0_some_model.rb

class SomeModel < ApplicationRecord
  serialize :client_secret, EncryptedStringSerializer.new
  serialize :credentials, EncryptedJsonSerializer.new
end

1_encrypted_string_serializer.rb

class EncryptedStringSerializer
  KEY = if Rails.env.test?
    SecureRandom.bytes(32)
  else
    Base64.decode64(Rails.application.credentials.identity[:storage_key_base64])
  end

  def load(serialized_value)
    encryptor.decrypt_and_verify(serialized_value, purpose: :storage) if serialized_value.present?
  end

  def dump(string)
    encryptor.encrypt_and_sign(string, purpose: :storage)
  end

  def encryptor
    @encyrptor ||= ActiveSupport::MessageEncryptor.new(KEY)
  end
end

2_encrypted_json_serializer.rb

class EncryptedJsonSerializer
  KEY = if Rails.env.test?
    SecureRandom.bytes(32)
  else
    Base64.decode64(Rails.application.credentials.identity[:storage_key_base64])
  end

  def load(serialized_value)
    encryptor.decrypt_and_verify(serialized_value, purpose: :storage) if serialized_value.present?
  end

  def dump(json)
    encryptor.encrypt_and_sign(json, purpose: :storage)
  end

  def encryptor
    @encyrptor ||= ActiveSupport::MessageEncryptor.new(KEY, serializer: MessagePackDeflater)
  end
end

3_message_pack_deflater.rb

require "msgpack"

class MessagePackDeflater
  def self.dump(v)
    Zlib::Deflate.deflate(MessagePack.dump(v))
  end

  def self.load(v)
    MessagePack.load(Zlib::Inflate.inflate(v))
  end
end