Allow encryption without compression

Add a `compress` option to ActiveRecord::Encryption::Encryptor, which
defaults to `true`. When set to `false`, the encryptor will never
compress the data.

This is useful for cases where the data is already compressed.

This can be used with the `encryptor` option in the model:

```ruby
class Record < ApplicationRecord
  encrypts :field, encryptor: ActiveRecord::Encryption::Encryptor.new(compress: false)
end
```

activerecord/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Add an option to `ActiveRecord::Encryption::Encryptor` to disable compression
+
+    Allow compression to be disabled by setting `compress: false`
+
+    ```ruby
+      class User
+        encrypts :name, encryptor: ActiveRecord::Encryption::Encryptor.new(compress: false)
+      end
+    ```
+
+    *Donal McBreen*
+
 *   Deprecate passing strings to `ActiveRecord::Tasks::DatabaseTasks.cache_dump_filename`.
 
     A `ActiveRecord::DatabaseConfigurations::DatabaseConfig` object should be passed instead.

activerecord/lib/active_record/encryption/encryptor.rb

@@ -12,6 +12,14 @@ module Encryption
     # It interacts with a KeyProvider for getting the keys, and delegate to
     # ActiveRecord::Encryption::Cipher the actual encryption algorithm.
     class Encryptor
+      # === Options
+      #
+      # * <tt>:compress</tt> - Boolean indicating whether records should be compressed before encryption.
+      #   Defaults to +true+.
+      def initialize(compress: true)
+        @compress = compress
+      end
+
       # Encrypts +clean_text+ and returns the encrypted result
       #
       # Internally, it will:
@@ -111,13 +119,17 @@ def serializer
 
         # Under certain threshold, ZIP compression is actually worse that not compressing
         def compress_if_worth_it(string)
-          if string.bytesize > THRESHOLD_TO_JUSTIFY_COMPRESSION
+          if compress? && string.bytesize > THRESHOLD_TO_JUSTIFY_COMPRESSION
             [compress(string), true]
           else
             [string, false]
           end
         end
 
+        def compress?
+          @compress
+        end
+
         def compress(data)
           Zlib::Deflate.deflate(data).tap do |compressed_data|
             compressed_data.force_encoding(data.encoding)

activerecord/test/cases/encryption/encryptor_test.rb

@@ -48,6 +48,15 @@ class ActiveRecord::Encryption::EncryptorTest < ActiveRecord::EncryptionTestCase
     assert cipher_text.bytesize < content.bytesize
   end
 
+  test "content is not compressed, when disabled" do
+    @encryptor = ActiveRecord::Encryption::Encryptor.new(compress: false)
+    content = SecureRandom.hex(5.kilobytes)
+    cipher_text = @encryptor.encrypt(content)
+
+    assert_encrypt_text content
+    assert cipher_text.bytesize > content.bytesize
+  end
+
   test "trying to encrypt custom classes raises a ForbiddenClass exception" do
     assert_raises ActiveRecord::Encryption::Errors::ForbiddenClass do
       @encryptor.encrypt(Struct.new(:name).new("Jorge"))