aliyun oss js直传且使用服务器端生成签名

 public class AliyunController : MyControllerBase
    {
        public string accessKeyId = System.Configuration.ConfigurationManager.AppSettings["aliyun_ak_id"];
        public string accessKeySecret = System.Configuration.ConfigurationManager.AppSettings["aliyun_ak_secret"];
        public string bucket = System.Configuration.ConfigurationManager.AppSettings["aliyun_bucket"];
        // GET: Aliyun
        public ContentResult GetPostObjectSignature(int courseId)
        {
            string host = "http://"+ bucket + ".oss-cn-shanghai.aliyuncs.com";

            const string endpoint = "http://oss-cn-shanghai.aliyuncs.com";

            var ossClient = new OssClient(endpoint, accessKeyId, accessKeySecret);

            var dir = "zhangsan/";//设置用户上传指定的前缀，必须以斜线结尾
            var expiration = DateTime.Now.AddMinutes(100);
            var policyConds = new PolicyConditions();
            policyConds.AddConditionItem(MatchMode.StartWith, PolicyConditions.CondKey, dir);//上传目录
            policyConds.AddConditionItem(PolicyConditions.CondContentLengthRange, 1, 1048576000);//允许上传的文件大小限制
            var postPolicy = ossClient.GeneratePostPolicy(expiration, policyConds);//给policyConds添加过期时间并json序列化（格式iso8601:"yyyy-MM-dd‘T‘HH:mm:ss.fff‘Z‘"）

            /*生成的Policy范例
                {"expiration":"2017-05-17T20:23:23Z","conditions":[["content-length-range",0,1048576000],["starts-with","$key","zhangsan"]]}
                */
            var base64Policy = Convert.ToBase64String(Encoding.UTF8.GetBytes(postPolicy));
            var signature = ComputeSignature(accessKeySecret, base64Policy);//生成签名

            TimeSpan ts = expiration - new DateTime(1970, 1, 1, 0, 0, 0, 0);
            var expire = Convert.ToInt64(ts.TotalSeconds);

            Dictionary<string, object> response = new Dictionary<string, object>();
            response["accessid"] = accessKeyId;
            response["host"] = host;
            response["policy"] = base64Policy;
            response["signature"] = signature;
            response["expire"] = expire;
            //这个参数是设置用户上传指定的前缀
            response["dir"] = dir;
            return Content(JsonConvert.SerializeObject(response));
        }
}