Which of the following statements is not true for a secure cryptographic hash system?

A secure cryptographic hash system is designed to produce a unique hash value for each distinct input, making option A not true in the context of a secure hashing function. While it's technically possible for different inputs to generate the same hash output—a phenomenon known as a hash collision—this is considered a vulnerability in a cryptographic hash function and should be avoided. A secure hash function aims to minimize the chances of collisions to the extent that finding two different inputs that produce the same hash value is computationally infeasible.

In contrast, option B is true because cryptographic hashes are designed to be irreversible; once data is hashed, it cannot be easily reverted back to its original form. Option C is also a true statement because a secure hash function should generate a unique hash value for every different input. Lastly, option D reflects another key characteristic of hash functions: they are deterministic, meaning that the same input will always produce the same hash output. These properties are crucial for ensuring data integrity and authentication in various security applications.