Decrypt AES-GCM from python in Javascript (128 bits)

decrypt_AESGCM_from_python.js

//Function to get to convert to bytes the base64 values from python

function base64ToUint8Array(base64) {
		var binaryString = atob(base64);
		var len = binaryString.length;
		var bytes = new Uint8Array(len);
		for (var i = 0; i < len; i++) {
			bytes[i] = binaryString.charCodeAt(i);
		}
		return bytes;
	}

//Base 64 values from python (needed to decode again the message)

const key_from_python_base64 = 'ESp/VIgEgyKm1vr/0fZqKacJx1jqWolLoL1VSlGSNgQ='; 
const cipherText_from_python_base64 = 'AX155UlOs78q1lf0PA==';
const nonce_from_python_base64 = 'es3G3LmMtLLwffuCl8aUlw==';
const tag_from_python_base64 = '9PrESaYl5QpvOVLpzlUrpA==';

const key = base64ToUint8Array(key_from_python_base64);
const ciphertext = base64ToUint8Array(cipherText_from_python_base64);
const nonce = base64ToUint8Array(nonce_from_python_base64);
const tag = base64ToUint8Array(tag_from_python_base64);


async function decryptAESGCM(ciphertext, key, nonce, tag) {
		const algo = { name: "AES-GCM", iv: nonce, tagLength: 128 };
		const cryptoKey = await crypto.subtle.importKey(
			"raw",
			key,
			algo,
			false,
			["decrypt"]
		);

		try {
			const decrypted = await crypto.subtle.decrypt(
				{
					name: "AES-GCM",
					iv: nonce,
					additionalData: new Uint8Array(),
					tagLength: 128,
				},
				cryptoKey,
				new Uint8Array([...ciphertext, ...tag]) // Concatenate ciphertext and tag
				);

			return new TextDecoder().decode(decrypted);
			} catch (e) {
					console.error("Decryption failed:", e);
					return null;
				}
	}

//Usage

	decryptAESGCM(ciphertext, key, nonce, tag).then((decryptedMessage) => {
	console.log("Decrypted message:", decryptedMessage);
	});