Yukihiro "Matz" Matsumoto 206b9f7477 mruby-bigint: pre-reduce base in mpz_powm_montgomery to satisfy REDC ...

REDC's input T must satisfy T < R*N; mpz_powm_montgomery() computed
T = base * R^2 without first reducing base modulo n. When base >= n,
T exceeds R*N and REDC silently truncates upper limbs, producing a
wrong base_mont and ultimately a wrong result.

Use mpz_mmod (general division path) to pre-reduce base mod n before
the multiplication by R^2.

Visible effect: (2**160).pow(2, (2**40)+1) returned 0 instead of 1.
A separate pre-existing issue in mpz_mod's Barrett path (broken
precondition check) means that path could return a wrong reduction
for large operands; mmod sidesteps that path entirely.

Co-authored-by: Claude <noreply@anthropic.com>

2026-05-21 17:07:51 +09:00

..

bigint.rb

mruby-bigint: pre-reduce base in mpz_powm_montgomery to satisfy REDC

2026-05-21 17:07:51 +09:00