func (m *SigningMethodHMAC) Verify(signingString, signature string, key interface{}) error {
	keyBytes, ok := key.([]byte)
	if !ok {
		return ErrInvalidKey
	}
	sig, err := DecodeSegment(signature)
	if err != nil {
		return err
	}
	if !m.Hash.Available() {
		return ErrHashUnavailable
	}
	hasher := hmac.New(m.Hash.New, keyBytes)
	hasher.Write([]byte(signingString))
	if !hmac.Equal(sig, hasher.Sum(nil)) {
		return ErrSignatureInvalid
	}
	return nil
}
Verify the signature of HSXXX tokens.  Returns nil if the signature is valid.
package jwt
import (
	"crypto"
	"crypto/hmac"
	"errors"
)
type SigningMethodHMAC struct {
	Name string
	Hash crypto.Hash
}
var (
	SigningMethodHS256  *SigningMethodHMAC
	SigningMethodHS384  *SigningMethodHMAC
	SigningMethodHS512  *SigningMethodHMAC
	ErrSignatureInvalid = errors.New("signature is invalid")
)
func init() {
	SigningMethodHS256 = &SigningMethodHMAC{"HS256", crypto.SHA256}
	RegisterSigningMethod(SigningMethodHS256.Alg(), func() SigningMethod {
		return SigningMethodHS256
	})
	SigningMethodHS384 = &SigningMethodHMAC{"HS384", crypto.SHA384}
	RegisterSigningMethod(SigningMethodHS384.Alg(), func() SigningMethod {
		return SigningMethodHS384
	})
	SigningMethodHS512 = &SigningMethodHMAC{"HS512", crypto.SHA512}
	RegisterSigningMethod(SigningMethodHS512.Alg(), func() SigningMethod {
		return SigningMethodHS512
	})
}
func (m *SigningMethodHMAC) Alg() string {
	return m.Name
}