To reproduce:

1. Create new project
2. Create new script
3. Add using System.Security.Cryptography
4. In Start method start typing ECDiffieHellmanCng
5. Observe intellisense
6. Observe error: "The type or namespace name 'ECDiffieHellmanCng' could not be found (are you missing a using directive or an assembly reference"

Expected: ECDiffieHellmanCng is found as a valid and existing class in System.Security.Cryptography
Actual: ECDiffieHellmanCng class is not found.

Reproduced on 5.4.4f1, 5.4.5f1, 5.5.2p4, 5.6.0f3, 2017.1.0a6

By design: The ECDiffieHellmanCng is not supported by any old or current versions of Mono, so Unity won't have support for it either