func (m *SigningMethodECDSA) Sign(signingString string, key interface{}) (string, error) {
	var ecdsaKey *ecdsa.PrivateKey
	switch k := key.(type) {
	case *ecdsa.PrivateKey:
		ecdsaKey = k
	default:
		return "", ErrInvalidKeyType
	}
	if !m.Hash.Available() {
		return "", ErrHashUnavailable
	}
	hasher := m.Hash.New()
	hasher.Write([]byte(signingString))
	if r, s, err := ecdsa.Sign(rand.Reader, ecdsaKey, hasher.Sum(nil)); err == nil {
		curveBits := ecdsaKey.Curve.Params().BitSize
		if m.CurveBits != curveBits {
			return "", ErrInvalidKey
		}
		keyBytes := curveBits / 8
		if curveBits%8 > 0 {
			keyBytes += 1
		}
		out := make([]byte, 2*keyBytes)
		r.FillBytes(out[0:keyBytes])  
		s.FillBytes(out[keyBytes:])   
		return EncodeSegment(out), nil
	} else {
		return "", err
	}
}
Sign implements token signing for the SigningMethod.
For this signing method, key must be an ecdsa.PrivateKey struct
package jwt
import (
	"crypto"
	"crypto/ecdsa"
	"crypto/rand"
	"errors"
	"math/big"
)
var (
	ErrECDSAVerification = errors.New("crypto/ecdsa: verification error")
)
type SigningMethodECDSA struct {
	Name      string
	Hash      crypto.Hash
	KeySize   int
	CurveBits int
}
var (
	SigningMethodES256 *SigningMethodECDSA
	SigningMethodES384 *SigningMethodECDSA
	SigningMethodES512 *SigningMethodECDSA
)
func init() {
	SigningMethodES256 = &SigningMethodECDSA{"ES256", crypto.SHA256, 32, 256}
	RegisterSigningMethod(SigningMethodES256.Alg(), func() SigningMethod {
		return SigningMethodES256
	})
	SigningMethodES384 = &SigningMethodECDSA{"ES384", crypto.SHA384, 48, 384}
	RegisterSigningMethod(SigningMethodES384.Alg(), func() SigningMethod {
		return SigningMethodES384
	})
	SigningMethodES512 = &SigningMethodECDSA{"ES512", crypto.SHA512, 66, 521}
	RegisterSigningMethod(SigningMethodES512.Alg(), func() SigningMethod {
		return SigningMethodES512
	})
}
func (m *SigningMethodECDSA) Alg() string {
	return m.Name
}
func (m *SigningMethodECDSA) Verify(signingString, signature string, key interface{}) error {
	var err error
	var sig []byte
	if sig, err = DecodeSegment(signature); err != nil {
		return err
	}
	var ecdsaKey *ecdsa.PublicKey
	switch k := key.(type) {
	case *ecdsa.PublicKey:
		ecdsaKey = k
	default:
		return ErrInvalidKeyType
	}
	if len(sig) != 2*m.KeySize {
		return ErrECDSAVerification
	}
	r := big.NewInt(0).SetBytes(sig[:m.KeySize])
	s := big.NewInt(0).SetBytes(sig[m.KeySize:])
	if !m.Hash.Available() {
		return ErrHashUnavailable
	}
	hasher := m.Hash.New()
	hasher.Write([]byte(signingString))
	if verifystatus := ecdsa.Verify(ecdsaKey, hasher.Sum(nil), r, s); verifystatus {
		return nil
	}
	return ErrECDSAVerification
}