Actually Windows does have a utility that encodes and decodes base64 - CERTUTIL

I'm not sure what version of Windows introduced this command.

To encode a file:

certutil -encode inputFileName encodedOutputFileName

To decode a file:

certutil -decode encodedInputFileName decodedOutputFileName

There are a number of available verbs and options available to CERTUTIL.

To get a list of nearly all available verbs:

certutil -?

To get help on a particular verb (-encode for example):

certutil -encode -?

To get complete help for nearly all verbs:

certutil -v -?

Mysteriously, the -encodehex verb is not listed with certutil -? or certutil -v -?. But it is described using certutil -encodehex -?. It is another handy function :-)

Update

Regarding David Morales' comment, there is a poorly documented type option to the -encodehex verb that allows creation of base64 strings without header or footer lines.

certutil [Options] -encodehex inFile outFile [type]

A type of 1 will yield base64 without the header or footer lines.

See https://www.dostips.com/forum/viewtopic.php?f=3&t=8521#p56536 for a brief listing of the available type formats. And for a more in depth look at the available formats, see https://www.dostips.com/forum/viewtopic.php?f=3&t=8521#p57918.

Not investigated, but the -decodehex verb also has an optional trailing type argument.

You can use the CERTUTIL command without installing external software (though it will need temp files):

@echo off
del /q /f "%temp%\b64"  >nul 2>nul
del /q /f "%temp%\decoded"  >nul 2>nul

set "base64string=YmFzZTY0c3RyaW5n"
echo -----BEGIN CERTIFICATE----->"%temp%\b64"
<nul set /p=%base64string% >>"%temp%\b64"
echo -----END CERTIFICATE----->>"%temp%\b64"

certutil /decode "%temp%\b64" "%temp%\decoded" >nul 2>nul


for /f "useback tokens=* delims=" %%# in ("%temp%\decoded")  do set "decoded=%%#"
echo %decoded%
del /q /f "%temp%\b64"  >nul 2>nul
del /q /f "%temp%\decoded"  >nul 2>nul

You can also use powershell which will be slower despite there will be no temp files:

set "base64string=YmFzZTY0c3RyaW5n"
for /f "tokens=* delims=" %%# in ('powershell [System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String("""%base64string%"""^)^)') do set "decoded=%%#"
echo %decoded%

You can try also with atob.bat :

call atob.bat YmFzZTY0c3RyaW5n decoded
echo %decoded%

Or with base64.bat

for /f "tokens=* delims=" %%# in ('base64.bat -decode YmFzZTY0c3RyaW5n') do set "decoded=%%#"
echo %decoded% 

Just use the base64 program from the coreutils package:

echo QWxhZGRpbjpvcGVuIHNlc2FtZQ== | base64 --decode

Or, to include the newline character

echo `echo QWxhZGRpbjpvcGVuIHNlc2FtZQ== | base64 --decode`

output (includes newline):

Aladdin:open sesame

Here's a PowerShell one-liner you can run from a cmd console that'll Base64 encode a string.

powershell "[convert]::ToBase64String([Text.Encoding]::UTF8.GetBytes(\"Hello world!\"))"

It's probably not as fast as npocmaka's solution, but you could set a console macro with it.

doskey btoa=powershell "[convert]::ToBase64String([Text.Encoding]::UTF8.GetBytes(\"$*\"))"
doskey atob=powershell "[Text.Encoding]::UTF8.GetString([convert]::FromBase64String(\"$*\"))"

btoa Hello world!
btoa This is fun.
btoa wheeeeee!
atob SGVsbG8gd29ybGQh

Be advised that doskey doesn't work in batch scripts -- only the console. If you want do use this in a batch script, make a function.

@echo off
setlocal

call :btoa b64[0] "Hello world!"
call :btoa b64[1] "This is fun."
call :btoa b64[2] "wheeeeee!"
call :atob b64[3] SGVsbG8gd29ybGQh

set b64
goto :EOF

:btoa <var_to_set> <str>
for /f "delims=" %%I in (
    'powershell "[convert]::ToBase64String([Text.Encoding]::UTF8.GetBytes(\"%~2\"))"'
) do set "%~1=%%I"
goto :EOF

:atob <var_to_set> <str>
for /f "delims=" %%I in (
    'powershell "[Text.Encoding]::UTF8.GetString([convert]::FromBase64String(\"%~2\"))"'
) do set "%~1=%%I"
goto :EOF

Or if you'd prefer a batch + JScript hybrid:

@if (@CodeSection==@Batch) @then
@echo off & setlocal

call :btoa b64[0] "Hello world!"
call :btoa b64[1] "This is fun."
call :btoa b64[2] "wheeeeee!"
call :atob b64[3] SGVsbG8gd29ybGQh

set b64
goto :EOF

:btoa <var_to_set> <str>
:atob <var_to_set> <str>
for /f "delims=" %%I in ('cscript /nologo /e:JScript "%~f0" %0 "%~2"') do set "%~1=%%I"
goto :EOF

