Understanding JWT for Connect apps

Atlassian Connect uses a technology called JWT (JSON Web Token) to authenticate apps. JSON Web Tokens (JWT) are a standard way of representing security claims between the app and the Atlassian host product. A JWT token is a signed JSON object that contains information which enables the receiver to authenticate the sender of the request.

The format of a JWT token is: <base64url-encoded header>.<base64url-encoded claims>.<signature>.

• The header specifies a very small amount of information that the receiver needs in order to parse and verify the JWT token.
• The claims are a list of assertions that the issuer is making: each asserts that a particular field has a specific value.
• The signature is computed by using an algorithm such as HMAC SHA-256 plus the header and claims sections.

For more information about the structure of a JWT token, see Manually creating a JWT.

Types of JWT token

Atlassian issues two types of JWT to Connect apps:

1. An iframe or server-to-server JWT, which includes a computed query string hash.
2. A context JWT which is not tied to a particular URL. This will include a fixed qsh claim

The Atlassian client frameworks also allow a JWT to be created and issued by the app, known as a session token. These operate similarly to context JWTs as they too are not tied to a particular URL. They will also have a fixed qsh value.

Creating a JWT token

The high-level steps in creating a JWT token are:

1. Create a header JSON object.
2. Convert the header JSON object to a UTF-8 encoded string and base64url encode it. That gives you encodedHeader.
3. Create a claims JSON object, including a query string hash.
4. Convert the claims JSON object to a UTF-8 encoded string and base64url encode it. That gives you encodedClaims.
5. Concatenate the encoded header, a period character (.) and the encoded claims set. That gives you: signingInput = encodedHeader+ "." + encodedClaims.
6. Compute the signature of signingInput using the JWT or cryptographic library of your choice. Then base64url encode it. That gives you encodedSignature.
7. Concatenate the signing input, another period character and the signature, which gives you the JWT token: jwtToken = signingInput + "." + encodedSignature

Once you have the JWT token, you can use it to make calls like the ones in these examples.

Headers example:

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POST https://<my-dev-environment>.atlassian.net/jira/rest/api/2/issue/AC-1/attachments
"Authorization" header value: "JWT <jwt-token>"

This is the preferred approach. Header values are typically not logged by intermediate proxies.

Query string example:

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GET https://<my-dev-environment>.atlassian.net/jira/rest/api/2/issue/AC-1.json?jwt=<jwt-token>

The query strings of requests may be logged by intermediate proxies which could expose the JWT, which, although it has a limited lifetime, is a secure token. For this reason, using the query string form of providing a JWT is deprecated.

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.io site provides handy web based forms for exploring Atlassian Connect JWT tokens. Bear in mind that JWTs are secure tokens and care should be taken disclosing them to 3rd party sites.

Java example

Here is an example of creating a JWT token, in Java using atlassian-jwt and nimbus-jwt (last tested with atlassian-jwt version 1.5.3 and nimbus-jwt version 2.16):

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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-addon"; //the key from the app descriptor
        String sharedSecret = "...";    //the sharedsecret key received
                                        //during the app installation handshake
        String method = "GET";
        String baseUrl = "https://<my-dev-environment>.atlassian.net/";
        String contextPath = "/";
        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

The high-level steps of decoding and verifying a JWT token are:

1. Extract the JWT token from the request's jwt query parameter or the authorization header.
2. Decode the JWT token without verification. This gives you a header JSON object, a claims JSON object, and a signature.
3. Extract the issuer (iss) claim from the decoded, unverified claims object. This is the clientKey for the tenant - an identifier for the Atlassian product making the call, which should have been stored by the app as part of the installation handshake.
4. Look up the sharedSecret for the clientKey, as stored by the app during the installation handshake
5. Verify the signature with the sharedSecret and the algorithm specified in the header's alg field. This should be the HS256 algorithm.
6. For JWTs with a computed qsh value, verify the query has not been tampered by Creating a query hash and comparing it against the qsh claim on the verified token.
7. For context JWTs and session tokens, the qsh claim will be present but will be a fixed value 'context-qsh'. Standard qsh validation will fail on such values. You must make a deliberate decision to accept such context JWTs and session tokens on a given endpoint. Do not accept context JWTs in module or lifecycle endpoints. You should only accept one type of JWT per endpoint.
8. Verify standard claims if present. Issuers include these to help you ensure that tokens you receive are used according to the intentions of the issuer and with the best possible results.

These steps must be executed before processing the request, and the request must be rejected if any of these steps fail.

Java example

Here is a minimal example of decoding and verifying a JWT token, in Java, using atlassian-jwt and nimbus-jwt (last tested with atlassian-jwt version 1.5.3 and nimbus-jwt version 2.16).

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[] base64UrlEncodedSegments = jwtToken.split('.');
String base64UrlEncodedHeader = base64UrlEncodedSegments[0];
String base64UrlEncodedClaims = base64UrlEncodedSegments[1];
String signature = base64UrlEncodedSegments[2];
String header = base64Urldecode(base64UrlEncodedHeader);
String claims = base64Urldecode(base64UrlEncodedClaims);

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);
}

Creating a query string hash

A query string hash is a signed canonical request for the URI of the API you want to call.

