Hallo I’m trying to figure out how to recivece a installation token from my GitHub App via PowerShell.

Generating JWT based on private PEM

Following code is used which is basically copied from the GitHub guide.

function Generate-JWT-FromGithub
{
    $client_id = <<APP_ID>>
    $private_key_path = <<LOKAL_PATH_TO_MY_PEM_FILE>>
    try {
        $header = [Convert]::ToBase64String([System.Text.Encoding]::UTF8.GetBytes((ConvertTo-Json -InputObject @{
            alg = "RS256"
            typ = "JWT"
            }))).TrimEnd('=').Replace('+', '-').Replace('/', '_');
        
            $payload = [Convert]::ToBase64String([System.Text.Encoding]::UTF8.GetBytes((ConvertTo-Json -InputObject @{
            iat = [System.DateTimeOffset]::UtcNow.AddSeconds(-10).ToUnixTimeSeconds()
            exp = [System.DateTimeOffset]::UtcNow.AddMinutes(10).ToUnixTimeSeconds()
                iss = $client_id
            }))).TrimEnd('=').Replace('+', '-').Replace('/', '_');

            $rsa = [System.Security.Cryptography.RSA]::Create()
            $rsa.ImportFromPem((Get-Content $private_key_path -Raw))
        
            $signature = [Convert]::ToBase64String($rsa.SignData([System.Text.Encoding]::UTF8.GetBytes("$header.$payload"), [System.Security.Cryptography.HashAlgorithmName]::SHA256, [System.Security.Cryptography.RSASignaturePadding]::Pkcs1)).TrimEnd('=').Replace('+', '-').Replace('/', '_')
            $jwt = "$header.$payload.$signature"
            return $jwt
    }
    catch {
        Handle-Error $_.Exception.Message
    }
}

Get Installation Token from GitHub API

function Get-InstallationToken {
    param(
        $JwtToken,        
        $InstallationId    
    )

    $Headers = @{
        Authorization = "Bearer $JwtToken"
        Accept        = "application/vnd.github.v3+json"
        "X-GitHub-Api-Version" = "2022-11-28"
    }
    $Url = "https://api.github.com/app/installations/$InstallationId/access_tokens"
    $Response = Invoke-RestMethod -Method Post -Uri $Url -Headers $Headers

    return $Response.token
}

Call in PowerShell 7

$InstallationId = <<ID_FROM_MY_GITHUB_APP>>
$JwtToken = Generate-JWT-FromGithub 
$InstallationToken = Get-InstallationToken -JwtToken $JwtToken -InstallationId $InstallationId

Following issue I’m facing

Response status code does not indicate success: 401 (Unauthorized).

I made sure that the PEM file is valid. Also I checked twice the APP_ID and Installation ID.
Results all the time in “Response status code does not indicate success: 401 (Unauthorized).”