support ed25519 signatures

[guylouis]

ed25519 are widely used (e.g: monero, solana) and it would make sense on keycard

two possibilties to evaluate:

• implement signature scheme within the applet directly
• use 3.1 javacards (depending on their availability) which provide hw support for this

[bitgamma]

software approach would be unfeasible in a pure javacard environment

[guylouis]

For sake of clarity, some more precision on this: eddsa (ed25519) can be implemented on a Javacard 3.0.5 but only for the sake of a proof of concept. Without lower layer support (that will come with javacard 3.1) some cryptographic operations won't be executed in constant time and this would be totally unsecure to use in a production environment.

See https://github.com/dufkan/JCEd25519?tab=readme-ov-file#warning-warning-warning

[guylouis]

got from very reliable source that JC 3.1 cards won't be in general availability before 2028