Feature request from kabel, posted on GitHub Jul 10, 2015
The mcrypt php extension and the underlying libmcrypt are unmaintained and there is a strong effort to remove them from the PHP core.  
This would be a good opportunity to port any password or data encryption to the better maintained openssl or password extensions.
Comment from pronto2000, posted on GitHub Jul 10, 2015
Pretty much every payment method there uses Mcrypt. I don't think there's a way to get rid of it that easily.
Comment from kabel, posted on GitHub Jul 10, 2015
I'm fairly certain there isn't anything the Magento app is doing with mcrypt that couldn't be trivially ported to openssl.
Comment from piotrekkaminski, posted on GitHub Jul 10, 2015
We are aware of that, internal ticket MAGETWO-39838. As this is not scheduled yet, if someone wants to contribute the change as a pull request, we would appreciate that.
Do you have any recommendation which library should be used instead?
If this is not PHP core functionality, it should be: