Setting private key permissions with PowerShell

Question

I have a PowerShell script that installs pfx certificate into the LocalMachine certificate store. The function looks like this:

function Add-Certificate {
param
(
    [Parameter(Position=1, Mandatory=$true)]
    [ValidateNotNullOrEmpty()]
    [string]$pfxPath,

    [Parameter(Position=2, Mandatory=$true)]
    [ValidateNotNullOrEmpty()]
    [string]$pfxPassword
)

    Write-Host "Installing certificate" -ForegroundColor Yellow

    try 
    {
        $pfxcert = new-object system.security.cryptography.x509certificates.x509certificate2
        $pfxcert.Import($pfxPath, $pfxPassword, [System.Security.Cryptography.X509Certificates.X509KeyStorageFlags]"PersistKeySet")

        $store = new-object system.security.cryptography.X509Certificates.X509Store -argumentlist "MY", LocalMachine
        $store.Open([System.Security.Cryptography.X509Certificates.OpenFlags]"ReadWrite");
        $store.Add($pfxcert);
        $store.Close();

        return $pfxcert
    }
    catch 
    {
        throw
    }
}

When I open the Certificate Manager to verify the installation I can see that it has installed correctly.

The next step in my process is to assign permissions to the certificate to a service account.

function Set-CertificatePermission
{
    param
    (
        [Parameter(Position=1, Mandatory=$true)]
        [ValidateNotNullOrEmpty()]
        [string]$pfxThumbPrint,

        [Parameter(Position=2, Mandatory=$true)]
        [ValidateNotNullOrEmpty()]
        [string]$serviceAccount
    )

    $cert = Get-ChildItem -Path cert:\LocalMachine\My | Where-Object -FilterScript { $PSItem.ThumbPrint -eq $pfxThumbPrint; };

    # Specify the user, the permissions and the permission type
    $permission = "$($serviceAccount)","Read,FullControl","Allow"
    $accessRule = New-Object -TypeName System.Security.AccessControl.FileSystemAccessRule -ArgumentList $permission;

    # Location of the machine related keys
    $keyPath = $env:ProgramData + "\Microsoft\Crypto\RSA\MachineKeys\";
    $keyName = $cert.PrivateKey.CspKeyContainerInfo.UniqueKeyContainerName;
    $keyFullPath = $keyPath + $keyName;

    try
    {
        # Get the current acl of the private key
        # This is the line that fails!
        $acl = Get-Acl -Path $keyFullPath;

        # Add the new ace to the acl of the private key
        $acl.AddAccessRule($accessRule);

        # Write back the new acl
        Set-Acl -Path $keyFullPath -AclObject $acl;
    }
    catch
    {
        throw $_;
    }
}

This function fails. Specifically, this function fails when trying to evaluate the Get-Acl command, with the following error: Get-Acl : Cannot find path 'C:\ProgramData\Microsoft\Crypto\RSA\MachineKeys\59f1e969a4f7e5de90224f68bc9be536_1d508f5e-0cbc-4eca-a402-3e55947faa3b'

As it turns out the key file has been installed into my roaming profile C:\Users\MyUserName\AppData\Roaming\Microsoft\Crypto\RSA\S-1-5-21-1259098847-1967870486-1845911597-155499

I'm sure there is something wrong with the Add-Certificate function, but I cannot figure out what it is. How do I force it to install the key file in the C:\ProgramData\Microsoft\Crypto\RSA\MachineKeys directory?