How to decode a Base64 string?

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 䐲券.

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 䐲券 etc.

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

There are no PowerShell-native commands for Base64 conversion - yet (as of PowerShell [Core] 7.1), but adding dedicated cmdlets has been suggested in GitHub issue #8620.
For now, direct use of .NET is needed.

Important:

• Base64 encoding is an encoding of binary data using bytes whose values are constrained to a well-defined 64-character subrange of the ASCII character set representing printable characters, devised at a time when sending arbitrary bytes was problematic, especially with the high bit set (byte values > 0x7f).

• Therefore, you must always specify explicitly what character encoding the Base64 bytes do / should represent.

Ergo:

• on converting TO Base64, you must first obtain a byte representation of the string you're trying to encode using the character encoding the consumer of the Base64 string expects.

• on converting FROM Base64, you must interpret the resultant array of bytes as a string using the same encoding that was used to create the Base64 representation.

Examples:

Note:

• The following examples convert to and from UTF-8 encoded strings:

• To convert to and from UTF-16LE ("Unicode") instead, substitute [Text.Encoding]::Unicode for [Text.Encoding]::UTF8

Convert TO Base64:

PS> [Convert]::ToBase64String([Text.Encoding]::UTF8.GetBytes('Motörhead'))
TW90w7ZyaGVhZA==

Convert FROM Base64:

PS> [Text.Encoding]::Utf8.GetString([Convert]::FromBase64String('TW90w7ZyaGVhZA=='))
Motörhead

This page shows up when you google how to convert to base64, so for completeness:

$b  = [System.Text.Encoding]::UTF8.GetBytes("blahblah")
[System.Convert]::ToBase64String($b)

Base64 encoding converts three 8-bit bytes (0-255) into four 6-bit bytes (0-63 aka base64). Each of the four bytes indexes an ASCII string which represents the final output as four 8-bit ASCII characters. The indexed string is typically 'A-Za-z0-9+/' with '=' used as padding. This is why encoded data is 4/3 longer.

Base64 decoding is the inverse process. And as one would expect, the decoded data is 3/4 as long.

While base64 encoding can encode plain text, its real benefit is encoding non-printable characters which may be interpreted by transmitting systems as control characters.

I suggest the original poster render $z as bytes with each bit having meaning to the application. Rendering non-printable characters as text typically invokes Unicode which produces glyphs based on your system's localization.

Base64decode("the answer to life the universe and everything") = 00101010

I had issues with spaces showing in between my output and there was no answer online at all to fix this issue. I literally spend many hours trying to find a solution and found one from playing around with the code to the point that I almost did not even know what I typed in at the time that I got it to work. Here is my fix for the issue: [System.Text.Encoding]::UTF8.GetString(([System.Convert]::FromBase64String($base64string)|?{$_}))

Still not a "built-in", but published to gallery, authored by MS:

https://github.com/powershell/textutility

TextUtility

• ConvertFrom-Base64
  Return a string decoded from base64.
• ConvertTo-Base64
  Return a base64 encoded representation of a string.

If anyone would like to do it with a pipe in Powershell (like a filter) (e.g. read file contents and decode it), it can be achieved with a one-liner like that:

Get-Content base64.txt | %{[Text.Encoding]::UTF8.GetString([Convert]::FromBase64String($_))}