Made some progress

app/services/follow_remote_account_service.rb

@@ -0,0 +1,57 @@
+class FollowRemoteAccountService
+  include ApplicationHelper
+
+  def call(uri)
+    username, domain = uri.split('@')
+    account = Account.where(username: username, domain: domain).first
+
+    return account unless account.nil?
+
+    account = Account.new(username: username, domain: domain)
+    data    = Goldfinger.finger("acct:#{uri}")
+
+    account.remote_url  = data.link('http://schemas.google.com/g/2010#updates-from').href
+    account.salmon_url  = data.link('salmon').href
+    account.public_key  = magic_key_to_pem(data.link('magic-public-key').href)
+    account.private_key = nil
+
+    account.secret       = SecureRandom.hex
+    account.verify_token = SecureRandom.hex
+
+    feed = get_feed(account.remote_url)
+    hubs = feed.xpath('//xmlns:link[@rel="hub"]')
+
+    return false if hubs.empty? || hubs.first.attribute('href').nil? || feed.at_xpath('/xmlns:author/xmlns:uri').nil?
+
+    account.uri     = feed.at_xpath('/xmlns:author/xmlns:uri').content
+    account.hub_url = hubs.first.attribute('href').value
+    account.save!
+
+    subscription = account.subscription(subscription_url(account))
+    subscription.subscribe
+  rescue Goldfinger::Error, HTTP::Error => e
+    false
+  end
+
+  private
+
+  def get_feed(url)
+    response = http_client.get(Addressable::URI.parse(url))
+    Nokogiri::XML(response)
+  end
+
+  def magic_key_to_pem(magic_key)
+    _, modulus, exponent = magic_key.split('.')
+    modulus, exponent = [modulus, exponent].map { |n| Base64.urlsafe_decode64(n).bytes.inject(0) { |num, byte| (num << 8) | byte } }
+
+    key   = OpenSSL::PKey::RSA.new
+    key.n = modulus
+    key.e = exponent
+
+    key.to_pem
+  end
+
+  def http_client
+    HTTP
+  end
+end