GC256C is the 256-bit GOST curve CryptoPro-B (id-GostR3410-2001-CryptoPro-B-ParamSet).
GC256C is a short Weierstrass curve y² = x³ + ax + b over a 256-bit prime field with a 256-bit prime-order subgroup and cofactor 1; its estimated security level is 128 bits. Status: secure, but regional or niche.
GC256C was defined for GOST R 34.10-2001 in RFC 4357 §11.4 and is kept as paramSetC by GOST R 34.10-2012 (RFC 9215). It is y² = x³ − 3x + b over the prime 2^255 + 3225, with prime order.
TLS lists it as group GC256C with signature scheme gostr34102012_256c (RFC 9189, RFC 9367).