# define and encode test data
$TestString = 'This is a test. A short test for encoding and padding.'
$Encoded = [Convert]::ToBase64String([Text.Encoding]::UTF8.GetBytes($TestString))

# insert random '='
$Length = $Encoded.Length
$RandomChar = 1..($Length - 3) | Get-Random
$Encoded = $Encoded.Insert($RandomChar,'=')

# strip out '='
$Stripped = $Encoded.Replace('=','')

# append appropriate padding
$ModulusValue = ($Stripped.length % 4)
    Switch ($ModulusValue) {
        '0' {$Padded = $Stripped}
        '1' {$Padded = $Stripped.Substring(0,$Stripped.Length - 1)}
        '2' {$Padded = $Stripped + ('=' * (4 - $ModulusValue))}
        '3' {$Padded = $Stripped + ('=' * (4 - $ModulusValue))}
    }

# base64 decode and output
$Decoded = [Text.Encoding]::UTF8.GetString([Convert]::FromBase64String($Padded))
Write-Output $Decoded