Parametrization and larger key sizes

[amiller]

Not enough thought has yet gone into choosing appropriate key sizes and e.g. the elliptic curves / pairings used for threshold crypto.

In the case of threshold encryption, at least, the individual instances are short-lived. However, unless keys are regenerated / redistributed (#5) occasionally, a long running instance of HoneyBadger may have long term keys.

Just poking around with Charm, I've found it difficult to get other curve choices to work. This can probably be addressed by using a different version of Charm or the PBC dependency of a different dependency.