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+![hawk Logo](https://raw.github.com/hueniverse/hawk/master/images/hawk.png)
+
+<img align="right" src="https://raw.github.com/hueniverse/hawk/master/images/logo.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: **6.x**
+
+Note: 6.x, 5.x, 4.x, 3.x, and 2.x are the same exact protocol as 1.1. The version increments reflect changes in the node API.
+
+[![Build Status](https://travis-ci.org/hueniverse/hawk.svg?branch=master)](https://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)
+- [**Implementations**](#implementations)
+- [**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 occurring 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
+const Http = require('http');
+const Hawk = require('hawk');
+
+
+// Credentials lookup function
+
+const credentialsFunc = function (id, callback) {
+
+    const credentials = {
+        key: 'werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn',
+        algorithm: 'sha256',
+        user: 'Steve'
+    };
+
+    return callback(null, credentials);
+};
+
+// Create HTTP server
+
+const handler = function (req, res) {
+
+    // Authenticate incoming request
+
+    Hawk.server.authenticate(req, credentialsFunc, {}, (err, credentials, artifacts) => {
+
+        // Prepare response
+
+        const payload = (!err ? `Hello ${credentials.user} ${artifacts.ext}` : 'Shoosh!');
+        const headers = { 'Content-Type': 'text/plain' };
+
+        // Generate Server-Authorization response header
+
+        const header = Hawk.server.header(credentials, artifacts, { payload, contentType: headers['Content-Type'] });
+        headers['Server-Authorization'] = header;
+
+        // Send the response back
+
+        res.writeHead(!err ? 200 : 401, headers);
+        res.end(payload);
+    });
+};
+
+// Start server
+
+Http.createServer(handler).listen(8000, 'example.com');
+```
+
+Client code:
+
+```javascript
+const Request = require('request');
+const Hawk = require('hawk');
+
+
+// Client credentials
+
+const credentials = {
+    id: 'dh37fgj492je',
+    key: 'werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn',
+    algorithm: 'sha256'
+}
+
+// Request options
+
+const requestOptions = {
+    uri: 'http://example.com:8000/resource/1?b=1&a=2',
+    method: 'GET',
+    headers: {}
+};
+
+// Generate Authorization request header
+
+const header = Hawk.client.header('http://example.com:8000/resource/1?b=1&a=2', 'GET', { credentials: credentials, ext: 'some-app-data' });
+requestOptions.headers.Authorization = header.field;
+
+// Send authenticated request
+
+Request(requestOptions, function (error, response, body) {
+
+    // Authenticate the server's response
+
+    const isValid = Hawk.client.authenticate(response, credentials, header.artifacts, { payload: body });
+
+    // Output results
+
+    console.log(`${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 synchronize 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: `hmac sha256`
+* Hash: `6R4rV5iE+NPoym+WwjeHzjAGXUtLNIxmo1vpMofpLAE=`
+
+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`
+* Algorithm: `sha256`
+* Hash: `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 its 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 constructed 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 `<script>` tag in [lib/browser.js](/lib/browser.js). It's also a [component](http://component.io/hueniverse/hawk).
+
+**Hawk** relies on the _Server-Authorization_ and _WWW-Authenticate_ headers in its response to communicate with the client.
+Therefore, in case of CORS requests, it is important to consider sending _Access-Control-Expose-Headers_ with the value
+_"WWW-Authenticate, Server-Authorization"_ on each response from your server. As explained in the
+[specifications](http://www.w3.org/TR/cors/#access-control-expose-headers-response-header), it will indicate that these headers
+can safely be accessed by the client (using getResponseHeader() on the XmlHttpRequest object). Otherwise you will be met with a
+["simple response header"](http://www.w3.org/TR/cors/#simple-response-header) which excludes these fields and would prevent the
+Hawk client from authenticating the requests.You can read more about the why and how in this
+[article](http://www.html5rocks.com/en/tutorials/cors/#toc-adding-cors-support-to-the-server)
+
+
+# Single URI Authorization
+
+There are cases in which limited and short-term access to a protected resource is granted to a third party which does not
+have access to the shared credentials. For example, displaying a protected image on a web page accessed by anyone. **Hawk**
+provides limited support for such URIs in the form of a _bewit_ - a URI query parameter appended to the request URI which contains
+the necessary credentials to authenticate the request.
