GC256A (id-tc26-gost-3410-2012-256-paramSetA) is the 256-bit GOST R 34.10-2012 curve with a twisted Edwards form.
GC256A is a short Weierstrass curve y² = x³ + ax + b over a 256-bit prime field with a 255-bit prime-order subgroup and cofactor 4; its estimated security level is 128 bits. Status: secure, but regional or niche.
Russia's GOST R 34.10-2012 signature standard uses curves maintained by the TC 26 standardisation committee. paramSetA for 256-bit keys, published in RFC 7836 §5.2, has cofactor 4 and was chosen to be birationally equivalent to a twisted Edwards curve, so implementations can use complete addition formulas.
It is the curve of DNSSEC algorithm 23 (RFC 9558) and appears in TLS as group GC256A with signature scheme gostr34102012_256a (RFC 9189 for TLS 1.2, RFC 9367 for TLS 1.3). Signatures pair it with the 256-bit Streebog hash (GOST R 34.11-2012).