Validate data from telegram web app (https://core.telegram.org/bots/webapps#validating-data-received-via-the-web-app) in C#.

ValidateWebData.cs

// Parse string initData from telegram.
var data = HttpUtility.ParseQueryString(initData);

// Put data in a alphabetically sorted dict.
var dataDict = new SortedDictionary<string, string>(
    data.AllKeys.ToDictionary(x => x!, x => data[x]!),
    StringComparer.Ordinal);

// Constant key to genrate secret key.
var constantKey = "WebAppData";

// https://core.telegram.org/bots/webapps#validating-data-received-via-the-web-app:
// Data-check-string is a chain of all received fields,
// sorted alphabetically.
// in the format key=<value>.
// with a line feed character ('\n', 0x0A) used as separator.
// e.g., 'auth_date=<auth_date>\nquery_id=<query_id>\nuser=<user>'
var dataCheckString = string.Join(
    '\n', dataDict.Where(x => x.Key != "hash") // Hash should be removed.
        .Select(x => $"{x.Key}={x.Value}")); // like auth_date=<auth_date> ..

// secrecKey is the HMAC-SHA-256 signature of the bot's token
// with the constant string WebAppData used as a key.
var secretKey = HMACSHA256.HashData(
    Encoding.UTF8.GetBytes(constantKey), // WebAppData
    Encoding.UTF8.GetBytes(_botConfig.BotToken)); // Bot's token

var generatedHash = HMACSHA256.HashData(
    secretKey,
    Encoding.UTF8.GetBytes(dataCheckString)); // data_check_string

// Convert received hash from telegram to a byte array.
var actualHash = Convert.FromHexString(dataDict["hash"]); // .NET 5.0 

// Compare our hash with the one from telegram.
if (actualHash.SequenceEqual(generatedHash))
{
    // Data from telegram.
}

For .NET Core 3
You can use byte[] HmacSHA256(byte[] code, byte[] data) { using (HMACSHA256 hmac = new HMACSHA256(code)) { var hmBytes = hmac.ComputeHash(data); return hmBytes; } } instead of HMACSHA256.HashData

There is no way this code can work because you use HMACSHA256 for hashing of token for secret key instead of SHA256.

There is no way this code can work because you use HMACSHA256 for hashing of token for secret key instead of SHA256.

data_check_string = ...
secret_key = HMAC_SHA256(<bot_token>, "WebAppData")
if (hex(HMAC_SHA256(data_check_string, secret_key)) == hash) {
  // data is from Telegram
}

Telegram docs noticed that it should be HMAC_SHA256.

I’m not really sure that it’s working at the moment, but it was.

@immmdreza Man! Thank you very much!

@immmdreza Man! Thank you very much!
🙏

data_check_string = ...
secret_key = SHA256(<bot_token>)
if (hex(HMAC_SHA256(data_check_string, secret_key)) == hash) {
// data is from Telegram
}