Frontend Password change

I think you are referring to two different things..

1) Verifying the request.
You should be using WP Nonces
to verify the request and protect it against XSS. That should be a practice for all your forms. you could also add additional layer of security by integrating a reCAPTCHA.

2) Data Encryption when you attempt to hash the password. Which in this case, your only option is to use SSL. Using it will secure all data transfer between client and server, additionally it goes beyond this single process of updating password.