Re: Question about the X25519 algo versus the

Hi Benoit,
Firstly, please note that there is a separate mailing list for the C#
API (https://bouncycastle.org/csharp/mailing_lists.html).

To your questions:

On 2/10/19 9:22 pm, Benoit Laforest wrote:
> We are considering using the X25519 curve for the DH key exchange in our
> project, between our server and our clients and we have a few interrogation.
> 
>   * What is the difference between directly using the
>     */X25519KeyPairGenerator/*and using  a DH key generator via
>     *GeneratorUtilities**.**GetKeyPairGenerator**(**“ECDH”**)*?

GeneratorUtilities is intended for code that is trying to be generic
over many different algorithms. If you just want to use X25519 key
exchange, then I would recommend using X25519KeyPairGenerator directly.


>   * Is using the *X22519* *key* pair generator meaning that we are using
>     the same “hardcoded” curve on both server and clients?

X25519 uses a hard-coded curve, yes (by design). If you have to choose
just one key exchange algorithm, it's a great choice.


>   * If we use the other option, with the *GeneratorUtilities*, then we
>     would generate our *P and G parameters*, we would hardcode them and
>     use them with the *DHKeyPairGenerator*but would this mean that we
>     are using a another type of curve?

Yes, DH in this context means specifically Finite-Field Diffie-Hellman.
X25519 is an elliptic curve based Diffie-Hellman, with the elliptic
curve part treated for the most part as an implementation detail.

Although you can generate your own (FF) DH parameters, you may be better
off using standardised parameters, e.g. see:
    Org.BouncyCastle.Crypto.Agreement.DHStandardGroups

You'd probably be better off not using (FF) DH though. X25519 is a much
better choice for new systems; or if it's not feasible, then ECDH
(Elliptic Curve Diffie-Hellman).

Regards,
Pete Dettman