diff --git a/actionpack/lib/action_dispatch/middleware/cookies.rb b/actionpack/lib/action_dispatch/middleware/cookies.rb
index f2f3150b56..4b992a186d 100644
--- a/actionpack/lib/action_dispatch/middleware/cookies.rb
+++ b/actionpack/lib/action_dispatch/middleware/cookies.rb
@@ -372,7 +372,7 @@ module ActionDispatch
 
         handle_options(options)
 
-        if @cookies[name.to_s] != value or options[:expires]
+        if @cookies[name.to_s] != value || options[:expires]
           @cookies[name.to_s] = value
           @set_cookies[name.to_s] = options
           @delete_cookies.delete(name.to_s)
@@ -576,8 +576,8 @@ module ActionDispatch
             "Read the upgrade documentation to learn more about this new config option."
         end
 
-        secret = key_generator.generate_key(request.encrypted_cookie_salt || '')
-        sign_secret = key_generator.generate_key(request.encrypted_signed_cookie_salt || '')
+        secret = key_generator.generate_key(request.encrypted_cookie_salt || "")[0, ActiveSupport::MessageEncryptor.key_len]
+        sign_secret = key_generator.generate_key(request.encrypted_signed_cookie_salt || "")
         @encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret, digest: digest, serializer: ActiveSupport::MessageEncryptor::NullSerializer)
       end
 
diff --git a/actionpack/test/dispatch/cookies_test.rb b/actionpack/test/dispatch/cookies_test.rb
index dfcef14344..5d828f2f0e 100644
--- a/actionpack/test/dispatch/cookies_test.rb
+++ b/actionpack/test/dispatch/cookies_test.rb
@@ -603,7 +603,7 @@ class CookiesTest < ActionController::TestCase
     secret = key_generator.generate_key(encrypted_cookie_salt)
     sign_secret = key_generator.generate_key(encrypted_signed_cookie_salt)
 
-    marshal_value = ActiveSupport::MessageEncryptor.new(secret, sign_secret, serializer: Marshal).encrypt_and_sign("bar")
+    marshal_value = ActiveSupport::MessageEncryptor.new(secret[0, ActiveSupport::MessageEncryptor.key_len], sign_secret, serializer: Marshal).encrypt_and_sign("bar")
     @request.headers["Cookie"] = "foo=#{marshal_value}"
 
     get :get_encrypted_cookie
@@ -612,7 +612,7 @@ class CookiesTest < ActionController::TestCase
     assert_not_equal "bar", cookies[:foo]
     assert_equal "bar", cookies.encrypted[:foo]
 
-    encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret, serializer: JSON)
+    encryptor = ActiveSupport::MessageEncryptor.new(secret[0, ActiveSupport::MessageEncryptor.key_len], sign_secret, serializer: JSON)
     assert_equal "bar", encryptor.decrypt_and_verify(@response.cookies["foo"])
   end
 
@@ -624,7 +624,7 @@ class CookiesTest < ActionController::TestCase
     encrypted_signed_cookie_salt = @request.env["action_dispatch.encrypted_signed_cookie_salt"]
     secret = key_generator.generate_key(encrypted_cookie_salt)
     sign_secret = key_generator.generate_key(encrypted_signed_cookie_salt)
-    json_value = ActiveSupport::MessageEncryptor.new(secret, sign_secret, serializer: JSON).encrypt_and_sign("bar")
+    json_value = ActiveSupport::MessageEncryptor.new(secret[0, ActiveSupport::MessageEncryptor.key_len], sign_secret, serializer: JSON).encrypt_and_sign("bar")
     @request.headers["Cookie"] = "foo=#{json_value}"
 
     get :get_encrypted_cookie
@@ -636,6 +636,19 @@ class CookiesTest < ActionController::TestCase
     assert_nil @response.cookies["foo"]
   end
 
+  def test_compat_encrypted_cookie_using_64_byte_key
+    # Cookie generated with 64 bytes secret
+    message = ["566d4e75536d686e633246564e6b493062557079626c566d51574d30515430394c53315665564a694e4563786555744f57537454576b396a5a31566a626e52525054303d2d2d34663234333330623130623261306163363562316266323335396164666364613564643134623131"].pack("H*")
+    @request.headers["Cookie"] = "foo=#{message}"
+
+    get :get_encrypted_cookie
+
+    cookies = @controller.send :cookies
+    assert_not_equal "bar", cookies[:foo]
+    assert_equal "bar", cookies.encrypted[:foo]
+    assert_nil @response.cookies["foo"]
+  end
+
   def test_accessing_nonexistent_encrypted_cookie_should_not_raise_invalid_message
     get :set_encrypted_cookie
     assert_nil @controller.send(:cookies).encrypted[:non_existant_attribute]
@@ -799,8 +812,8 @@ class CookiesTest < ActionController::TestCase
     key_generator = @request.env["action_dispatch.key_generator"]
     secret = key_generator.generate_key(@request.env["action_dispatch.encrypted_cookie_salt"])
     sign_secret = key_generator.generate_key(@request.env["action_dispatch.encrypted_signed_cookie_salt"])
-    encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret)
-    assert_equal 'bar', encryptor.decrypt_and_verify(@response.cookies["foo"])
+    encryptor = ActiveSupport::MessageEncryptor.new(secret[0, ActiveSupport::MessageEncryptor.key_len], sign_secret)
+    assert_equal "bar", encryptor.decrypt_and_verify(@response.cookies["foo"])
   end
 
