Subresource Integrity

Think of a tamper-evident seal on a medicine bottle. You check the seal before you swallow anything. If it is broken, you do not take it, whoever put it on the shelf.

What the tag looks like

<script src="https://cdn.example/lib.js"
        integrity="sha384-iht3RcshTShmtRmXjSLQ8CjUTiVO41jHRJzgK44j1lSJHvfC4YAeuY4ZOoMtzhi5"
        crossorigin="anonymous"></script>

The value is the algorithm name, a dash, then the hash written in Base64 (a way of writing bytes using letters, digits, + and /). The crossorigin="anonymous" part is needed for files from another site: the browser fetches them without cookies and the server must allow it with an Access-Control-Allow-Origin header, otherwise the check cannot run and the file is blocked. SRI works in all major browsers (checked October 2026).

Computing one in Python

import base64, hashlib
data = b"console.log('hi')"
digest = hashlib.sha384(data).digest()          # 48 raw bytes
print("sha384-" + base64.b64encode(digest).decode("ascii"))

hashlib.new(name, data) does the same for any algorithm given by name, such as "sha256" or "sha512". Note .digest() (raw bytes, what SRI wants) versus .hexdigest() (hex text, which would be wrong here).

How the browser checks it

1. Split the attribute on spaces. You may list several hashes, for example while switching file versions.
2. Keep only tokens with an algorithm it knows: sha256, sha384, sha512. Ignore the rest. (A token may end in ?options; drop that part.)
3. If nothing usable is left, it does no check at all and uses the file. A typo such as sha385-... therefore silently switches protection off. Always test that a wrong hash really blocks the file.
4. Otherwise, take the strongest algorithm present (sha512 beats sha384 beats sha256) and look only at tokens with that algorithm. If any of them matches the file's hash, the file is used; if none does, it is blocked.

A common misconception: "SRI keeps the file private". It does not hide anything, and the third party still sees the request. SRI only guarantees the code is the code you approved. It also cannot work for files that change all the time, like an ad script; for those, the honest answer is often not to embed them.

# Shell (OpenSSL)
openssl dgst -sha384 -binary lib.js | openssl base64 -A

// JavaScript (Node)
const crypto = require("crypto");
const sri = "sha384-" + crypto.createHash("sha384").update(fs.readFileSync("lib.js")).digest("base64");

// Go
sum := sha512.Sum384(data)
sri := "sha384-" + base64.StdEncoding.EncodeToString(sum[:])

// PHP
$sri = 'sha384-' . base64_encode(hash('sha384', $data, true));

// C#
var sri = "sha384-" + Convert.ToBase64String(SHA384.HashData(data));