Change encryption to CTR mode and use better signature

The code is still using the ECB mode, so it should be changed.

I just changed the encryption mode. I did not change how the signature is generated.

With the OpenSSL extension, it would be possible to use an AEAD cipher mode, which would allow to encrypt and generate the signature with a single function call.

Removing the signature, or using an AEAD cipher mode can be done in a follow-up issue, for example the one to use the OpenSSL extension.

As for changing the encryption mode, the merge request is ready for review.