We don't enable the ECDSA recovery module, because we don't recommend
ECDSA recovery for new protocols. In particular, the recovery API is
prone to misuse: It invites the caller to forget to check the public
key (and the verification function always returns 1).
In general, we also don't recommend ordinary ECDSA for new protocols.
But disabling the ECDSA functions is not possible because they're not
in a module, and let's be honest: disabling ECDSA would mean to ignore
reality blatantly.
Instead of using eta=-delta, use zeta=-(delta+1/2) to represent
delta. This variant only needs at most 590 iterations for 256-bit
inputs rather than 724 (by convex hull bounds analysis).
This adds a long comment explaining the algorithm and implementation choices by building
it up step by step in Python.
Comments in the code are also reworked/added, with references to the long explanation.
Tim Ruffing
Jonas Nick
Elliott Jin
Pieter Wuille