Although the internet was originally designed as a system to withstand attacks by hostile agents, it developed in a co-operative environment of relatively trusted entities. Alas, those days are long gone. Spam mail, denial of service attacks, phishing attempts and so on are indicative that anyone using the internet does so at their own risk.
Applications have to be built to work correctly in hostile situations. "correctly" no longer means just getting the functional aspects of the program correct, but also means ensuring privacy and integrity of data transferred, access only to legitimate users and other issues.
This of course makes your programs much more complex. There are difficult and subtle computing problems involved in making applications secure. Attempts to do it yourself (such as making up your own encryption libraries) are usually doomed to failure. Instead, you need to make use of libraries designed by security professionals.