We only need is_identity() in tests and can implement it on the
concrete EccPoint type. This method is flagged off by #[cfg(test)].
Co-authored-by: Daira Hopwood <daira@jacaranda.org>
Selectors previously used in the witness_scalar_* APIs, such as
q_scalar_fixed and q_scalar_fixed_short, are now removed. The
remaining selectors have been renamed for clarity.
The coordinates check for scalars decomposed using a running sum
has been moved into the mul_fixed.rs file, instead of being
duplicated in both mul_fixed::base_field_elem and mul_fixed::short.
The decompose_scalar_fixed() method is now only used in
mul_fixed::full_width, and has been moved there.
In the Orchard protocol, only the NullifierK fixed base in used in
scalar multiplication with a base field element.
The mul_fixed_base_field_elem() API does not have to accept fixed
bases other than NullifierK; conversely, NullifierK does not have
to work with the full-width mul_fixed() API.
The mul_fixed regions use complete addition on the last window,
and incomplete addition on all other windows. However, the complete
addition does not depend on any offsets in the incomplete addition
region, and can be separated into a disjoint region. Since incomplete
addition uses only four advice columns, while complete addition uses
nine, separating the regions would allow the layouter to optimise
their placement.
Co-authored-by: Jack Grigg <jack@electriccoin.co>
In Orchard nullifier derivation, we multiply the fixed base
K^Orchard by a value encoded as a base field element. This commit
introduces an API that allows using a base field element as the
"scalar" in fixed-base scalar multiplication.
The API currently assumes that the base field element is output by
another instruction (i.e. there is no instruction to directly
witness it).
Fixed-base scalar mul makes use of the add_incomplete and add
instructions internally. The full-width and short signed share
some common logic, which is captured in chip::mul_fixed.rs.
The signed short variant introduces additional logic to handle
the scalar's sign. This is done in the submodule mul_fixed::short.
therealyingtong