GenerateKey generates a public and private key pair.

KeyExchangeA 协商第二部，用户A调用，返回共享密钥k.

KeyExchangeB **************************Key agreement algorithm**************************// KeyExchangeB 协商第二部，用户B调用， 返回共享密钥k.

Sign signs a hash (which should be the result of hashing a larger message) using the private key, priv.

SignASN1 signs a hash (which should be the result of hashing a larger message) using the private key, priv.

SM2 returns a Curve which implements SM2 The cryptographic operations are implemented using constant-time algorithms.

Verify verifies the signature in r, s of hash using the public key, pub.

VerifyASN1 verifies the ASN.1 encoded signature, sig, of hash using the public key, pub.

PrivateKey represents an ECDSA private key.

PublicKey represents an ECDSA public key.