@end // end batch / begin JScript hybrid code
var htmlfile = WSH.CreateObject('htmlfile');
htmlfile.write('<meta http-equiv="x-ua-compatible" content="IE=10" />');
WSH.Echo(htmlfile.parentWindowWSH.Arguments(0).substr(1)));

Edit: batch + VBScript hybrid for @Hackoo:

<!-- : batch portion
@echo off & setlocal

call :btoa b64[0] "Hello world!"
call :btoa b64[1] "This is fun."
call :btoa b64[2] "wheeeeee!"
call :atob b64[3] SGVsbG8gd29ybGQh

set b64
goto :EOF

:btoa <var_to_set> <str>
:atob <var_to_set> <str>
for /f "delims=" %%I in ('cscript /nologo "%~f0?.wsf" %0 "%~2"') do set "%~1=%%I"
goto :EOF

: VBScript -->
<job>
    <script language="VBScript">
        Set htmlfile = WSH.CreateObject("htmlfile")
        htmlfile.write("<meta http-equiv='x-ua-compatible' content='IE=10' />")
        if WSH.Arguments(0) = ":btoa" then
            WScript.Echo htmlfile.parentWindow.btoa(WSH.Arguments(1))
        else
            WScript.Echo htmlfile.parentWindow.atob(WSH.Arguments(1))
        end if
    </script>
</job>

In general, HTML can't be parsed properly with regular expressions, but if you have a specific limited format then it could work.

Given a simple format like

<body>
<img src="data:image/jpeg;base64,DpFDPGOIg3renreGR43LGLJKds==">
<img src="data:image/jpeg;base64,DpFDPGOIg3renreGR43LGLJKds=="><img src="data:image/jpeg;base64,DpFaPGOIg3renreGR43LGLJKds==">
<div><img src="data:image/jpeg;base64,DpFdPGOIg3renreGR43LGLJKds=="></div>
</body>

the following can pull out the data

i=0; awk 'BEGIN{RS="<"} /="data:image\/jpeg;base64,[^\"]*"/ { match($0, /="data:image\/jpeg;base64,([^\"]*)"/, data); print data[1]; }' test.html | while read d; do echo $d  | base64 -d > $i.jpg; i=$(($i+1)); done

To break that down:

i=0 Keep a counter so we can output different filenames for each image.

awk 'BEGIN{RS="<"} Run awk with the Record Separator changed from the default newline to <, so we always treat each HTML element as a separate record.

/="data:image\/jpeg;base64,[^\"]*"/ Only run the following commands on records that have embedded base64 jpeg data.

{ match($0, /="data:image\/jpeg;base64,([^\"]*)"/, data); print data[1]; }' Pull out the data itself, the part matched with parentheses between the comma and the trailing quotation mark, then print it.

test.html Just the input filename.

| while read d; do Pipe the output base64 data to a loop. read will put each line into d until there's no more input.

echo $d | base64 -d > img$i.jpg; Pass the current image through the base64 decoder and store the output to a file.

i=$(($i+1)); Increment to change the next filename.

done Done.

There are a few things that could probably be done better here:

• There should be a way to get the line-match regexp to capture the base64 data directly, instead of repeating the regexp in a call to the match() function, but I couldn't get it to work.
• I don't like the technique of reading a pipe into the variable d, only to echo it back out to another pipe - it would be nicer to just pipe straight through - but base64 doesn't know to only use one line of the input.
• For some reason I have not yet figured out, incrementing the counter directly where it's used (i.e. echo $d | base64 -d > img$((i++)).jpg) only wrote to the first file, even though echo $d > img$((i++)).b64 correctly wrote the encoded data to multiple files. Rather than waiting on working that out, I've just split the increment into its own command.

Actually Windows does have a utility that encodes and decodes base64 - CERTUTIL

I'm not sure what version of Windows introduced this command.

To encode a file:

certutil -encode inputFileName encodedOutputFileName 

To decode a file:

certutil -decode encodedInputFileName decodedOutputFileName 

There are a number of available verbs and options available to CERTUTIL.

To get a list of nearly all available verbs:

certutil -? 

To get help on a particular verb (-encode for example):

certutil -encode -? 

To get complete help for nearly all verbs:

certutil -v -? 

Mysteriously, the -encodehex verb is not listed with certutil -? or certutil -v -?. But it is described using certutil -encodehex -?. It is another handy function :-)

Update

Regarding David Morales' comment, there is a poorly documented type option to the -encodehex verb that allows creation of base64 strings without header or footer lines.

certutil [Options] -encodehex inFile outFile [type] 

A type of 1 will yield base64 without the header or footer lines.

See https://www.dostips.com/forum/viewtopic.php?f=3&t=8521#p56536 for a brief listing of the available type formats. And for a more in depth look at the available formats, see https://www.dostips.com/forum/viewtopic.php?f=3&t=8521#p57918.

Not investigated, but the -decodehex verb also has an optional trailing type argument.