functions/Convert-DryADEncryptedBase64StringToClearText.ps1

<#
    This is an AD Config module for use with DryDeploy, or by itself.
    Copyright (C) 2021 Bjørn Henrik Formo (bjornhenrikformo@gmail.com)
    LICENSE: https://raw.githubusercontent.com/bjoernf73/dry.module.ad/main/LICENSE
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.
 
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    GNU General Public License for more details.
 
    You should have received a copy of the GNU General Public License along
    with this program; if not, write to the Free Software Foundation, Inc.,
    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
#>

Function Convert-DryADEncryptedBase64StringToClearText {
    [CmdletBinding()]
    [OutputType([System.String])]
    Param(
        [Parameter(Mandatory)]
        [ValidateNotNullOrEmpty()]
        [String] $EncryptedBase64String
    )
    try {
        # Try to find a certificate in the LocalMachine\My (Personal) Store with
        # - a private key accessible
        # - of type SHA256 RSA (ECDH does not work)
        # - 'Server Authentiaction' as part of the Enhanced Key Usage
        $Cert = Get-ChildItem -Path Cert:\LocalMachine\My -ErrorAction Stop | 
            Where-Object { 
            ($_.HasPrivateKey -eq $True) -and 
            ($_.SignatureAlgorithm.FriendlyName -eq 'SHA256RSA') -and
            (@(($_.EnhancedKeyUsageList).FriendlyName) -contains 'Server Authentication')  
            }

        # If multiple, use first
        If ($Cert -is [Array]) {
            $Cert = $Cert[0]
        }
        
        If ($Cert) {
            $EncryptedByteArray = [Convert]::FromBase64String($EncryptedBase64String)
            $ClearText = [System.Text.Encoding]::UTF8.GetString($Cert.PrivateKey.Decrypt($EncryptedByteArray, $true))
        }
        Else {
            Throw "Server Authentication Certificate with Private Key not found!"
        }
        Return $ClearText
    }
    Catch {
        $PSCmdlet.ThrowTerminatingError($_)
    }
}