one ecc key-pair for both encryption and signature?

[Robert J. Hansen]

> There is anequivalence given (two functions) in the Ed25519 wikipedia page,
> but I don't know if this allows the same curve used in both algorithms.

Yes, in the same way that if you torture a DSA key long enough you can 
get the Elgamal encryption algorithm out of it.  But once you do that, 
*you're no longer using DSA*.

Likewise, Edwards DSA can be tortured into becoming a Curve25519 key. 
But once you do that, *you're no longer using Edwards DSA*.