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qsh = `sign(canonical-request)`
canonical-request = `canonical-method + '&' + canonical-URI + '&' + canonical-query-string`

A canonical request is a normalised representation of the URI. Here is an example. For the following URL, assuming you want to do a "GET" operation:

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"https://<my-dev-environment>.atlassian.net/path/to/service?zee_last=param&repeated=parameter 1&first=param&repeated=parameter 2"

The canonical request is

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"GET&/path/to/service&first=param&repeated=parameter%201,parameter%202&zee_last=param"

To create a query string hash, follow the detailed instructions below:

1. Compute canonical method
   • Simply the upper-case of the method name (e.g. "GET" or "PUT")
2. Append the character '&'
3. Compute canonical URI
   • Discard the protocol, server, port, context path and query parameters from the full URL.
   • For requests targeting apps discard the baseUrl in the app descriptor.
   • Removing the context path allows a reverse proxy to redirect incoming requests for "jira.example.com/getsomething" to "example.com/jira/getsomething" without breaking authentication. The requester cannot know that the reverse proxy will prepend the context path "/jira" to the originally requested path "/getsomething"
   • Empty-string is not permitted; use "/" instead.
   • Url-encode any '&' characters in the path.
   • Do not suffix with a '/' character unless it is the only character. e.g.
     • Canonical URI of "https://example.atlassian.net/wiki/some/path/?param=value" is "/some/path"
     • Canonical URI of "https://example.atlassian.net" is "/"
4. Append the character '&'
5. Compute canonical query string
   • The query string will use percent-encoding.
   • Sort the parameters primarily by their percent-encoded names and secondarily by their percent-encoded values.
     • Include all the parameters from query string if it is a GET request.
     • Include all the post data in payload if it is a POST request.
   • Sorting is by codepoint: sort(["a", "A", "b", "B"]) => ["A", "B", "a", "b"]
   • For each parameter append its percent-encoded name, the '=' character and then its percent-encoded value. If the parameter has no value, the '=' character must still be included: &a=&b=foo&c=
   • In the case of repeated parameters, append the encoded ',' character (i.e., "%2C") and subsequent percent-encoded values.
   • Ignore the jwt parameter, if present.
   • Some particular values to be aware of: - A whitespace character is encoded as "%20", - "," as "%2C", - "+" as "%2B", - "*" as "%2A" and - "~" as "~".
     (These values used for consistency with OAuth1.)
6. Convert the canonical request string to bytes
   • The encoding used to represent characters as bytes is UTF-8
7. Hash the canonical request bytes using the SHA-256 algorithm
   • e.g. The SHA-256 hash of "foo" is "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae"

As discussed above, context JWTs and session tokens will have a qsh claim, but it will contain a fixed value and will not validate against the above procedure.

Manually creating a JWT

A JWT token looks like this:

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eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJleHAiOjEzODY4OTkxMzEsImlzcyI6ImppcmE6MTU0ODk1OTUiLCJxc2giOiI4MDYzZmY0Y2ExZTQxZGY3YmM5MGM4YWI2ZDBmNjIwN2Q0OTFjZjZkYWQ3YzY2ZWE3OTdiNDYxNGI3MTkyMmU5IiwiaWF0IjoxMzg2ODk4OTUxfQ.uKqU9dTB6gKwG6jQCuXYAiMNdfNRw98Hw_IWuA5MaMo

The basic format is:

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<base64url-encoded header>.<base64url-encoded claims>.<base64url-encoded signature>

In other words:

• You create a header object, with the JSON format. Then you encode it in base64url
• You create a claims object, with the JSON format. Then you encode it in base64url
• You create a signature for the URI (we'll get into that later). Then you encode it in base64url
• 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:

Header

The header object declares the type of the encoded object and the algorithm used for the cryptographic signature. Atlassian Connect always uses the same values for these. The typ property will be "JWT" and the alg property will be "HS256".

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

Claims

The claims object contains security information about the message you're transmitting. The attributes of this object provide information to ensure the authenticity of the claim. The information includes the issuer, when the token was issued, when the token will expire, and other contextual information, described below.

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{
    "iss": "jira:1234567",
    "iat": 1300819370,
    "exp": 1300819380,
    "qsh": "8063ff4ca1e41df7bc90c8ab6d0f6207d491cf6dad7c66ea797b4614b71922e9",
    "sub": "123456:1234abcd-1234-abcd-1234-1234abcd1234"
}

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).

Signature

The signature of the token is a combination of a hashing algorithm combined with the header and claims sections of the token. This provides a way to verify that the claims and headers haven't been been compromised during transmission. The signature will also detect if a different secret is used for signing. In the JWT spec, there are multiple algorithms you can use to create the signature, but Atlassian Connect uses the HMAC SHA-256 algorithm. If the JWT token has no specified algorithm, you should discard that token as they're not able to be signature verified.

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 =
            "https://<my-dev-environment>.atlassian.net/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;
    }
}