WIP Encrypt outgoing emails when possible

Sometimes, the pubkey might not be imported in the local keychain
(anymore), but at this point in the code it had been successfully
imported at least once before. So we just (re-)import every time for it
to never fail.

app/mailers/application_mailer.rb

@@ -1,3 +1,89 @@
 class ApplicationMailer < ActionMailer::Base
   layout 'mailer'
+
+  private
+
+    def send_mail
+      @template ||= "#{self.class.name.underscore}/#{caller[0][/`([^']*)'/, 1]}"
+      headers['Message-ID'] = message_id
+
+      if @user.pgp_pubkey.present?
+        mail(to: @user.email, subject: "...", content_type: pgp_content_type) do |format|
+          format.text { render plain: pgp_content }
+        end
+      else
+        mail(to: @user.email, subject: @subject) do |format|
+          format.text { render @template }
+        end
+      end
+    end
+
+    def from_address
+      ENV.fetch('SMTP_FROM_ADDRESS', 'accounts@localhost')
+    end
+
+    def from_domain
+      Mail::Address.new(from_address).domain
+    end
+
+    def message_id
+      @message_id ||= "#{SecureRandom.uuid}@#{from_domain}"
+    end
+
+    def boundary
+      @boundary ||= SecureRandom.hex(8)
+    end
+
+    def pgp_content_type
+      "multipart/encrypted; protocol=\"application/pgp-encrypted\"; boundary=\"------------#{boundary}\""
+    end
+
+    def pgp_nested_content
+      message_content = render_to_string(template: @template)
+      message_content_base64 = Base64.encode64(message_content)
+      nested_boundary = SecureRandom.hex(8)
+
+      <<~NESTED_CONTENT
+        Content-Type: multipart/mixed; boundary="------------#{nested_boundary}"; protected-headers="v1"
+        Subject: #{@subject}
+        From: <#{from_address}>
+        To: #{@user.display_name || @user.cn} <#{@user.email}>
+        Message-ID: <#{message_id}>
+
+        --------------#{nested_boundary}
+        Content-Type: text/plain; charset=UTF-8; format=flowed
+        Content-Transfer-Encoding: base64
+
+        #{message_content_base64}
+
+        --------------#{nested_boundary}--
+      NESTED_CONTENT
+    end
+
+    def pgp_content
+      encrypted_content = UserManager::PgpEncrypt.call(user: @user, text: pgp_nested_content)
+      encrypted_base64 = Base64.encode64(encrypted_content.to_s)
+
+      <<~EMAIL_CONTENT
+        This is an OpenPGP/MIME encrypted message (RFC 4880 and 3156)
+        --------------#{boundary}
+        Content-Type: application/pgp-encrypted
+        Content-Description: PGP/MIME version identification
+
+        Version: 1
+
+        --------------#{boundary}
+        Content-Type: application/octet-stream; name="encrypted.asc"
+        Content-Description: OpenPGP encrypted message
+        Content-Disposition: inline; filename="encrypted.asc"
+
+        -----BEGIN PGP MESSAGE-----
+
+        #{encrypted_base64}
+
+        -----END PGP MESSAGE-----
+
+        --------------#{boundary}--
+      EMAIL_CONTENT
+    end
 end

app/mailers/custom_mailer.rb

@@ -18,6 +18,6 @@ class CustomMailer < ApplicationMailer
     @user = params[:user]
     @subject = params[:subject]
     @body = params[:body]
-    mail(to: @user.email, subject: @subject)
+    send_mail
   end
 end

app/mailers/notification_mailer.rb

@@ -3,7 +3,7 @@ class NotificationMailer < ApplicationMailer
     @user = params[:user]
     @amount_sats = params[:amount_sats]
     @subject = "Sats received"
-    mail to: @user.email, subject: @subject
+    send_mail
   end
 
   def remotestorage_auth_created
@@ -15,19 +15,19 @@ class NotificationMailer < ApplicationMailer
       "#{access} #{directory}"
     end
     @subject = "New app connected to your storage"
-    mail to: @user.email, subject: @subject
+    send_mail
   end
 
   def new_invitations_available
     @user = params[:user]
     @subject = "New invitations added to your account"
-    mail to: @user.email, subject: @subject
+    send_mail
   end
 
   def bitcoin_donation_confirmed
     @user = params[:user]
     @donation = params[:donation]
     @subject = "Donation confirmed"
-    mail to: @user.email, subject: @subject
+    send_mail
   end
 end

app/models/user.rb

@@ -174,7 +174,8 @@ class User < ApplicationRecord
 
   def gnupg_key
     return nil unless pgp_pubkey.present?
-    @gnupg_key ||= GPGME::Key.get(pgp_fpr)
+    GPGME::Key.import(pgp_pubkey)
+    GPGME::Key.get(pgp_fpr)
   end
 
   def pgp_pubkey_contains_user_address?

app/services/user_manager/pgp_encrypt.rb

@@ -0,0 +1,19 @@
+require 'gpgme'
+
+module UserManager
+  class PgpEncrypt < UserManagerService
+    def initialize(user:, text:)
+      @user = user
+      @text = text
+    end
+
+    def call
+      crypto = GPGME::Crypto.new
+      crypto.encrypt(
+        @text,
+        recipients: @user.gnupg_key,
+        always_trust: true
+      )
+    end
+  end
+end