code: func NewECDSAVerifier() Verifier {
  	return verifyFn(func(checksum, signature []byte, hash crypto.Hash, publicKey crypto.PublicKey) error {
  		key, ok := publicKey.(*ecdsa.PublicKey)
  		if !ok {
  			return errors.New("not a valid ECDSA public key")
  		}
  		var rs rsDER
  		if _, err := asn1.Unmarshal(signature, &rs); err != nil {
  			return err
  		}
  		if !ecdsa.Verify(key, checksum, rs.R, rs.S) {
  			return errors.New("failed to verify ecsda signature")
  		}
  		return nil
  	})
  }
docs: // NewECDSAVerifier returns a Verifier that uses the ECDSA algorithm to verify updates.