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hawk Logo

Hawk is an HTTP authentication scheme using a message authentication code (MAC) algorithm to provide partial HTTP request cryptographic verification. For more complex use cases such as access delegation, see Oz.

Current version: 0.1.0

Build Status

Table of Content

Introduction

Hawk is an HTTP authentication scheme providing a method for making authenticated HTTP requests with partial cryptographic verification of the request, covering the HTTP method, request URI, and host.

Similar to the HTTP Basic access authentication scheme, the Hawk scheme utilizes a set of client credentials which include an identifier and key. However, in contrast with the Basic scheme, the key is never included in authenticated requests but is used to calculate a request MAC value which is included instead.

The Hawk scheme requires the establishment of a shared symmetric key between the client and the server, which is beyond the scope of this module. Typically, the shared credentials are established via an initial TLS-protected phase or derived from some other shared confidential information available to both the client and the server.

The primary design goals of this mechanism are to:

  • simplify and improve HTTP authentication for services that are unwilling or unable to employ TLS for every request,
  • secure the shared credentials against leakage when sent over a secure channel to the wrong server (e.g., when the client uses some form of dynamic configuration to determine where to send an authenticated request), and
  • mitigate the exposure of credentials sent to a malicious server over an unauthenticated secure channel due to client failure to validate the server's identity as part of its TLS handshake.

Unlike the HTTP Digest authentication scheme, Hawk provides limited protection against replay attacks which does not require prior interaction with the server. Instead, the client provides a timestamp which the server can use to prevent replay attacks outside a narrow time window. Also unlike Digest, this mechanism is not intended to protect the key itself (user's password in Digest) because the client and server both have access to the key material in the clear.

Usage Example

Server code:

var Http = require('http');
var Hawk = require('../lib/hawk');


// Credentials lookup function

var credentialsFunc = function (id, callback) {

    var credentials = {
        key: 'werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn',
        algorithm: 'hmac-sha-256',
        user: 'Steve'
    };

    return callback(null, credentials);
};

// Create HTTP server

var handler = function (req, res) {

    Hawk.authenticate(req, credentialsFunc, function (err, isAuthenticated, credentials, ext) {

        res.writeHead(isAuthenticated ? 200 : 401, { 'Content-Type': 'text/plain' });
        res.end(isAuthenticated ? 'Hello ' + credentials.user : 'Shoosh!');
    });
};

Http.createServer(handler).listen(8000, 'example.com');

Client code:

var Request = require('request');
var Hawk = require('../lib/hawk');


// Client credentials

var credentials = {
    id: 'dh37fgj492je',
    key: 'werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn',
    algorithm: 'hmac-sha-256'
}

// Send authenticated request

var options = {
    uri: 'http://example.com:8000/resource/1?b=1&a=2',
    method: 'GET',
    headers: {
        authorization: Hawk.getAuthorizationHeader(credentials, 'GET', '/resource/1?b=1&a=2', 'example.com', 8000, 'some-app-data')
    }
};

Request(options, function (error, response, body) {

    console.log(response.statusCode + ': ' + body);
});

Protocol Example

The client attempts to access a protected resource without authentication, sending the following HTTP request to the resource server:

GET /resource/1?b=1&a=2 HTTP/1.1
Host: example.com:8000

The resource server returns the following authentication challenge:

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Hawk

The client has previously obtained a set of Hawk credentials for accessing resources on the "http://example.com/" server. The Hawk credentials issued to the client include the following attributes:

  • Key identifier: dh37fgj492je
  • Key: werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn
  • Algorithm: hmac-sha-256

The client generates the authentication header by calculating a timestamp (e.g. the number of seconds since January 1, 1970 00:00:00 GMT), generates a nonce, and constructs the normalized request string (newline separated values):

1353832234
j4h3g2
GET
/resource/1?b=1&a=2
example.com
8000
some-app-data

The request MAC is calculated using the specified algorithm "hmac-sha-256" and the key over the normalized request string. The result is base64-encoded to produce the request MAC:

hpf5lg0G0rtKrT04CiRf0Q+IDjkGkyvKdMjtqu1XV/s=

The client includes the Hawk key identifier, timestamp, and request MAC with the request using the HTTP "Authorization" request header field:

GET /resource/1?b=1&a=2 HTTP/1.1
Host: example.com:8000
Authorization: Hawk id="dh37fgj492je", ts="1353832234", ext="some-app-data", mac="hpf5lg0G0rtKrT04CiRf0Q+IDjkGkyvKdMjtqu1XV/s="

The server validates the request by calculating the request MAC again based on the request received and verifies the validity and scope of the Hawk credentials. If valid, the server responds with the requested resource.

Security Considerations

The greatest sources of security risks are usually found not in Hawk but in the policies and procedures surrounding its use. Implementers are strongly encouraged to assess how this module addresses their security requirements. This section includes an incomplete list of security considerations that must be reviewed and understood before deploying Hawk on the server.

