Sample code on how to post an xml / base64 encoded file as a SAMLResponse to a server (Saml assertion)

Posting SAML Assertion to from PowerShell

Add-Type -AssemblyName System.Web;

$samlFile = "C:\Tmp\Assertion.xml"
$serviceProviderUri = "http://MYAPP/Sso/Acs"

function Get-SamlPayload($file){
    $f = Get-ChildItem $file;
    # If the file is an xml, content must be encoded.
    if($f.Extension -match ".xml"){
        $payload = ([xml](Get-Content $file)).InnerXml;        
        $payload = [System.Convert]::ToBase64String($payload.ToCharArray());
    } else{
        $payload = Get-Content $file;
    }        
    # Modify the payload, by encoding invalid characters to post.
    $payload = $payload -replace "[+]" , "%2B" -replace "=" , "%3D";
    return $payload;
}

$data = ("SAMLResponse={0}" -f (Get-SamlPayload -file $samlFile));

$headers = @{
    "Content-Type"= "application/x-www-form-urlencoded";
    "Accept"="text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8";
    "Accept-Encoding"="gzip, deflate";
    "Accept-Language"="en-US,en;q=0.8,nb;q=0.6"
    "Content-Length"=$data.Length;
};

Invoke-WebRequest $serviceProviderUri -method "POST" -Body $data -Headers $headers;