature of signingInput using the JWT or cryptographic library of your choice. Then base64 encode it. That gives you encodedSignature.
• Concatenate the signing input, another period character and the signature, which gives you the JWT token. jwtToken = signingInput + "." + encodedSignature.

Example

Here is an example in Java using gson, commons-codec, and the Java security and crypto libraries:

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public class JwtClaims {
     protected String iss;
     protected long iat;
     protected long exp;
     protected String qsh;
     protected String sub;
    // + getters/setters/constructors
}

[...]

public class JwtHeader {
    protected String alg;
    protected String typ;
     // + getters/setters/constructors
}

[...]

import static org.apache.commons.codec.binary.Base64.encodeBase64URLSafeString;
import static org.apache.commons.codec.binary.Hex.encodeHexString;
import java.io.UnsupportedEncodingException;
import java.security.*;
import javax.crypto.*;
import javax.crypto.spec.SecretKeySpec;
import com.google.gson.Gson;

public class JwtBuilder {

    public static String generateJWTToken(String requestUrl, String canonicalUrl,
        String key, String sharedSecret)
                 throws NoSuchAlgorithmException, UnsupportedEncodingException,
                 InvalidKeyException {

        JwtClaims claims = new JwtClaims();
        claims.setIss(key);
        claims.setIat(System.currentTimeMillis() / 1000L);
        claims.setExp(claims.getIat() + 180L);

        claims.setQsh(getQueryStringHash(canonicalUrl));
        String jwtToken = sign(claims, sharedSecret);
        return jwtToken;
    }

    private static String sign(JwtClaims claims, String sharedSecret)
            throws InvalidKeyException, NoSuchAlgorithmException {
         String signingInput = getSigningInput(claims, sharedSecret);
         String signed256 = signHmac256(signingInput, sharedSecret);
         return signingInput + "." + signed256;
     }

     private static String getSigningInput(JwtClaims claims, String sharedSecret)
            throws InvalidKeyException, NoSuchAlgorithmException {
         JwtHeader header = new JwtHeader();
         header.alg = "HS256";
         header.typ = "JWT";
         Gson gson = new Gson();
         String headerJsonString = gson.toJson(header);
         String claimsJsonString = gson.toJson(claims);
         String signingInput = encodeBase64URLSafeString(headerJsonString
                 .getBytes())
                 + "."
                 + encodeBase64URLSafeString(claimsJsonString.getBytes());
         return signingInput;
     }

     private static String signHmac256(String signingInput, String sharedSecret)
            throws NoSuchAlgorithmException, InvalidKeyException {
        SecretKey key = new SecretKeySpec(sharedSecret.getBytes(), "HmacSHA256");
        Mac mac = Mac.getInstance("HmacSHA256");
        mac.init(key);
        return encodeBase64URLSafeString(mac.doFinal(signingInput.getBytes()));
    }

    private static String getQueryStringHash(String canonicalUrl)
            throws NoSuchAlgorithmException,UnsupportedEncodingException {
        MessageDigest md = MessageDigest.getInstance("SHA-256");
         md.update(canonicalUrl.getBytes("UTF-8"));
         byte[] digest = md.digest();
         return encodeHexString(digest);
     }
 }

[...]

public class Sample {
    public String getUrlSample() throws Exception {
        String requestUrl =
            "http://localhost:2990/jira/rest/atlassian-connect/latest/license";
        String canonicalUrl = "GET&/rest/atlassian-connect/latest/license&";
        String key = "...";     //from the app descriptor
                            //and received during installation handshake
        String sharedSecret = "..."; //received during installation Handshake

        String jwtToken = JwtBuilder.generateJWTToken(
            requestUrl, canonicalUrl, key, sharedSecret);
        String restAPIUrl = requestUrl + "?jwt=" + jwtToken;
        return restAPIUrl;
    }
}