Ed25519 is a publick key signature system used for signing and verifying data using generated public and private keys from a given seed/secret. Its simillar to Curve25519 system both uses elliptic curve but with slightly different approach. System generates a public and private key from a given seed.
The following are the Ed25519 implementations currently in use. Each implementation differs in performance, size of private key and order of signature generation.
To know how the above three implementations works, read more at https://blog.mozilla.org/warner/2011/11/29/ed25519-keys/
Currently there is a github repository Chaos.Nacl, an equivalent implementation of Nacl in C#. This doesn't provide portable libraries. I have added a new project Chaos.Nacl.PCL by forking the original one. This PCL library supports Xamarin IOS, Andriod, Windows Universal apps, Asp.Net 5 core, Windows phone, and Windows 8.
You can download PCL project from my repository https://www.github.com/rajbandi/chaos.nacl