[SOLVED] Extract the shared secret from class ECDiffieHellmanCng

Issue

I am currently developing an SSH client and it is necessary that said client is able to exchange keys with the server via ECDH KEX (NIST-256, 384 and 521).

I did some (actually a lot) of research, found the .NET class ECDiffieHellmanCng, and was able to extract and import the public key of the server into the class.

The problem, however, is that I can't extract the shared secret without deriving it (ECDiffieHellmanCng.DeriveKeyMaterial(CngKey otherpartyPublicKey)).

Is there a way to directly access the shared secret ("k" as it's called in the RFC papers)?

Here is page 7 from the RFC of the ECDH implementation and why I need the shared secret:

The exchange hash H is computed as the hash of the concatenation of the following.

string V_C, client's identification string (CR and LF excluded)

string V_S, server's identification string (CR and LF excluded)

string I_C, payload of the client's SSH_MSG_KEXINIT

string I_S, payload of the server's SSH_MSG_KEXINIT

string K_S, server's public host key

string Q_C, client's ephemeral public key octet string

string Q_S, server's ephemeral public key octet string

mpint K, shared secret <-- this is why I need the pure secret before any derivation

Thanks for any solutions or hints!

Solution

Even after a lot of research i couldn't find a way to do it so the answer is no - you can not extract the secret.

My solution for the big picture was to discard the ECDiffieHellmanCng class altogether and instead wrap the OpenSSH library in C#.

Hope this at least helps someone else with the same idea.

Answered By - Oachkatzl