🚀[Feature]: Add functionality to ease the use of this for Function Apps

[MariusStorhaug]

Description

Stuff like:

• Connect using GitHub App -> Connect-GitHubAccount -ClientID ... -PrivateKey ...
• Gather info from a request (as a bot). Map all events into objects.
• Check signature from WebHook

$secret = '1231456789564321654987654321654987' # This should be from a KeyVault

function Test-Signature {
    param (
        [string]$PayloadBody,
        [string]$Secret,
        [string]$ReceivedSignature
    )

    $HMAC = New-Object System.Security.Cryptography.HMACSHA256
    $HMAC.Key = [Text.Encoding]::UTF8.GetBytes($Secret)
    $GeneratedSignatureBytes = $HMAC.ComputeHash([Text.Encoding]::UTF8.GetBytes($PayloadBody))
    $GeneratedSignature = 'sha256=' + ( -join ($GeneratedSignatureBytes | ForEach-Object { $_.ToString('x2') }))

    Write-Verbose "Received signature:  $ReceivedSignature"
    Write-Verbose "Generated signature: $GeneratedSignature"

    return Compare-SecureString -String1 $ReceivedSignature -String2 $GeneratedSignature
}

function Compare-SecureString {
    param (
        [string]$String1,
        [string]$String2
    )

    Write-Information 'Comparing:'
    Write-Information " - $String1"
    Write-Information " - $String2"

    if ($String1.Length -ne $String2.Length) {
        return $false
    }

    $Diff = 0
    for ($i = 0; $i -lt $String1.Length; $i++) {
        $Diff += [int][char]$String1[$i] -bxor [int][char]$String2[$i]
    }

    Write-Information "Diff: $Diff"
    return $Diff -eq 0
}
# Compare the signatures
$requestBody = $Request.Body | ConvertTo-Json -Depth 100 -Compress
if (Test-Signature -PayloadBody $requestBody -SecretToken $key -ReceivedSignature $Request.Headers['X-Hub-Signature-256']) {
    Write-Information 'Signatures match.'
    $response = [HttpResponseContext]@{
        StatusCode = [HttpStatusCode]::OK
    }
} else {
    Write-Information 'Signatures do not match.'
    $response = [HttpResponseContext]@{
        StatusCode = [System.Net.HttpStatusCode]::Unauthorized
    }
}

Push-OutputBinding -Name Response -Value (
    $response
)