Using RSA to encrypt a message in JS and decrypt in Python

Question

I want to encrypt a message using RSA with a provided PEM public key in Javascript, using SubtleCrypto window.crypto.subtle and then decode it with Python (PyCryptodome) in the back-end. However, I get a ValueError: Incorrect decryption.. I'm not sure if the data is being correctly handled though. Here is my code:

JavaScript:

var publicKey;
var pemPublicKey = `public.pem key with stripped header and footer and newlines (just the base64 data)`;

function base64ToArrayBuffer(b64) {
    var byteString = window.atob(b64);
    var byteArray = new Uint8Array(byteString.length);
    for (var i = 0; i < byteString.length; i++) { byteArray[i] = byteString.charCodeAt(i); }
    return byteArray;
}

function arrayBufferToBase64(buffer) {
    var binary = '';
    var bytes = new Uint8Array(buffer);
    var len = bytes.byteLength;
    for (var i = 0; i < len; i++) { binary += String.fromCharCode(bytes[i]); }
    return window.btoa(binary);
}

window.crypto.subtle.importKey(
    "spki",
    base64ToArrayBuffer(pemPublicKey),
    { name: "RSA-OAEP", hash: { name: "SHA-256" } },
    false,
    ["encrypt"])
    .then(function (key) {
        publicKey = key
    })

console.log(publicKey)

var enc = new TextEncoder()
var encmessage = enc.encode("test14")

var encryptedData;
window.crypto.subtle.encrypt({
    name: "RSA-OAEP"
}, publicKey, encmessage).then(function (encrypted) { encryptedData = encrypted })

var encodedData = arrayBufferToBase64(encryptedData);
console.log(encodedData)

What the code above does is convert the public PEM key, generate a CryptoKey object out of it (using crypto.subtle.importKey) and then encrypts a simple message "test14".

Python backend:

import base64
from Cryptodome.PublicKey import RSA
from Cryptodome.Cipher import AES, PKCS1_OAEP

with open('private.pem', 'r') as f: keypair = RSA.import_key(f.read())
decryptor = PKCS1_OAEP.new(keypair)
decrypted = decryptor.decrypt(base64.b64decode(encrypted))  # encrypted is the data that is returned by JavaScript code
print(decrypted)

Maarten Bodewes · Accepted Answer · 2021-12-28 13:11:10Z

2

Directly from the documentation of Crypto.Cipher.PKCS1_OAEP.new(key, hashAlgo=None, mgfunc=None, label='', randfunc=None):

...

hashAlgo (hash object) - The hash function to use. This can be a module under Crypto.Hash or an existing hash object created from any of such modules. If not specified, Crypto.Hash.SHA1 is used.

...

answered Dec 28, 2021 at 13:11

Maarten Bodewes

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3 Comments

musava_ribica Over a year ago

For some reason I thought SHA-256 was the default. Changing to SHA-1 in Javascript code fixed it, thanks!

Maarten Bodewes Over a year ago

SHA-1 is the (slightly unfortunate) default in the PKCS#1 v2.x standards. It's secure for this type of function but SHA-256 would have been the better choice.

kelalaka Over a year ago

And my general advice is never to use the default value always define it, especially in Cryptographic implementation.

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