Even if you devised a perfectly secure system - using quantum signalling, or something - there's still a key on file somewhere, or stuck on a postit note next to someone's desk. Etc.

Even if not, security services will demand a back door, which can be exploited.

Security is relative. Most security is non-existent. A few applications pretend to offer 'almost good enough', with hope rather than certainty.

All information has a market value, and if the cost of breaking security is higher than the value, you're safe, up to a point.

But some hackers like breaking into things just because they can. So 'secure' is pretty much meaningless in absolute terms, and certainly not something you can rely on with any confidence.