Rental credit card deposit

There is no 100% guaranteed secure method, other than the trust you have in the individual or merchant you've given your CC details to.

This is no different than securing hotel reservations, or rental car reservations, or purchasing airline tickets, or train tickets, etc.

Were you perhaps asking what is a secure way to give your credit card details? If the receiver doesn't use a secure method on its website you could either phone them to give the details or you could email with separate emails the first part of the numbers then second part of the numbers, then expiration date and then "code" if they need it. No method is secure if the receiver abuses it but at least harder to be hacked on the internet by a third party.

We've always given the information needed using a telephone call. They're really not that expensive compared to the overall cost of the trip. Never had anyone refuse to do it this way and never had any problems.