diff --git a/lib/rack/attack.rb b/lib/rack/attack.rb
index 1a5c50d..a2342f5 100644
--- a/lib/rack/attack.rb
+++ b/lib/rack/attack.rb
@@ -31,7 +31,7 @@ module Rack
     autoload :Allow2Ban,            'rack/attack/allow2ban'
 
     class << self
-      attr_accessor :enabled, :notifier
+      attr_accessor :enabled, :notifier, :discriminator_normalizer
       attr_reader :configuration
 
       def instrument(request)
@@ -79,6 +79,9 @@ module Rack
     # Set defaults
     @enabled = true
     @notifier = ActiveSupport::Notifications if defined?(ActiveSupport::Notifications)
+    @discriminator_normalizer = lambda do |discriminator|
+      discriminator.to_s.strip.downcase
+    end
     @configuration = Configuration.new
 
     attr_reader :configuration
diff --git a/lib/rack/attack/throttle.rb b/lib/rack/attack/throttle.rb
index 3b80d9e..a1e21a7 100644
--- a/lib/rack/attack/throttle.rb
+++ b/lib/rack/attack/throttle.rb
@@ -22,8 +22,7 @@ module Rack
       end
 
       def matched_by?(request)
-        discriminator = block.call(request)
-
+        discriminator = discriminator_for(request)
         return false unless discriminator
 
         current_period  = period_for(request)
@@ -49,6 +48,14 @@ module Rack
 
       private
 
+      def discriminator_for(request)
+        discriminator = block.call(request)
+        if discriminator && Rack::Attack.discriminator_normalizer
+          discriminator = Rack::Attack.discriminator_normalizer.call(discriminator)
+        end
+        discriminator
+      end
+
       def period_for(request)
         period.respond_to?(:call) ? period.call(request) : period
       end
diff --git a/spec/rack_attack_throttle_spec.rb b/spec/rack_attack_throttle_spec.rb
index dcb1e41..feb599c 100644
--- a/spec/rack_attack_throttle_spec.rb
+++ b/spec/rack_attack_throttle_spec.rb
@@ -144,3 +144,47 @@ describe 'Rack::Attack.throttle with block retuning nil' do
     end
   end
 end
+
+describe 'Rack::Attack.throttle with discriminator_normalizer' do
+  before do
+    @period = 60
+    @emails = [
+      "person@example.com",
+      "PERSON@example.com ",
+      " person@example.com\r\n  ",
+    ]
+    Rack::Attack.cache.store = ActiveSupport::Cache::MemoryStore.new
+    Rack::Attack.throttle('logins/email', limit: 4, period: @period) do |req|
+      if req.path == '/login' && req.post?
+        req.params['email']
+      end
+    end
+  end
+
+  it 'should not differentiate requests when discriminator_normalizer is enabled' do
+    post_logins
+    key = "rack::attack:#{Time.now.to_i / @period}:logins/email:person@example.com"
+    _(Rack::Attack.cache.store.read(key)).must_equal 3
+  end
+
+  it 'should differentiate requests when discriminator_normalizer is disabled' do
+    begin
+      prev = Rack::Attack.discriminator_normalizer
+      Rack::Attack.discriminator_normalizer = nil
+
+      post_logins
+      @emails.each do |email|
+        key = "rack::attack:#{Time.now.to_i / @period}:logins/email:#{email}"
+        _(Rack::Attack.cache.store.read(key)).must_equal 1
+      end
+    ensure
+      Rack::Attack.discriminator_normalizer = prev
+    end
+  end
+
+  def post_logins
+    @emails.each do |email|
+      post '/login', email: email
+    end
+  end
+end