MAC Keys Transmission

Hawk does not provide any mechanism for obtaining or transmitting the set of shared credentials required. Any mechanism used to obtain Hawk credentials must ensure that these transmissions are protected using transport-layer mechanisms such as TLS.

Confidentiality of Requests

While Hawk provides a mechanism for verifying the integrity of HTTP requests, it provides no guarantee of request confidentiality. Unless other precautions are taken, eavesdroppers will have full access to the request content. Servers should carefully consider the types of data likely to be sent as part of such requests, and employ transport-layer security mechanisms to protect sensitive resources.

Spoofing by Counterfeit Servers

Hawk makes no attempt to verify the authenticity of the server. A hostile party could take advantage of this by intercepting the client's requests and returning misleading or otherwise incorrect responses. Service providers should consider such attacks when developing services using this protocol, and should require transport-layer security for any requests where the authenticity of the resource server or of server responses is an issue.

Plaintext Storage of Credentials

The Hawk key functions the same way passwords do in traditional authentication systems. In order to compute the request MAC, the server must have access to the key in plaintext form. This is in contrast, for example, to modern operating systems, which store only a one-way hash of user credentials.

If an attacker were to gain access to these keys - or worse, to the server's database of all such keys - he or she would be able to perform any action on behalf of any resource owner. Accordingly, it is critical that servers protect these keys from unauthorized access.

Entropy of Keys

Unless a transport-layer security protocol is used, eavesdroppers will have full access to authenticated requests and request MAC values, and will thus be able to mount offline brute-force attacks to recover the key used. Servers should be careful to assign keys which are long enough, and random enough, to resist such attacks for at least the length of time that the Hawk credentials are valid.

For example, if the credentials are valid for two weeks, servers should ensure that it is not possible to mount a brute force attack that recovers the key in less than two weeks. Of course, servers are urged to err on the side of caution, and use the longest key reasonable.

It is equally important that the pseudo-random number generator (PRNG) used to generate these keys be of sufficiently high quality. Many PRNG implementations generate number sequences that may appear to be random, but which nevertheless exhibit patterns or other weaknesses which make cryptanalysis or brute force attacks easier. Implementers should be careful to use cryptographically secure PRNGs to avoid these problems.

Coverage Limitations

The request MAC only covers the HTTP Host header and does not cover any other headers which can often affect how the request body is interpreted by the server (i.e. Content-Type). If the server behavior is influenced by the presence or value of such headers, an attacker can manipulate the request header without being detected. Implementers should use the ext feature to pass application-specific information via the Authorization header which is protected by the request MAC.

Frequently Asked Questions

Where is the protocol specification?

If you are looking for some prose explaining how all this works, there isn't any. Hawk is being developed as an open source project instead of a standard. In other words, the code is the specification.

Is it done?

Far from it. Until this module reaches version 1.0.0 it is considered experimental and is likely to change. This also means your feedback and contribution are very welcome. Feel free to open issues with questions and suggestions.

Does Hawk have anything to do with OAuth?

Short answer: no.

Hawk was originally proposed as the OAuth MAC Token specification. However, the OAuth working group in its consistent incompetence failed to produce a final, usable solution to address one of the most popular use cases of OAuth 1.0 - using it to authenticate simple client-server transactions (i.e. two-legged).

Hawk provides a simple HTTP authentication scheme for making client-server requests. It does not address the OAuth use case of delegating access to a third party. If you are looking for an OAuth alternative, check out Oz.

Where can I find Hawk implementations in other languages?

At this time, Hawk is only implemented in JavaScript as a node.js module. Check this space for future support of other languages (and such contributions are always welcome).

Why isn't the algorithm part of the challenge or dynamically negotiated?

The algorithm used is closely related to the key issued as different algorithms require different key sizes (and other requirements). While some keys can be used for multiple algorithm, the protocol is designed to closely bind the key and algorithm together as part of the issued credentials.

Why is Host the only header covered by the request MAC?

It is really had to include other headers. Headers can be changed by proxies and other intermediaries and there is no well-established way to normalize them. The only straight-forward solution is to include the headers in some blob (say, bas64 encoded JSON) and include that with the request, an approach taken by JWT and other such formats. However, that design violates the HTTP header boundaries, repeats information, and introduces other security issues because firewalls will not be aware of these “hidden” headers. In addition, any information repeated must be compared to the duplicated information in the header and therefore only moves the problem elsewhere.

Why not just use HTTP Digest?

Digest requires pre-negotiation to establish a nonce. This means you can't just make a request - you must first send a protocol handshake to the server. This pattern has become unacceptable for most web services, especially mobile where extra round-trip are costly. While Hawk includes support for sending a challenge when a request lacks authentication, it does not require it.

Acknowledgements

Hawk is a derivative work of the HTTP MAC Authentication Scheme proposal Co-authored by Ben Adida, Adam Barth, and Eran Hammer, which in turn was based on the OAuth 1.0 community specification.

Special thanks to Ben Laurie for his always insightful feedback and advice.

The Hawk logo was created by Chris Carrasco.

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