Check are the base64 images gzipped or not

Question

I am working on a web app, the flow is something like that: the browser getting images from a server as a base64 data, then JavaScript encoding those images etc. (as a web server we are using apache with mod_deflate' module) I want to check are those getting images compressed (gzipped) or not.

Well, the problem is that there are no HTTP headers for the base64 images, so how can I check that? any idea? thanks.

What do you mean "there are no HTTP headers for the base64 images?" If you're getting a response, there are some headers and they should tell you if the body is compressed, but I'm not sure what you're asking for. — ssube, Oct 04 '16 at 16:56

Ma3x · Answer 1 · 2016-10-04T17:27:45.843

If you are requesting the image data from a server the response will always have some headers and if the contents are compressed (gzipped) you will see the header content-encoding: gzip.

However, if you want to know if the actual (base64 encoded) data is compressed (gzipped) you would decode the first 4 characters of the base64 string to get at least the first 2 bytes. If the first two bytes are 1F 8B you are dealing with gzipped data.

If the first 2 bytes are 50 4B it's zipped data. If the first 3 bytes are FF D8 FF it's jpg/jpeg data. If the first 4 bytes are 89 50 4E 47 it's png data.

See: https://en.wikipedia.org/wiki/List_of_file_signatures

score 1 · Answer 2 · answered Oct 04 '16 at 18:23

You are fetching the image from the server in base64 format so now decode the image which is in base64 format and store the output in a variable. Now lets see the decoded string and check the starting 10-12 characters of the decoded string because file extensions are stored in the starting of the file.

For example, if you want to understand the above logic open an jpeg/png image in notepad and observe the first 10-12 character in the file and you can find their respective file extensions in that file.

So I'm also doing the same thing here I will search for GZIP string in the decoded file, if its found then the file is in GZIP Format else its not.It may be a bit confusing. Let me show you the code for this : `

var encodedData; //This one was fetched from the server
var decodedData = atob(encodedData); //atob() decodes the string which is in base64 format
var extension = "GZIP"
var IndexOfGZIP = decodedData.IndexOf(extension) //Checking for GZIP word in decoded data

//If it is equal to -1 it says that GZIP is not found in data
if( IndexOfGZIP !== -1 ){ 

    //Normally the file extensions are found in the starting of the file only and hence I'm taking only first 11 characters into consideration.
    if( IndexOfGZIP >= 0 && IndexOfGZIP <=10 ){
        console.log("This is a GZIP file only")
    }else{
        console.log("File is not in GZIP Format")
    }
}else{
    console.log("File is not in GZIP Format")
}

` Hope this helps you.

Check are the base64 images gzipped or not

2 Answers2