TelegramLoginWidget.cs

// using System;
// using System.Text;
// using System.Security.Cryptography;

static bool CheckAuthorization(Dictionary<string, string> fields, string token, string hash)
        {
            int allowedTimeOffset = 30; // Or whatever you choose
            if (!fields.ContainsKey("auth_date")) return false;
            if (long.TryParse(fields["auth_date"], out long timestamp))
            {
                if (DateTime.UtcNow.Subtract(new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)).TotalSeconds - timestamp > allowedTimeOffset)
                    return false;
            }
            else return false;
            
            var sortedFields = fields.OrderBy(i => i.Key).ToArray();
            StringBuilder data_check_string = new StringBuilder();
            for (int i = 0; i < sortedFields.Length; i++)
            {
                data_check_string.Append(sortedFields[i].Key);
                data_check_string.Append('=');
                data_check_string.Append(sortedFields[i].Value);
                if (i != sortedFields.Length - 1)
                {
                    data_check_string.Append('\n');
                }
            }

            using (HMACSHA256 hmac = new HMACSHA256())
            {
                using (SHA256 sha256 = SHA256.Create())
                {
                    hmac.Key = sha256.ComputeHash(Encoding.UTF8.GetBytes(token));
                }
                byte[] signature = hmac.ComputeHash(Encoding.UTF8.GetBytes(data_check_string.ToString()));
                string hexSignature = BitConverter.ToString(signature).Replace("-", "");

                return hexSignature == hash.ToUpper();
            }
        }