Private keys, public keys, and signatures on the Ethereum blockchain are all computed using Elliptic Curve Cryptography (ECC). The 21,000 base gas fee in Ethereum goes toward the "elliptic curve operation to recover the sender address from the signature" as outlined in the Ethereum Wiki. "elliptic" is composed of elliptic curves filled with the elliptic curve formula defined by the secp256k1 standard used by Ethereum: y^2 % p =(x^3 + 7) % p.