Remove sign-in token authentication, instead send e-mail about new sign-in (#17970)

app/lib/suspicious_sign_in_detector.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+class SuspiciousSignInDetector
+  IPV6_TOLERANCE_MASK = 64
+  IPV4_TOLERANCE_MASK = 16
+
+  def initialize(user)
+    @user = user
+  end
+
+  def suspicious?(request)
+    !sufficient_security_measures? && !freshly_signed_up? && !previously_seen_ip?(request)
+  end
+
+  private
+
+  def sufficient_security_measures?
+    @user.otp_required_for_login?
+  end
+
+  def previously_seen_ip?(request)
+    @user.ips.where('ip <<= ?', masked_ip(request)).exists?
+  end
+
+  def freshly_signed_up?
+    @user.current_sign_in_at.blank?
+  end
+
+  def masked_ip(request)
+    masked_ip_addr = begin
+      ip_addr = IPAddr.new(request.remote_ip)
+
+      if ip_addr.ipv6?
+        ip_addr.mask(IPV6_TOLERANCE_MASK)
+      else
+        ip_addr.mask(IPV4_TOLERANCE_MASK)
+      end
+    end
+
+    "#{masked_ip_addr}/#{masked_ip_addr.prefix}"
+  end
+end