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Makes it dead easy to do HTTP Digest authentication.

Simple Digest example

require 'digest/md5'
class PostsController < ApplicationController
  REALM = "SuperSecret"
  USERS = {"dhh" => "secret", #plain text password
           "dap" => Digest::MD5.hexdigest(["dap",REALM,"secret"].join(":"))}  #ha1 digest password

  before_action :authenticate, except: [:index]

  def index
    render plain: "Everyone can see me!"
  end

  def edit
    render plain: "I'm only accessible if you know the password"
  end

  private
    def authenticate
      authenticate_or_request_with_http_digest(REALM) do |username|
        USERS[username]
      end
    end
end

Notes

The authenticate_or_request_with_http_digest block must return the user's password or the ha1 digest hash so the framework can appropriately hash to check the user's credentials. Returning nil will cause authentication to fail.

Storing the ha1 hash: MD5(username:realm:password), is better than storing a plain password. If the password file or database is compromised, the attacker would be able to use the ha1 hash to authenticate as the user at this realm, but would not have the user's password to try using at other sites.

In rare instances, web servers or front proxies strip authorization headers before they reach your application. You can debug this situation by logging all environment variables, and check for HTTP_AUTHORIZATION, amongst others.

Namespace
Methods
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Instance Public methods
authenticate(request, realm, &password_procedure)

Returns false on a valid response, true otherwise

# File actionpack/lib/action_controller/metal/http_authentication.rb, line 179
def authenticate(request, realm, &password_procedure)
  request.authorization && validate_digest_response(request, realm, &password_procedure)
end
authentication_header(controller, realm)
# File actionpack/lib/action_controller/metal/http_authentication.rb, line 237
def authentication_header(controller, realm)
  secret_key = secret_token(controller.request)
  nonce = self.nonce(secret_key)
  opaque = opaque(secret_key)
  controller.headers["WWW-Authenticate"] = %Q(Digest realm="#{realm}", qop="auth", algorithm=MD5, nonce="#{nonce}", opaque="#{opaque}")
end
authentication_request(controller, realm, message = nil)
# File actionpack/lib/action_controller/metal/http_authentication.rb, line 244
def authentication_request(controller, realm, message = nil)
  message ||= "HTTP Digest: Access denied.\n"
  authentication_header(controller, realm)
  controller.response_body = message
  controller.status = 401
end
decode_credentials(header)
# File actionpack/lib/action_controller/metal/http_authentication.rb, line 230
def decode_credentials(header)
  ActiveSupport::HashWithIndifferentAccess[header.to_s.gsub(/^Digest\s+/, '').split(',').map do |pair|
    key, value = pair.split('=', 2)
    [key.strip, value.to_s.gsub(/^"|"$/,'').delete('\')]
  end]
end
decode_credentials_header(request)
# File actionpack/lib/action_controller/metal/http_authentication.rb, line 226
def decode_credentials_header(request)
  decode_credentials(request.authorization)
end
encode_credentials(http_method, credentials, password, password_is_ha1)
# File actionpack/lib/action_controller/metal/http_authentication.rb, line 221
def encode_credentials(http_method, credentials, password, password_is_ha1)
  credentials[:response] = expected_response(http_method, credentials[:uri], credentials, password, password_is_ha1)
  "Digest " + credentials.sort_by {|x| x[0].to_s }.map {|v| "#{v[0]}='#{v[1]}'" }.join(', ')
end
expected_response(http_method, uri, credentials, password, password_is_ha1=true)

Returns the expected response for a request of http_method to uri with the decoded credentials and the expected password Optional parameter password_is_ha1 is set to true by default, since best practice is to store ha1 digest instead of a plain-text password.

# File actionpack/lib/action_controller/metal/http_authentication.rb, line 211
def expected_response(http_method, uri, credentials, password, password_is_ha1=true)
  ha1 = password_is_ha1 ? password : ha1(credentials, password)
  ha2 = ::Digest::MD5.hexdigest([http_method.to_s.upcase, uri].join(':'))
  ::Digest::MD5.hexdigest([ha1, credentials[:nonce], credentials[:nc], credentials[:cnonce], credentials[:qop], ha2].join(':'))
end
ha1(credentials, password)
# File actionpack/lib/action_controller/metal/http_authentication.rb, line 217
def ha1(credentials, password)
  ::Digest::MD5.hexdigest([credentials[:username], credentials[:realm], password].join(':'))
end
nonce(secret_key, time = Time.now)

Uses an MD5 digest based on time to generate a value to be used only once.

