Bobby L. Craig

![hawk Logo](https://raw.github.com/hueniverse/hawk/master/images/hawk.png) **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](https://github.com/hueniverse/oz). Current version: **1.0** [![Build Status](https://secure.travis-ci.org/hueniverse/hawk.png)](http://travis-ci.org/hueniverse/hawk) # Table of Content - [**Introduction**](#introduction) - [Replay Protection](#replay-protection) - [Usage Example](#usage-example) - [Protocol Example](#protocol-example) - [Payload Validation](#payload-validation) - [Response Payload Validation](#response-payload-validation) - [Browser Support and Considerations](#browser-support-and-considerations)

- [**Single URI Authorization**](#single-uri-authorization) - [Usage Example](#bewit-usage-example)

- [**Security Considerations**](#security-considerations) - [MAC Keys Transmission](#mac-keys-transmission) - [Confidentiality of Requests](#confidentiality-of-requests) - [Spoofing by Counterfeit Servers](#spoofing-by-counterfeit-servers) - [Plaintext Storage of Credentials](#plaintext-storage-of-credentials) - [Entropy of Keys](#entropy-of-keys) - [Coverage Limitations](#coverage-limitations) - [Future Time Manipulation](#future-time-manipulation) - [Client Clock Poisoning](#client-clock-poisoning) - [Bewit Limitations](#bewit-limitations) - [Host Header Forgery](#host-header-forgery)

- [**Frequently Asked Questions**](#frequently-asked-questions)

