powershell - How to decode a Base64 string?

Question

I have a normal string in Powershell that is from a text file containing Base64 text; it is stored in $x. I am trying to decode it as such:

$z = [System.Text.Encoding]::Unicode.GetString([System.Convert]::FromBase64String($x));

This works if $x was a Base64 string created in Powershell (but it's not). And this does not work on the $x Base64 string that came from a file, $z simply ends up as something like ?券.

What am I missing? For example, $x could be YmxhaGJsYWg= which is Base64 for blahblah.

In a nutshell, YmxhaGJsYWg= is in a text file then put into a string in this Powershell code and I try to decode it but end up with ?券 etc.

Answer 1

Isn't encoding taking the text TO base64 and decoding taking base64 BACK to text? You seem be mixing them up here. When I decode using this online decoder I get:

BASE64: blahblah
UTF8: nVnV

not the other way around. I can't reproduce it completely in PS though. See sample below:

PS > [System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String("blahblah"))
nV?nV?

PS > [System.Convert]::ToBase64String([System.Text.Encoding]::UTF8.GetBytes("nVnV"))
blZuVg==

EDIT I believe you're using the wrong encoder for your text. The encoded base64 string is encoded from UTF8(or ASCII) string.

PS > [System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String("YmxhaGJsYWg="))
blahblah

PS > [System.Text.Encoding]::Unicode.GetString([System.Convert]::FromBase64String("YmxhaGJsYWg="))
汢桡汢桡

PS > [System.Text.Encoding]::ASCII.GetString([System.Convert]::FromBase64String("YmxhaGJsYWg="))
blahblah