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.

user2766136 · Accepted Answer · 2013-09-19 23:27:11Z

28

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

answered Sep 19, 2013 at 23:27

user2766136

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2 Comments

Ben Over a year ago

I suggest -Encoding "UTF-8" instead

VeeTheSecond Over a year ago

Looks like it's -Encoding "UTF8"

NYCdotNet · Accepted Answer · 2019-02-21 15:38:48Z

22

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

answered Feb 21, 2019 at 15:38

NYCdotNet

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1 Comment

Joachim Otahal Over a year ago

Encoding Byte - that is the best answer I needed right now, since some sent .tar.gz.base64 - there actually "never" is an ascii target. Unless someone wants to endanger his data, even if it is ascii.

groenhen · Accepted Answer · 2019-12-26 16:52:40Z

22

On Windows 10, using PowerShell you can do:

certutil -decode in.b64 out.txt

edited Dec 26, 2019 at 16:52

groenhen

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answered Dec 26, 2019 at 13:43

marc

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1 Comment

Sagebrush Gardener Over a year ago

This is the best answer. System.Convert is ridiculously slow and never finishes with a 10Mb file. The same file takes about one second with certutil.

Random_Guy · Accepted Answer · 2021-09-30 09:18:08Z

4

Old question, new answer. Solution that preserv newlines

// Generate Content
ssh-keygen -t rsa -b 2048 -f example_rsa
// Encode
[System.Convert]::ToBase64String([System.Text.Encoding]::UTF8.GetBytes($(Get-Content -Path example_rsa -Encoding utf8 -Raw))) >> encoded
// Decode
[System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String($(Get-Content -Path encoded -Encoding utf8 -Raw))) >> decoded

answered Sep 30, 2021 at 9:18

Random_Guy

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2 Comments

Liviu Over a year ago

Unfortunately, both commands will write the BOM to the file ... see stackoverflow.com/questions/5596982/… Use [IO.File]::WriteAllLinesto avoid it.

GuySoft Over a year ago

If the base64 is binary data, for example a file. I had to use this: [System.Convert]::FromBase64String($VAR) | Set-Content file.zip -Encoding Byte

Stoffi · Accepted Answer · 2018-11-26 20:17:48Z

1

I know it's an old question, but i found another answer. You can use unicode at the base64 conversion, it will fit after.

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

answered Nov 26, 2018 at 20:17

Stoffi

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Comments

TeamDman · Accepted Answer · 2022-07-20 13:10:39Z

This function should behave the same as the linux base64 command, at least in regards to encoding and decoding. It can also accept values from pipes.

function base64
{
    [CmdletBinding()]
    param(
        [Parameter(Mandatory, ValueFromPipeline)]
        [string]
        $str,
        [switch]
        $decode = $false
    )
    if ($decode)
    {
        return [System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String($str))
    }
    else
    {
        return [System.Convert]::ToBase64String([System.Text.Encoding]::UTF8.GetBytes($str))
    }
}

PS > base64 -decode YXNk
asd
PS > base64 YXNk
WVhOaw==
PS > echo YXNk | base64
WVhOaw==
PS > echo YXNk | base64 -decode
asd

Note that Get-Content returns an Object[] by default

PS > $z=Get-Content .\README.md; $z.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array

Piping this into the func above will only convert the first line.
To convert an entire file, instead use Get-Content -Raw

Get-Content -Raw README.md | base64

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