   def test_legacy_json_signed_cookie_is_read_and_transparently_upgraded_by_signed_json_cookie_jar_if_both_secret_token_and_secret_key_base_are_set
@@ -838,8 +851,8 @@ class CookiesTest < ActionController::TestCase
     key_generator = @request.env["action_dispatch.key_generator"]
     secret = key_generator.generate_key(@request.env["action_dispatch.encrypted_cookie_salt"])
     sign_secret = key_generator.generate_key(@request.env["action_dispatch.encrypted_signed_cookie_salt"])
-    encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret, serializer: JSON)
-    assert_equal 'bar', encryptor.decrypt_and_verify(@response.cookies["foo"])
+    encryptor = ActiveSupport::MessageEncryptor.new(secret[0, ActiveSupport::MessageEncryptor.key_len], sign_secret, serializer: JSON)
+    assert_equal "bar", encryptor.decrypt_and_verify(@response.cookies["foo"])
   end
 
   def test_legacy_json_signed_cookie_is_read_and_transparently_upgraded_by_signed_json_hybrid_jar_if_both_secret_token_and_secret_key_base_are_set
@@ -877,8 +890,8 @@ class CookiesTest < ActionController::TestCase
     key_generator = @request.env["action_dispatch.key_generator"]
     secret = key_generator.generate_key(@request.env["action_dispatch.encrypted_cookie_salt"])
     sign_secret = key_generator.generate_key(@request.env["action_dispatch.encrypted_signed_cookie_salt"])
-    encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret, serializer: JSON)
-    assert_equal 'bar', encryptor.decrypt_and_verify(@response.cookies["foo"])
+    encryptor = ActiveSupport::MessageEncryptor.new(secret[0, ActiveSupport::MessageEncryptor.key_len], sign_secret, serializer: JSON)
+    assert_equal "bar", encryptor.decrypt_and_verify(@response.cookies["foo"])
   end
 
   def test_legacy_marshal_signed_cookie_is_read_and_transparently_upgraded_by_signed_json_hybrid_jar_if_both_secret_token_and_secret_key_base_are_set
@@ -916,8 +929,8 @@ class CookiesTest < ActionController::TestCase
     key_generator = @request.env["action_dispatch.key_generator"]
     secret = key_generator.generate_key(@request.env["action_dispatch.encrypted_cookie_salt"])
     sign_secret = key_generator.generate_key(@request.env["action_dispatch.encrypted_signed_cookie_salt"])
-    encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret, serializer: JSON)
-    assert_equal 'bar', encryptor.decrypt_and_verify(@response.cookies["foo"])
+    encryptor = ActiveSupport::MessageEncryptor.new(secret[0, ActiveSupport::MessageEncryptor.key_len], sign_secret, serializer: JSON)
+    assert_equal "bar", encryptor.decrypt_and_verify(@response.cookies["foo"])
   end
 
   def test_legacy_signed_cookie_is_treated_as_nil_by_signed_cookie_jar_if_tampered
diff --git a/activesupport/lib/active_support/message_encryptor.rb b/activesupport/lib/active_support/message_encryptor.rb
index 721efea789..32e67a38db 100644
--- a/activesupport/lib/active_support/message_encryptor.rb
+++ b/activesupport/lib/active_support/message_encryptor.rb
@@ -18,6 +18,8 @@ module ActiveSupport
   #   encrypted_data = crypt.encrypt_and_sign('my secret data')              # => "NlFBTTMwOUV5UlA1QlNEN2xkY2d6eThYWWh..."
   #   crypt.decrypt_and_verify(encrypted_data)                               # => "my secret data"
   class MessageEncryptor
+    DEFAULT_CIPHER = "aes-256-cbc"
+
     module NullSerializer #:nodoc:
       def self.load(value)
         value
@@ -64,6 +66,11 @@ module ActiveSupport
       _decrypt(verifier.verify(value))
     end
 
+    # Given a cipher, returns the key length of the cipher to help generate the key of desired size
+    def self.key_len(cipher = DEFAULT_CIPHER)
+      OpenSSL::Cipher.new(cipher).key_len
+    end
+
     private
 
     def _encrypt(value)
diff --git a/activesupport/test/message_encryptor_test.rb b/activesupport/test/message_encryptor_test.rb
index a1ff4c1d3e..02831c77cb 100644
--- a/activesupport/test/message_encryptor_test.rb
+++ b/activesupport/test/message_encryptor_test.rb
@@ -48,6 +48,16 @@ class MessageEncryptorTest < ActiveSupport::TestCase
     assert_equal @data, @encryptor.decrypt_and_verify(message)
   end
 
+  def test_backwards_compat_for_64_bytes_key
+    # 64 bit key
+    secret = ["3942b1bf81e622559ed509e3ff274a780784fe9e75b065866bd270438c74da822219de3156473cc27df1fd590e4baf68c95eeb537b6e4d4c5a10f41635b5597e"].pack("H*")
+    # Encryptor with 32 bit key, 64 bit secret for verifier
+    encryptor = ActiveSupport::MessageEncryptor.new(secret[0..31], secret)
+    # Message generated with 64 bit key
+    message = "eHdGeExnZEwvMSt3U3dKaFl1WFo0TjVvYzA0eGpjbm5WSkt5MXlsNzhpZ0ZnbWhBWFlQZTRwaXE1bVJCS2oxMDZhYVp2dVN3V0lNZUlWQ3c2eVhQbnhnVjFmeVVubmhRKzF3WnZyWHVNMDg9LS1HSisyakJVSFlPb05ISzRMaXRzcFdBPT0=--831a1d54a3cda8a0658dc668a03dedcbce13b5ca"
+    assert_equal "data", encryptor.decrypt_and_verify(message)[:some]
+  end
+
   def test_alternative_serialization_method
     prev = ActiveSupport.use_standard_json_time_format
     ActiveSupport.use_standard_json_time_format = true