- [**Acknowledgements**](#acknowledgements) # Introduction **Hawk** is an HTTP authentication scheme providing mechanisms for making authenticated HTTP requests with partial cryptographic verification of the request and response, covering the HTTP method, request URI, host, and optionally the request payload. Similar to the HTTP [Digest access authentication schemes](http://www.ietf.org/rfc/rfc2617.txt), **Hawk** uses a set of client credentials which include an identifier (e.g. username) and key (e.g. password). Likewise, just as with the Digest scheme, the key is never included in authenticated requests. Instead, it is used to calculate a request MAC value which is included in its place. However, **Hawk** has several differences from Digest. In particular, while both use a nonce to limit the possibility of replay attacks, in **Hawk** the client generates the nonce and uses it in combination with a timestamp, leading to less "chattiness" (interaction with the server). Also unlike Digest, this scheme is not intended to protect the key itself (the password in Digest) because the client and server must both have access to the key material in the clear. The primary design goals of this scheme are to: * simplify and improve HTTP authentication for services that are unwilling or unable to deploy TLS for all resources, * secure credentials against leakage (e.g., when the client uses some form of dynamic configuration to determine where to send an authenticated request), and * avoid 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. In addition, **Hawk** supports a method for granting third-parties temporary access to individual resources using a query parameter called _bewit_ (in falconry, a leather strap used to attach a tracking device to the leg of a hawk). 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. ## Replay Protection Without replay protection, an attacker can use a compromised (but otherwise valid and authenticated) request more than once, gaining access to a protected resource. To mitigate this, clients include both a nonce and a timestamp when making requests. This gives the server enough information to prevent replay attacks. The nonce is generated by the client, and is a string unique across all requests with the same timestamp and key identifier combination. The timestamp enables the server to restrict the validity period of the credentials where requests occuring afterwards are rejected. It also removes the need for the server to retain an unbounded number of nonce values for future checks. By default, **Hawk** uses a time window of 1 minute to allow for time skew between the client and server (which in practice translates to a maximum of 2 minutes as the skew can be positive or negative). Using a timestamp requires the client's clock to be in sync with the server's clock. **Hawk** requires both the client clock and the server clock to use NTP to ensure synchronization. However, given the limitations of some client types (e.g. browsers) to deploy NTP, the server provides the client with its current time (in seconds precision) in response to a bad timestamp. There is no expectation that the client will adjust its system clock to match the server (in fact, this would be a potential attack vector). Instead, the client only uses the server's time to calculate an offset used only for communications with that particular server. The protocol rewards clients with synchronized clocks by reducing the number of round trips required to authenticate the first request. ## Usage Example Server code: ```javascript var Http = require('http'); var Hawk = require('hawk'); // Credentials lookup function var credentialsFunc = function (id, callback) { var credentials = { key: 'werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn', algorithm: 'sha256', user: 'Steve' }; return callback(null, credentials); }; // Create HTTP server var handler = function (req, res) { // Authenticate incoming request Hawk.server.authenticate(req, credentialsFunc, {}, function (err, credentials, artifacts) { // Prepare response var payload = (!err ? 'Hello ' + credentials.user + ' ' + artifacts.ext : 'Shoosh!'); var headers = { 'Content-Type': 'text/plain' }; // Generate Server-Authorization response header var header = Hawk.server.header(credentials, artifacts, { payload: payload, contentType: headers['Content-Type'] }); headers['Server-Authorization'] = header; // Send the response back res.writeHead(!err ? 200 : 401, headers); res.end(payload); }); }; // Start server Http.createServer(handler).listen(8000, 'example.com'); ``` Client code: ```javascript var Request = require('request'); var Hawk = require('hawk'); // Client credentials var credentials = { id: 'dh37fgj492je', key: 'werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn', algorithm: 'sha256' } // Request options var requestOptions = { uri: 'http://example.com:8000/resource/1?b=1&a=2', method: 'GET', headers: {} }; // Generate Authorization request header var header = Hawk.client.header('http://example.com:8000/resource/1?b=1&a=2', 'GET', { credentials: credentials, ext: 'some-app-data' }); requestOptions.headers.Authorization = header.field; // Send authenticated request Request(requestOptions, function (error, response, body) { // Authenticate the server's response var isValid = Hawk.client.authenticate(response, credentials, header.artifacts, { payload: body }); // Output results console.log(response.statusCode + ': ' + body + (isValid ? ' (valid)' : ' (invalid)')); }); ``` **Hawk** utilized the [**SNTP**](https://github.com/hueniverse/sntp) module for time sync management. By default, the local machine time is used. To automatically retrieve and synchronice the clock within the application, use the SNTP 'start()' method. ```javascript Hawk.sntp.start(); ``` ## 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 an 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: sha256 The client generates the authentication header by calculating a timestamp (e.g. the number of seconds since January 1, 1970 00:00:00 GMT), generating a nonce, and constructing the normalized request string (each value followed by a newline character): ``` hawk.1.header 1353832234 j4h3g2 GET /resource/1?b=1&a=2 example.com 8000 some-app-ext-data ``` The request MAC is calculated using HMAC with the specified hash algorithm "sha256" and the key over the normalized request string. The result is base64-encoded to produce the request MAC: ``` 6R4rV5iE+NPoym+WwjeHzjAGXUtLNIxmo1vpMofpLAE= ``` The client includes the **Hawk** key identifier, timestamp, nonce, application specific data, 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", nonce="j4h3g2", ext="some-app-ext-data", mac="6R4rV5iE+NPoym+WwjeHzjAGXUtLNIxmo1vpMofpLAE=" ``` 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. ### Payload Validation **Hawk** provides optional payload validation. When generating the authentication header, the client calculates a payload hash using the specified hash algorithm. The hash is calculated over the concatenated value of (each followed by a newline character): * `hawk.1.payload` * the content-type in lowercase, without any parameters (e.g. `application/json`) * the request payload prior to any content encoding (the exact representation requirements should be specified by the server for payloads other than simple single-part ascii to ensure interoperability) For example: * Payload: `Thank you for flying Hawk` * Content Type: `text/plain` * Hash (sha256): `Yi9LfIIFRtBEPt74PVmbTF/xVAwPn7ub15ePICfgnuY=` Results in the following input to the payload hash function (newline terminated values): ``` hawk.1.payload text/plain Thank you for flying Hawk ``` Which produces the following hash value: ``` Yi9LfIIFRtBEPt74PVmbTF/xVAwPn7ub15ePICfgnuY= ``` The client constructs the normalized request string (newline terminated values): ``` hawk.1.header 1353832234 j4h3g2 POST /resource/1?a=1&b=2 example.com 8000 Yi9LfIIFRtBEPt74PVmbTF/xVAwPn7ub15ePICfgnuY= some-app-ext-data ``` Then calculates the request MAC and includes the **Hawk** key identifier, timestamp, nonce, payload hash, application specific data, and request MAC, with the request using the HTTP `Authorization` request header field: ``` POST /resource/1?a=1&b=2 HTTP/1.1 Host: example.com:8000 Authorization: Hawk id="dh37fgj492je", ts="1353832234", nonce="j4h3g2", hash="Yi9LfIIFRtBEPt74PVmbTF/xVAwPn7ub15ePICfgnuY=", ext="some-app-ext-data", mac="aSe1DERmZuRl3pI36/9BdZmnErTw3sNzOOAUlfeKjVw=" ``` It is up to the server if and when it validates the payload for any given request, based solely on it's security policy and the nature of the data included. If the payload is available at the time of authentication, the server uses the hash value provided by the client to construct the normalized string and validates the MAC. If the MAC is valid, the server calculates the payload hash and compares the value with the provided payload hash in the header. In many cases, checking the MAC first is faster than calculating the payload hash. However, if the payload is not available at authentication time (e.g. too large to fit in memory, streamed elsewhere, or processed at a different stage in the application), the server may choose to defer payload validation for later by retaining the hash value provided by the client after validating the MAC. It is important to note that MAC validation does not mean the hash value provided by the client is valid, only that the value included in the header was not modified. Without calculating the payload hash on the server and comparing it to the value provided by the client, the payload may be modified by an attacker. ## Response Payload Validation **Hawk** provides partial response payload validation. The server includes the `Server-Authorization` response header which enables the client to authenticate the response and ensure it is talking to the right server. **Hawk** defines the HTTP `Server-Authorization` header as a response header using the exact same syntax as the `Authorization` request header field. The header is contructed using the same process as the client's request header. The server uses the same credentials and other artifacts provided by the client to constructs the normalized request string. The `ext` and `hash` values are replaced with new values based on the server response. The rest as identical to those used by the client. The result MAC digest is included with the optional `hash` and `ext` values: ``` Server-Authorization: Hawk mac="XIJRsMl/4oL+nn+vKoeVZPdCHXB4yJkNnBbTbHFZUYE=", hash="f9cDF/TDm7TkYRLnGwRMfeDzT6LixQVLvrIKhh0vgmM=", ext="response-specific" ``` ## Browser Support and Considerations A browser script is provided for including using a `