diff --git a/activesupport/lib/active_support/message_encryptor.rb b/activesupport/lib/active_support/message_encryptor.rb
index bffdfc6201..7773611e11 100644
--- a/activesupport/lib/active_support/message_encryptor.rb
+++ b/activesupport/lib/active_support/message_encryptor.rb
@@ -76,12 +76,12 @@ module ActiveSupport
       encrypted_data = cipher.update(@serializer.dump(value))
       encrypted_data << cipher.final
 
-      [encrypted_data, iv].map {|v| ::Base64.strict_encode64(v)}.join("--")
+      "#{::Base64.strict_encode64 encrypted_data}--#{::Base64.strict_encode64 iv}"
     end
 
     def _decrypt(encrypted_message)
       cipher = new_cipher
-      encrypted_data, iv = encrypted_message.split("--").map {|v| ::Base64.decode64(v)}
+      encrypted_data, iv = encrypted_message.split("--").map {|v| ::Base64.strict_decode64(v)}
 
       cipher.decrypt
       cipher.key = @secret
@@ -91,7 +91,7 @@ module ActiveSupport
       decrypted_data << cipher.final
 
       @serializer.load(decrypted_data)
-    rescue OpenSSLCipherError, TypeError
+    rescue OpenSSLCipherError, TypeError, ArgumentError
       raise InvalidMessage
     end
 
diff --git a/activesupport/lib/active_support/message_verifier.rb b/activesupport/lib/active_support/message_verifier.rb
index e0cd92ae3c..a35d5980fe 100644
--- a/activesupport/lib/active_support/message_verifier.rb
+++ b/activesupport/lib/active_support/message_verifier.rb
@@ -37,7 +37,11 @@ module ActiveSupport
 
       data, digest = signed_message.split("--")
       if data.present? && digest.present? && secure_compare(digest, generate_digest(data))
-        @serializer.load(::Base64.decode64(data))
+        begin
+          @serializer.load(::Base64.strict_decode64(data))
+        rescue ArgumentError
+          raise InvalidSignature
+        end
       else
         raise InvalidSignature
       end
diff --git a/activesupport/test/message_encryptor_test.rb b/activesupport/test/message_encryptor_test.rb
index 203156baa1..b6c0a08b05 100644
--- a/activesupport/test/message_encryptor_test.rb
+++ b/activesupport/test/message_encryptor_test.rb
@@ -66,6 +66,17 @@ class MessageEncryptorTest < ActiveSupport::TestCase
     ActiveSupport.use_standard_json_time_format = prev
   end
 
+  def test_message_obeys_strict_encoding
+    bad_encoding_characters = "\n!@#"
+    message, iv = @encryptor.encrypt_and_sign("This is a very \n\nhumble string"+bad_encoding_characters)
+
+    assert_not_decrypted("#{::Base64.encode64 message.to_s}--#{::Base64.encode64 iv.to_s}")
+    assert_not_verified("#{::Base64.encode64 message.to_s}--#{::Base64.encode64 iv.to_s}")
+
+    assert_not_decrypted([iv,  message] * bad_encoding_characters)
+    assert_not_verified([iv,  message] * bad_encoding_characters)
+  end
+
   private
 
   def assert_not_decrypted(value)
@@ -81,7 +92,7 @@ class MessageEncryptorTest < ActiveSupport::TestCase
   end
 
   def munge(base64_string)
-    bits = ::Base64.decode64(base64_string)
+    bits = ::Base64.strict_decode64(base64_string)
     bits.reverse!
     ::Base64.strict_encode64(bits)
   end