What characteristic defines an effective cryptosystem?

An effective cryptosystem is fundamentally defined by the strength of its security, which is primarily dependent on the secrecy of the keys used in the encryption and decryption processes. When the keys are kept secret and are sufficiently complex, they ensure that unauthorized parties cannot decrypt the information without access to these keys. This principle is rooted in the concept of "Kerckhoffs's principle," which states that a cryptographic system should remain secure even if everything about the system except the key is public knowledge. Thus, even if the algorithm is known, the security of the system will remain intact as long as the key remains secret.

The other characteristics mentioned do not contribute to the effectiveness of a cryptosystem in the same way. The encryption process being time-consuming does not necessarily indicate a secure system; rather, efficiency can be crucial in practical applications. Similarly, complex user training can create barriers to proper implementation and usage rather than enhancing security. Lastly, while keeping an algorithm secret may seem beneficial, many secure systems rely on public algorithms whose security is based on the complexity and secrecy of the keys, rather than the obscurity of the algorithm itself.