+
+Because of the significant security risks involved in issuing such access, bewit usage is purposely limited only to GET requests
+and for a finite period of time. Both the client and server can issue bewit credentials, however, the server should not use the same
+credentials as the client to maintain clear traceability as to who issued which credentials.
+
+In order to simplify implementation, bewit credentials do not support single-use policy and can be replayed multiple times within
+the granted access timeframe.
+
+
+## Bewit Usage Example
+
+Server code:
+
+```javascript
+const Http = require('http');
+const Hawk = require('hawk');
+
+
+// Credentials lookup function
+
+const credentialsFunc = function (id, callback) {
+
+    const credentials = {
+        key: 'werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn',
+        algorithm: 'sha256'
+    };
+
+    return callback(null, credentials);
+};
+
+// Create HTTP server
+
+const handler = function (req, res) {
+
+    Hawk.uri.authenticate(req, credentialsFunc, {}, (err, credentials, attributes) => {
+
+        res.writeHead(!err ? 200 : 401, { 'Content-Type': 'text/plain' });
+        res.end(!err ? 'Access granted' : 'Shoosh!');
+    });
+};
+
+Http.createServer(handler).listen(8000, 'example.com');
+```
+
+Bewit code generation:
+
+```javascript
+const Request = require('request');
+const Hawk = require('hawk');
+
+
+// Client credentials
+
+const credentials = {
+    id: 'dh37fgj492je',
+    key: 'werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn',
+    algorithm: 'sha256'
+}
+
+// Generate bewit
+
+const duration = 60 * 5;      // 5 Minutes
+const bewit = Hawk.uri.getBewit('http://example.com:8000/resource/1?b=1&a=2', { credentials: credentials, ttlSec: duration, ext: 'some-app-data' });
+const uri = 'http://example.com:8000/resource/1?b=1&a=2' + '&bewit=' + bewit;
+
+// Output URI
+
+console.log('URI: ' + uri);
+```
+
+
+# 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.
+Many of the protections provided in **Hawk** depends on whether and how they are used.
+
+### 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** provides limited verification of the server authenticity. When receiving a response back from the server, the server
+may choose to include a response `Server-Authorization` header which the client can use to verify the response. However, it is up to
+the server to determine when such measure is included, to up to the client to enforce that policy.
+
+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 optionally the `Content-Type` header. It does not cover any other headers
+which can often affect how the request body is interpreted by the server. If the server behavior is influenced by the presence
+or value of such headers, an attacker can manipulate the request headers 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.
+
+The response authentication, when performed, only covers the response payload, content-type, and the request information
+provided by the client in its request (method, resource, timestamp, nonce, etc.). It does not cover the HTTP status code or
+any other response header field (e.g. `Location`) which can affect the client's behaviour.
+
+### Future Time Manipulation
+
+The protocol relies on a clock sync between the client and server. To accomplish this, the server informs the client of its
+current time when an invalid timestamp is received.
+
+If an attacker is able to manipulate this information and cause the client to use an incorrect time, it would be able to cause
+the client to generate authenticated requests using time in the future. Such requests will fail when sent by the client, and will
+not likely leave a trace on the server (given the common implementation of nonce, if at all enforced). The attacker will then
+be able to replay the request at the correct time without detection.
+
+The client must only use the time information provided by the server if:
+* it was delivered over a TLS connection and the server identity has been verified, or
+* the `tsm` MAC digest calculated using the same client credentials over the timestamp has been verified.
+
+### Client Clock Poisoning
+
+When receiving a request with a bad timestamp, the server provides the client with its current time. The client must never use
+the time received from the server to adjust its own clock, and must only use it to calculate an offset for communicating with
+that particular server.
+
+### Bewit Limitations
+
+Special care must be taken when issuing bewit credentials to third parties. Bewit credentials are valid until expiration and cannot
+be revoked or limited without using other means. Whatever resource they grant access to will be completely exposed to anyone with
+access to the bewit credentials which act as bearer credentials for that particular resource. While bewit usage is limited to GET
+requests only and therefore cannot be used to perform transactions or change server state, it can still be used to expose private
+and sensitive information.
