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?

Answer 1

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.