I use Gmail to exchange confidential and important documents with my customers. The documents are read by an unauthorized person (during sending the email) that I don't know him/her. How can I solve the problem?
E-mail is not designed to protect the confidentiality of the information sent, hence additional security mechanisms are needed.
To ensure that only the specific recipient will be able to access the information sent, you could use public-key cryptography. A free and open-source implementation is GNU Privacy Guard (GPG).
GPG (and similar implementations) is able to preserve:
- the confidentiality of information by encrypting it;
- the integrity of information by signing it.
It's important to notice that your customers must generate their own pair of keys (public and private), otherwise you won't be able to encrypt information to be shared only with them.
If this is not possible, you may have to rely on different (and less ideal) ways. A possible solution is to provide your customers with an FTP directory, where the data will be placed, and to share the password via other channels (e.g. SMS). Another solution, but not as secure as the previous ones, is to encrypt a zip file containing the information and send the password to the customer via SMS or any other non-email channel.