Compute Github Webhook SHA1 from github secret for PowerShell / C#.

ComputeGithubWebhookSha1WithSecret.cs

var target = "YOUR GITHUB WEBHOOK JSON";
var key = "GITHUB WEBHOOK SECRET";

var data = System.Text.Encoding.UTF8.GetBytes(target);
var keyData = System.Text.Encoding.UTF8.GetBytes(key);
using (var hmac = new System.Security.Cryptography.HMACSHA1(keyData))
{
    var bs = hmac.ComputeHash(data);
    var sha1 = System.BitConverter.ToString(bs).ToLower().Replace("-","");
    Console.WriteLine($"YOUR WEBHOOK JSON HASH (SHA1) is : {sha1}");
}

ComputeGithubWebhookSha1WithSecret.ps1

# Serialize your webhook json to file, or passdirectly from data
$target = cat -Raw -Path .\GITHUBWEBHOOK.json;
# or use raw string.
# $target = "YOUR GITHUB WEBHOOK JSON";
$key = "GITHUB WEBHOOK SECRET";

$data = [System.Text.Encoding]::UTF8.GetBytes($target);
$keyData = [System.Text.Encoding]::UTF8.GetBytes($key);
$hmac = [System.Security.Cryptography.HMACSHA1]::new($keyData);
$bs = $hmac.ComputeHash($data);
$hmac.Dispose();
$sha1 = [System.BitConverter]::ToString($bs).ToLower().Replace("-","");

Write-Host "YOUR WEBHOOK JSON HASH (SHA1) is : $sha1"