Decode Base64 String in Powershell

Question

I have a Base64 encoded file and I am trying the following to decode it:

[System.Text.Encoding]::ASCII.GetString([System.Convert]::FromBase64String($data)) 

However it generates the following error:

Exception calling "FromBase64String" with "1" argument(s): "The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding 
characters, or an illegal character among the padding characters.

Although it is Base64, completely. Any ideas?

*Please note that it is Base64, but not encoded by PowerShell.

Answer 1

If you would like to decode a Base64 string to text this will do:

$EncodedText = 'SGVsbG8gV29ybGQh'
$DecodedText = [System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String($EncodedText))
$DecodedText

If you need to decode a Base64 string to a binary file this will do:

$base64string = ''
$FileName = "file.png"
[IO.File]::WriteAllBytes($FileName, [Convert]::FromBase64String($base64string))

Answer 2

Some applications require the Base64 string to end with one or two equal signs as the padding character. For a quick test you can add one or two "=" characters at the end of your variable $data like this:

[System.Text.Encoding]::ASCII.GetString([System.Convert]::FromBase64String($data+"="))

and try decoding again.

See Decoding Base64 with padding