GC256B is the 256-bit GOST curve known since 2006 as CryptoPro-A (id-GostR3410-2001-CryptoPro-A-ParamSet).
GC256B 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.
GC256B was defined for GOST R 34.10-2001 and published in RFC 4357 §11.4 as id-GostR3410-2001-CryptoPro-A-ParamSet; GOST R 34.10-2012 keeps it as paramSetB (RFC 9215). It is the prime-order curve y² = x³ − 3x + 166 over the prime 2^256 − 617.
The same parameters are registered for key exchange as CryptoPro-XchA, which is why the curve has two OIDs. DNSSEC algorithm 12 (RFC 5933, now deprecated) used it, and TLS lists it as group GC256B (RFC 9189, RFC 9367).