HTTP signatures (#4146)

* Add Request class with HTTP signature generator

Spec: https://tools.ietf.org/html/draft-cavage-http-signatures-06

* Add HTTP signature verification concern

* Add test for SignatureVerification concern

* Add basic test for Request class

* Make PuSH subscribe/unsubscribe requests use new Request class

Accidentally fix lease_seconds not being set and sent properly, and
change the new minimum subscription duration to 1 day

* Make all PuSH workers use new Request class

* Make Salmon sender use new Request class

* Make FetchLinkService use new Request class

* Make FetchAtomService use the new Request class

* Make Remotable use the new Request class

* Make ResolveRemoteAccountService use the new Request class

* Add more tests

* Allow +-30 seconds window for signed request to remain valid

* Disable time window validation for signed requests, restore 7 days
as PuSH subscription duration (which was previous default due to a bug)

app/lib/request.rb

@@ -0,0 +1,70 @@
+# frozen_string_literal: true
+
+class Request
+  REQUEST_TARGET = '(request-target)'
+
+  include RoutingHelper
+
+  def initialize(verb, url, options = {})
+    @verb    = verb
+    @url     = Addressable::URI.parse(url).normalize
+    @options = options
+    @headers = {}
+
+    set_common_headers!
+  end
+
+  def on_behalf_of(account)
+    raise ArgumentError unless account.local?
+    @account = account
+  end
+
+  def add_headers(new_headers)
+    @headers.merge!(new_headers)
+  end
+
+  def perform
+    http_client.headers(headers).public_send(@verb, @url.to_s, @options)
+  end
+
+  def headers
+    (@account ? @headers.merge('Signature' => signature) : @headers).without(REQUEST_TARGET)
+  end
+
+  private
+
+  def set_common_headers!
+    @headers[REQUEST_TARGET] = "#{@verb} #{@url.path}"
+    @headers['User-Agent']   = user_agent
+    @headers['Host']         = @url.host
+    @headers['Date']         = Time.now.utc.httpdate
+  end
+
+  def signature
+    key_id    = @account.to_webfinger_s
+    algorithm = 'rsa-sha256'
+    signature = Base64.strict_encode64(@account.keypair.sign(OpenSSL::Digest::SHA256.new, signed_string))
+
+    "keyId=\"#{key_id}\",algorithm=\"#{algorithm}\",headers=\"#{signed_headers}\",signature=\"#{signature}\""
+  end
+
+  def signed_string
+    @headers.map { |key, value| "#{key.downcase}: #{value}" }.join("\n")
+  end
+
+  def signed_headers
+    @headers.keys.join(' ').downcase
+  end
+
+  def user_agent
+    @user_agent ||= "#{HTTP::Request::USER_AGENT} (Mastodon/#{Mastodon::Version}; +#{root_url})"
+  end
+
+  def timeout
+    { write: 10, connect: 10, read: 10 }
+  end
+
+  def http_client
+    HTTP.timeout(:per_operation, timeout).follow
+  end
+end