The threat model must take the entire system into account-not just the data to be protected, but the people who will use the system and how they will use it. Too often, designers don't take the time to build accurate threat models or analyze the real risks. Threat models allow both product designers and consumers to determine what security measures they need. You can't design a secure system unless you understand what it has to be secure against. System design Design work is the mainstay of the science of cryptography, and it is very specialized.
Good cryptographers know that nothing substitutes for extensive peer review and years of analysis.
Cryptographic system design is also an art. A designer must strike a balance between security and accessibility, anonymity and accountability, privacy and availability. There is an enormous difference between a mathematical algorithm and its concrete implementation in hardware or software. Cryptographic system designs are fragile. Just because a protocol is logically secure doesn't mean it will stay secure when a designer starts defining message structures and passing bits around.