X448 vs. ED448?

[Mike]

On the genpkey manual page, what's the difference between these arguments to
the -algorithm option?

[Viktor Dukhovni]

On Wed, Jul 09, 2025 at 03:26:48AM +0000, Mike wrote:

> On the genpkey manual page, what's the difference between these arguments to
> the -algorithm option?

These are two different algorithms:

- Ed448: An elliptic curve signature algorithm, based on a 448-bit
curve in Edwards form.

- X448: An elliptic curve key agreement algorithm, based on a 448-bit
curve in "Edwards form".

In typical applications you'd want the Ed448 key. Use of "fixed (EC)DH"
public key certificates is far from common (e.g., not used in TLS 1.3,
and no longer supported even for TLS 1.2 since OpenSSL 1.1.0.

Key exchange is almost always ephemeral, so you wouldn't normally need
go use genpkey with X448.

--
Viktor. 🇺🇦 Слава Україні!