Decoding base64 with powershell

Question

I'm attempting to decode an attachment in an email file from base64 and save it to disk.

For testing purposes, this is my code. Where input.txt contains just the base64 encoded data, which is an HTML file.

$file = "C:\input.txt"
$data = Get-Content $file
[System.Text.Encoding]::ASCII.GetString([System.Convert]::FromBase64String($data)) > out.html

The decoding works fine, and it generates a new file that contains all of the lines, and is visibly identical to the original attachment. The problem is that the output file is twice the size (actually (filesize*2)+6 bytes, in this case).

Am I decoding this improperly? I've also tried UTF8 instead of ASCII... same result.

Answer 1

Well I got it working. Who knew Out-File re-encoded to Unicode by default? Solved by doing the following:

$file = "C:\input.txt"
$data = Get-Content $file
[System.Text.Encoding]::ASCII.GetString([System.Convert]::FromBase64String($data)) | Out-File -Encoding "ASCII" out.html

Answer 2

This one-liner preserves the original encoding of the base64 encoded file, so it will work with binary files such as a PDF or ZIP. Change ".\input.txt" and output.bin as needed - this will take .\input.txt, base 64 decode it, and then write the bytes out to output.bin exactly as they were when the file was encoded.

$file = ".\input.txt"; [System.Convert]::FromBase64String((Get-Content $file)) | Set-Content output.bin -Encoding Byte