+
+### Host Header Forgery
+
+Hawk validates the incoming request MAC against the incoming HTTP Host header. However, unless the optional `host` and `port`
+options are used with `server.authenticate()`, a malicious client can mint new host names pointing to the server's IP address and
+use that to craft an attack by sending a valid request that's meant for another hostname than the one used by the server. Server
+implementors must manually verify that the host header received matches their expectation (or use the options mentioned above).
+
+# Frequently Asked Questions
+
+### Where is the protocol specification?
+
+If you are looking for some prose explaining how all this works, **this is it**. **Hawk** is being developed as an open source
+project instead of a standard. In other words, the [code](/hueniverse/hawk/tree/master/lib) is the specification. Not sure about
+something? Open an issue!
+
+### Is it done?
+
+As of version 0.10.0, **Hawk** is feature-complete. However, 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.
+
+### Where can I find **Hawk** implementations in other languages?
+
+**Hawk**'s only reference implementation is provided in JavaScript as a node.js module. However, it has been ported to other languages.
+The full list is maintained [here](https://github.com/hueniverse/hawk/issues?labels=port&state=closed). Please add an issue if you are
+working on another port. A cross-platform test-suite is in the works.
+
+### 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 and Content-Type the only headers covered by the request MAC?
+
+It is really hard to include other headers. Headers can be changed by proxies and other intermediaries and there is no
+well-established way to normalize them. Many platforms change the case of header field names and values. The only
+straight-forward solution is to include the headers in some blob (say, base64 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.
+
+### Why bother with all this nonce and timestamp business?
+
+**Hawk** is an attempt to find a reasonable, practical compromise between security and usability. OAuth 1.0 got timestamp
+and nonces halfway right but failed when it came to scalability and consistent developer experience. **Hawk** addresses
+it by requiring the client to sync its clock, but provides it with tools to accomplish it.
+
+In general, replay protection is a matter of application-specific threat model. It is less of an issue on a TLS-protected
+system where the clients are implemented using best practices and are under the control of the server. Instead of dropping
+replay protection, **Hawk** offers a required time window and an optional nonce verification. Together, it provides developers
+with the ability to decide how to enforce their security policy without impacting the client's implementation.
+
+### What are `app` and `dlg` in the authorization header and normalized mac string?
+
+The original motivation for **Hawk** was to replace the OAuth 1.0 use cases. This included both a simple client-server mode which
+this module is specifically designed for, and a delegated access mode which is being developed separately in
+[Oz](https://github.com/hueniverse/oz). In addition to the **Hawk** use cases, Oz requires another attribute: the application id `app`.
+This provides binding between the credentials and the application in a way that prevents an attacker from tricking an application
+to use credentials issued to someone else. It also has an optional 'delegated-by' attribute `dlg` which is the application id of the
+application the credentials were directly issued to. The goal of these two additions is to allow Oz to utilize **Hawk** directly,
+but with the additional security of delegated credentials.
+
+### What is the purpose of the static strings used in each normalized MAC input?
+
+When calculating a hash or MAC, a static prefix (tag) is added. The prefix is used to prevent MAC values from being
+used or reused for a purpose other than what they were created for (i.e. prevents switching MAC values between a request,
+response, and a bewit use cases). It also protects against exploits created after a potential change in how the protocol
+creates the normalized string. For example, if a future version would switch the order of nonce and timestamp, it
+can create an exploit opportunity for cases where the nonce is similar in format to a timestamp.
+
+### 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). As you can guess, the OAuth working group is still hard
+at work to produce more garbage.
+
+**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](https://github.com/hueniverse/oz).
+
+# Implementations
+
+- [Logibit Hawk in F#/.Net](https://github.com/logibit/logibit.hawk/)
+- [Tent Hawk in Ruby](https://github.com/tent/hawk-ruby)
+- [Wealdtech in Java](https://github.com/wealdtech/hawk)
+- [Kumar's Mohawk in Python](https://github.com/kumar303/mohawk/)
+- [Hiyosi in Go](https://github.com/hiyosi/hawk)
+
+# Acknowledgements
+
+**Hawk** is a derivative work of the [HTTP MAC Authentication Scheme](http://tools.ietf.org/html/draft-hammer-oauth-v2-mac-token-05) 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](http://chriscarrasco.com).