secp521r1

secp521r1 (P-521, note: 521 not 512) is the largest NIST curve, providing approximately 256-bit security. It offers the highest security margin of any commonly supported curve, suitable for ultra-long-term secrecy requirements (50+ years). The unusual 521-bit size (not 512) was chosen for mathematical reasons related to Mersenne primes, which simplify modular arithmetic. Despite the name, the actual security is around 256 bits. Performance is very slow (5-10x slower than X25519), making it impractical for most applications. Browser support exists but usage is extremely rare outside specialized military/government contexts.