Since the changes to the DH implementations that were merged with
30faf04e92 ("Merge branch 'multi-ke-backport'"), most implementations
don't support deriving different shared secrets for the same private key
by calling set_public_key() with another public key anymore (some prevent
it explicitly, but reusing DH private keys is not something we want to
support anyway). So we can't reuse the DH object on one side for every
round.
Tobias Brunner
a59a6d4783