A. Keys are selected based on the longest bit supported by the system’s algorithms.
B. Public keys are exchanged only with trusted communication partners.
C. Private keys are accessible only to the user to which they were assigned.
D. Private keys must be used only once before being replaced.
The correct answer is C.
Domain: 6.1. In an asymmetric system, specifically in a public cryptography system, the security it provides rests on the complexity of its algorithms, a lack of flaws, and the privacy maintained over the private key. If anyone other than the original subject gains access to the private keys, the security of the system is degraded or lost. Keys only need to be long enough for reasonable protection, not necessarily the longest supported by an algorithm. Public keys can be freely and openly exchanged with everyone. Private keys can be reused multiple times within a reasonable time frame (often 1–10 years) based on their key length.