微信支付的二次签名

-(NSString *)createMD5SingForPayWithAppID:(NSString *)appid_key partnerid:(NSString *)partnerid_key prepayid:(NSString *)prepayid_key package:(NSString *)package_key noncestr:(NSString *)noncestr_key timestamp:(UInt32)timestamp_key{
    
    
    NSMutableDictionary *signParams = [NSMutableDictionary dictionary];
    [signParams setObject:appid_key forKey:@"appid"];//微信appid 例如wxfb132134e5342
    [signParams setObject:noncestr_key forKey:@"noncestr"];//随机字符串
    [signParams setObject:package_key forKey:@"package"];//扩展字段  参数为 Sign=WXPay
    [signParams setObject:partnerid_key forKey:@"partnerid"];//商户账号
    [signParams setObject:prepayid_key forKey:@"prepayid"];//此处为统一下单接口返回的预支付订单号
    [signParams setObject:[NSString stringWithFormat:@"%u",timestamp_key] forKey:@"timestamp"];//时间戳
    
    NSMutableString *contentString  =[NSMutableString string];
    NSArray *keys = [signParams allKeys];
    //按字母顺序排序
    NSArray *sortedArray = [keys sortedArrayUsingComparator:^NSComparisonResult(id obj1, id obj2) {
        return [obj1 compare:obj2 options:NSNumericSearch];
    }];
    //拼接字符串
    for (NSString *categoryId in sortedArray) {
        if (   ![[signParams objectForKey:categoryId] isEqualToString:@""]
            && ![[signParams objectForKey:categoryId] isEqualToString:@"sign"]
            && ![[signParams objectForKey:categoryId] isEqualToString:@"key"]
            )
        {
            [contentString appendFormat:@"%@=%@&", categoryId, [signParams objectForKey:categoryId]];
        }
    }
    //添加商户密钥key字段  API 密钥
    [contentString appendFormat:@"key=%@", @"你的key"];
    NSString *result = [self MD5ForLower32Bate:contentString];//md5加密
    return result;
}


-(NSString *)MD5ForLower32Bate:(NSString *)str{
    
    //要进行UTF8的转码
    const char* input = [str UTF8String];
    unsigned char result[CC_MD5_DIGEST_LENGTH];
    CC_MD5(input, (CC_LONG)strlen(input), result);
    
    NSMutableString *digest = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
    for (NSInteger i = 0; i < CC_MD5_DIGEST_LENGTH; i++) {
        [digest appendFormat:@"%02x", result[i]];
    }
    
    return digest;
}