Javascript atob is not decoding the base64 string properly

Question

I'm trying to decode a javascript base64 string. The encoded string is from a dataUri. It is

<a href="data:application/octet-stream;charset=utf-16le;base64,//5mAG8AbwAgAGIAYQByAAoA">text file</a>

I'm trying to decode "//5mAG8AbwAgAGIAYQByAAoA" which is supposed to give "foo bar" as output. When I click the anchor tag it downloads a file as expected with the same expected string "foo bar" in it.

If I use atob('//5mAG8AbwAgAGIAYQByAAoA') I'm not getting the desired output

var a=atob('//5mAG8AbwAgAGIAYQByAAoA');
console.log(a);

I've even tried to give "utf8" as second arguement and no luck :(

How to extract the original encoded string from this dataUri? Thanks :)

btoa('foo bar') gives me "Zm9vIGJhcg==", then atob("Zm9vIGJhcg==") gives me foo bar are you sur about your base64 string ? — Guillaume Badi
– Guillaume Badi, Commented Aug 1, 2017 at 13:35
I'm trying to decode the base64 string in this dataUri:<a href="data:application/octet-stream;charset=utf-16le;base64,//5mAG8AbwAgAGIAYQByAAoA">text file</a> I believe that 5m... is the correct according to what given in en.wikipedia.org/wiki/Data_URI_scheme — Bharat
– Bharat, Commented Aug 1, 2017 at 13:39
5mAG8AbwAgAGIAYQBy‌ is not a Base64 version of foo bar; it is the Base64 version of æ`ðð (+ SO trimmed some invalid chars); I'd argue that's more of an auth string. — Michael Fedora
– Michael Fedora, Commented Aug 1, 2017 at 15:19

Michael Fedora · Accepted Answer · 2017-08-01 15:24:31Z

1

// isn't valid in the function atob: it will only take valid Base64 strings, which contain numbers,letters, and sometimes a ==. Strip those first and then run it:

> atob('5mAG8AbwAgAGIAYQByAAoA') // console input
< "æ`ðð" // console output; SO stripped some chars

As @Guillaume Badi said in the comments, your Base64 string is not foo bar. To be honest, it looks more like an authentication key/tag, which usually aren't human readable anyways.

answered Aug 1, 2017 at 15:24

Michael Fedora

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1 Comment

Bharat Over a year ago

Okay but there must be some way to decode this...I'm getting the correct string in the downloaded file...

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