ArgumentError after upgrading Ruby 2.4

I upgraded to Ruby 2.4 and started getting this error:

ArgumentError in ConfessionsController#index

key must be 32 bytes

  # cipher = new_cipher
  cipher.encrypt
  cipher.key = @secret
       # Rely on OpenSSL for the initialization vector
       iv = cipher.random_iv

Image of error

I read the whole internet about it, and people posted about changing the encryption keys. But I don't even have such a files, so I have no idea what to change and how to fix this. Could someone please help?