A server-specified data string which should be uniquely generated each time a 401 response is made. It is recommended that this string be base64 or hexadecimal data. Specifically, since the string is passed in the header lines as a quoted string, the double-quote character is not allowed.

The contents of the nonce are implementation dependent. The quality of the implementation depends on a good choice. A nonce might, for example, be constructed as the base 64 encoding of

time-stamp H(time-stamp ":" ETag ":" private-key)

where time-stamp is a server-generated time or other non-repeating value, ETag is the value of the HTTP ETag header associated with the requested entity, and private-key is data known only to the server. With a nonce of this form a server would recalculate the hash portion after receiving the client authentication header and reject the request if it did not match the nonce from that header or if the time-stamp value is not recent enough. In this way the server can limit the time of the nonce's validity. The inclusion of the ETag prevents a replay request for an updated version of the resource. (Note: including the IP address of the client in the nonce would appear to offer the server the ability to limit the reuse of the nonce to the same client that originally got it. However, that would break proxy farms, where requests from a single user often go through different proxies in the farm. Also, IP address spoofing is not that hard.)

An implementation might choose not to accept a previously used nonce or a previously used digest, in order to protect against a replay attack. Or, an implementation might choose to use one-time nonces or digests for POST, PUT, or PATCH requests and a time-stamp for GET requests. For more details on the issues involved see Section 4 of this document.

The nonce is opaque to the client. Composed of Time, and hash of Time with secret key from the Rails session secret generated upon creation of project. Ensures the time cannot be modified by client.

# File actionpack/lib/action_controller/metal/http_authentication.rb, line 289
def nonce(secret_key, time = Time.now)
  t = time.to_i
  hashed = [t, secret_key]
  digest = ::Digest::MD5.hexdigest(hashed.join(":"))
  ::Base64.strict_encode64("#{t}:#{digest}")
end
opaque(secret_key)

Opaque based on random generation - but changing each request?

# File actionpack/lib/action_controller/metal/http_authentication.rb, line 308
def opaque(secret_key)
  ::Digest::MD5.hexdigest(secret_key)
end
secret_token(request)
# File actionpack/lib/action_controller/metal/http_authentication.rb, line 251
def secret_token(request)
  key_generator  = request.env["action_dispatch.key_generator"]
  http_auth_salt = request.env["action_dispatch.http_auth_salt"]
  key_generator.generate_key(http_auth_salt)
end
validate_digest_response(request, realm, &password_procedure)

Returns false unless the request credentials response value matches the expected value. First try the password as a ha1 digest password. If this fails, then try it as a plain text password.

# File actionpack/lib/action_controller/metal/http_authentication.rb, line 186
def validate_digest_response(request, realm, &password_procedure)
  secret_key  = secret_token(request)
  credentials = decode_credentials_header(request)
  valid_nonce = validate_nonce(secret_key, request, credentials[:nonce])

  if valid_nonce && realm == credentials[:realm] && opaque(secret_key) == credentials[:opaque]
    password = password_procedure.call(credentials[:username])
    return false unless password

    method = request.env['rack.methodoverride.original_method'] || request.env['REQUEST_METHOD']
    uri    = credentials[:uri]

    [true, false].any? do |trailing_question_mark|
      [true, false].any? do |password_is_ha1|
        _uri = trailing_question_mark ? uri + "?" : uri
        expected = expected_response(method, _uri, credentials, password, password_is_ha1)
        expected == credentials[:response]
      end
    end
  end
end
validate_nonce(secret_key, request, value, seconds_to_timeout=5*60)

Might want a shorter timeout depending on whether the request is a PATCH, PUT, or POST, and if client is browser or web service. Can be much shorter if the Stale directive is implemented. This would allow a user to use new nonce without prompting user again for their username and password.

# File actionpack/lib/action_controller/metal/http_authentication.rb, line 301
def validate_nonce(secret_key, request, value, seconds_to_timeout=5*60)
  return false if value.nil?
  t = ::Base64.decode64(value).split(":").first.to_i
  nonce(secret_key, t) == value && (t - Time.now.to_i).abs <= seconds_to_timeout
end