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HMAC authentication for Rails and HTTP Clients

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Logins and passwords are for humans. Communication between applications need to be protected through different means.

ApiAuth is a Ruby gem designed to be used both in your client and server HTTP-based applications. It implements the same authentication methods (HMAC-SHA1) used by Amazon Web Services.

The gem will sign your requests on the client side and authenticate that signature on the server side. If your server resources are implemented as a Rails ActiveResource, it will integrate with that. It will even generate the secret keys necessary for your clients to sign their requests.

Since it operates entirely using HTTP headers, the server component does not have to be written in the same language as the clients.

How it works

  1. A canonical string is first created using your HTTP headers containing the
    , request path and the date/time stamp. If
    are not present, then a blank string is used in their place. If the timestamp isn't present, a valid HTTP date is automatically added to the request. The canonical string is computed as follows:
canonical_string = "#{http method},#{content-type},#{X-Authorization-Content-SHA256},#{request URI},#{timestamp}"


canonical_string = 'POST,application/json,,request_path,Tue, 30 May 2017 03:51:43 GMT'
  1. This string is then used to create the signature which is a Base64 encoded SHA1 HMAC, using the client's private secret key.

  2. This signature is then added as the

    HTTP header in the form:
Authorization = APIAuth "#{client access id}:#{signature from step 2}"

A cURL request would look like:

curl -X POST --header 'Content-Type: application/json' --header "Date: Tue, 30 May 2017 03:51:43 GMT" --header "Authorization: ${AUTHORIZATION}"`
  1. On the server side, the SHA1 HMAC is computed in the same way using the request headers and the client's secret key, which is known to only the client and the server but can be looked up on the server using the client's access id that was attached in the header. The access id can be any integer or string that uniquely identifies the client. The signed request expires after 15 minutes in order to avoid replay attacks.



This gem require Ruby >= 2.5 and Rails >= 5.1 if you use rails.

For older version of Ruby or Rails, please use ApiAuth v2.1 and older.

IMPORTANT: v2.0.0 is backwards incompatible with the default settings of v1.x to address a security vulnerability. See for security update information.


The gem doesn't have any dependencies outside of having a working OpenSSL configuration for your Ruby VM. To install:

[sudo] gem install api-auth

Please note the dash in the name versus the underscore.


ApiAuth supports many popular HTTP clients. Support for other clients can be added as a request driver.

Here is the current list of supported request objects:

  • Net::HTTP
  • ActionDispatch::Request
  • Curb (Curl::Easy)
  • RestClient
  • Faraday
  • HTTP

HTTP Client Objects

Here's a sample implementation of signing a request created with RestClient.

Assuming you have a client access id and secret as follows:

@access_id = "1044"
@secret_key = ApiAuth.generate_secret_key

A typical RestClient PUT request may look like:

headers = { 'X-Authorization-Content-SHA256' => "dWiCWEMZWMxeKM8W8Yuh/TbI29Hw5xUSXZWXEJv63+Y=",
  'Content-Type' => "text/plain",
  'Date' => "Mon, 23 Jan 1984 03:29:56 GMT"

@request = url: "/resource.xml?foo=bar&bar=foo", headers: headers, method: :put )

To sign that request, simply call the

method as follows:
@signed_request = ApiAuth.sign!(@request, @access_id, @secret_key)

The proper

request header has now been added to that request object and it's ready to be transmitted. It's recommended that you sign the request as one of the last steps in building the request to ensure the headers don't change after the signing process which would cause the authentication check to fail on the server side.

If you are signing a request for a driver that doesn't support automatic http method detection (like Curb or httpi), you can pass the http method as an option into the sign! method like so:

@signed_request = ApiAuth.sign!(@request, @access_id, @secret_key, :override_http_method => "PUT")

If you want to use another digest existing in

, you can pass the http method as an option into the sign! method like so:
@signed_request = ApiAuth.sign!(@request, @access_id, @secret_key, :digest => 'sha256')

With the

option, the
header will be change from:
Authorization = APIAuth 'client access id':'signature'


Authorization = APIAuth-HMAC-DIGEST_NAME 'client access id':'signature'

ActiveResource Clients

ApiAuth can transparently protect your ActiveResource communications with a single configuration line:

class MyResource < ActiveResource::Base
  with_api_auth(access_id, secret_key)

This will automatically sign all outgoing ActiveResource requests from your app.


ApiAuth also works with Flexirest (used to be ActiveRestClient, but that is now unsupported) in a very similar way. Simply add this configuration to your Flexirest initializer in your app and it will automatically sign all outgoing requests.

Flexirest::Base.api_auth_credentials(@access_id, @secret_key)


ApiAuth provides some built in methods to help you generate API keys for your clients as well as verifying incoming API requests.

To generate a Base64 encoded API key for a client:


To validate whether or not a request is authentic:

ApiAuth.authentic?(signed_request, secret_key)


method uses the digest specified in the
header. For example SHA256 for:
Authorization = APIAuth-HMAC-SHA256 'client access id':'signature'

And by default SHA1 if the HMAC-DIGEST is not specified.

If you want to force the usage of another digest method, you should pass it as an option parameter:

ApiAuth.authentic?(signed_request, secret_key, :digest => 'sha256')

For security, requests dated older or newer than a certain timespan are considered inauthentic.

This prevents old requests from being reused in replay attacks, and also ensures requests can't be dated into the far future.

The default span is 15 minutes, but you can override this:

ApiAuth.authentic?(signed_request, secret_key, :clock_skew => 60) # or 1.minute in ActiveSupport

If you want to sign custom headers, you can pass them as an array of strings in the options like so:

ApiAuth.authentic?(signed_request, secret_key, headers_to_sign: %w[HTTP_HEADER_NAME])

With the specified headers values being at the end of the canonical string in the same order.

If your server is a Rails app, the signed request will be the


In order to obtain the secret key for the client, you first need to look up the client's access_id. ApiAuth can pull that from the request headers for you:


Once you've looked up the client's record via the access id, you can then verify whether or not the request is authentic. Typically, the access id for the client will be their record's primary key in the DB that stores the record or some other public unique identifier for the client.

Here's a sample method that can be used in a

if your server is a Rails app:
before_action :api_authenticate

def api_authenticate @current_account = Account.find_by_access_id(ApiAuth.access_id(request)) head(:unauthorized) unless @current_account && ApiAuth.authentic?(request, @current_account.secret_key) end


ApiAuth uses bundler for gem dependencies and RSpec for testing. Developing the gem requires that you have all supported HTTP clients installed. Bundler will take care of all that for you.

To run the tests:

Install the dependencies for a particular Rails version by specifying a gemfile in

BUNDLE_GEMFILE=gemfiles/rails_5.gemfile bundle install

Run the tests with those dependencies:

BUNDLE_GEMFILE=gemfiles/rails_5.gemfile bundle exec rake

If you'd like to add support for additional HTTP clients, check out the already implemented drivers in

for reference. All of the public methods for each driver are required to be implemented by your driver.



Copyright (c) 2014 Mauricio Gomes. See LICENSE.txt for further details.

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