package com.android.tencentvpn.util;
import io.jsonwebtoken.Jwts;
import io.jsonwebtoken.SignatureAlgorithm;
import java.security.Key;
import java.util.Date;
import javax.crypto.spec.SecretKeySpec;

public class SignGenerate {
    private static final long EXPIRE_TIME = 72 * 60 * 60 * 1000; // expiration time, milliseconds
    /**
     * @param deviceName   unique device identifier controlled by the business
     * @param SecretKey   application key returned by the cloud api
     * @return   The signature string required by the multi-network sdk   Call via setSign finally
     *
     */
    public static String generateSign(String deviceName, String SecretKey) {
        try {
            Date now = new Date();
            Date expireDate = new Date(now.getTime() + EXPIRE_TIME);
            Key key =
                    new SecretKeySpec(SecretKey.getBytes(), SignatureAlgorithm.HS256.getJcaName());
            return Jwts.builder()
                    .claim("deviceName", deviceName)
                    .notBefore(now)
                    .expiration(expireDate)
                    .signWith(key)
                    .compact();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return "";
    }
}

Add pod 'JWT' to the podfile for JWT library integration 
// .h file:
@interface SignGenerator : NSObject
/**
* @deviceName unique device identifier controlled by the business
* @param SecretKey application key returned by the cloud api
* @return The signature string required by the multi-network sdk Call via setSign finally
*/
 + (NSString *)generateSignWithDeviceName:(NSString *)deviceName secretKey:(NSString *)secretKey;
@end
// .m file
#import "SignGenerator.h"
#import <JWT/JWT.h>
@implementation SignGenerator
+ (NSString *)generateSignWithDeviceName:(NSString *)deviceName secretKey:(NSString *)secretKey {
    // 1. Set expiration time (after 72 hours)
        NSDate *currentDate = [NSDate date];
        NSDate *expireDate = [currentDate dateByAddingTimeInterval:72 * 60 * 60];
        // 2. Build Claims (payload)
        NSDictionary *claims = @{
            @"deviceName": deviceName,
            @"nbf": @((long)[currentDate timeIntervalSince1970]), // Not Before
            @"exp": @((long)[expireDate timeIntervalSince1970])   // Expiration
        };
        NSData *keyData = [secretKey dataUsingEncoding:NSUTF8StringEncoding];
        // 3. Use HS256 algorithm and key signature
        JWTBuilder *builder = [JWTBuilder encodePayload:claims]
            .secretData(keyData)
            .algorithmName(@"HS256"); //algorithm name
        NSString *jwt = builder.encode;
        // 4. error handling
        if (builder.jwtError) {
            NSLog(@"JWT generation failed: %@", builder.jwtError);
            return @"";
        }
        return jwt;
}
@end

package main
import (
	"fmt"
	"github.com/dgrijalva/jwt-go/v4"
	"time"
)
func main() {
	sign, expireTime, err := GenerateSign("tencent-test")
	if err != nil {
		panic(err)
	}
	println(sign, expireTime)
}
// SignClaims JwtCustomClaims
type SignClaims struct {
	jwt.StandardClaims
	DeviceName string `json:"deviceName"`
}
var (
	ExpireTime = 72                                                     // expiration time, hours
	SecretKey = []byte("shCrVLKq/Hp6UdtVpw8lBqu6ZDcLwOABf2Az67e/pjI=") // APP key
)
// GenerateSign generate signature
func GenerateSign(deviceName string) (string, int64, error) {
	expireTime := time.Now().Add(time.Duration(ExpireTime) * time.Hour)
	claims := &SignClaims{
		StandardClaims: jwt.StandardClaims{
			NotBefore: jwt.Now(),
			ExpiresAt: jwt.At(expireTime),
		},
		DeviceName: deviceName,
	}
	println(fmt.Sprintf("%#v", claims))

	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
	sign, err := token.SignedString(SecretKey)
	if err != nil {
		return "", 0, err
	}
	return sign, expireTime.Unix(), nil
}