How to create Base64 encode SHA256 string in Javascript?

Question

I want to log script hashes to implement content security policy. I have been able to generate the hash in python with the following code:

import hashlib
import base64

string='''
//<![CDATA[
var theForm = document.forms['ctl00'];
if (!theForm) {
    theForm = document.ctl00;
}
function __doPostBack(eventTarget, eventArgument) {
    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
        theForm.__EVENTTARGET.value = eventTarget;
        theForm.__EVENTARGUMENT.value = eventArgument;
        theForm.submit();
    }
}
//]]>
'''
# encode as UTF-8
string_UTF8 = string.encode('utf-8')

# hash the message
hash_string = hashlib.sha256(string_UTF8).digest()

# base64 encode
result = base64.b64encode(hash_string)

print('sha256-' + result.decode('utf-8'))

How can I do this with Javascript?

That looks like it would work, but I specifically want to do this on the fly, in a web browser, for scripts on the loaded page. — cmdln
– cmdln, Commented Oct 7, 2021 at 18:02

Chris_F · Accepted Answer · 2021-10-07 19:25:59Z

8

const string = `
//<![CDATA[
var theForm = document.forms['ctl00'];
if (!theForm) {
    theForm = document.ctl00;
}
function __doPostBack(eventTarget, eventArgument) {
    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
        theForm.__EVENTTARGET.value = eventTarget;
        theForm.__EVENTARGUMENT.value = eventArgument;
        theForm.submit();
    }
}
//]]>
`

async function hashFromString(string) {
    const hash = await crypto.subtle.digest("SHA-256", (new TextEncoder()).encode(string))
    return "sha256-" + btoa(String.fromCharCode(...new Uint8Array(hash)))
}

hashFromString(string).then(console.log)

Edit: I realize now that while not stated in your question it's likely that you are using Node.js therefor this answer that uses browser APIs may be of less use.

edited Oct 7, 2021 at 19:25

answered Oct 7, 2021 at 19:18

Chris_F

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1 Comment

cmdln Over a year ago

Exactly what I was looking for and no need for JQuery plugins

cmdln · Accepted Answer · 2021-10-07 19:01:30Z

0

I ended up using this JQuery plugin https://github.com/angeal185/jquery-hash The following code gets the correct answer:

<script>
  var s = "\n//<![CDATA[\nvar theForm = document.forms['ctl00'];\nif (!theForm) {\n    theForm = document.ctl00;\n}\nfunction __doPostBack(eventTarget, eventArgument) {\n    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {\n        theForm.__EVENTTARGET.value = eventTarget;\n        theForm.__EVENTARGUMENT.value = eventArgument;\n        theForm.submit();\n    }\n}\n//]]>\n"
  $.hash(s, '256', 'base64',function(i){
  console.log(i)
  });
</script>

answered Oct 7, 2021 at 19:01

cmdln

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