decrypt.py

from Cryptodome.Cipher import AES

KEY = '30c732878042aa2ea569efc1c42ab17d'


def gcm_decrypt(key, encrypted_data):
    nonce, tag, ciphertext = encrypted_data[:16], encrypted_data[16:32], encrypted_data[32:]
    cipher = AES.new(key, AES.MODE_GCM, nonce)

    return cipher.decrypt_and_verify(ciphertext, tag)


if __name__ == '__main__':
    data = raw_input()

    print gcm_decrypt(KEY, data)

gcm_encrypt.js

var fs = require('fs');
var crypto = require('crypto'),
    algorithm = 'aes-256-gcm',
    key = '30c732878042aa2ea569efc1c42ab17d';


function encrypt(text, key) {
  var encoding = 'binary';
  var tag;
  var output;

  crypto.pseudoRandomBytes(16, function (err, iv) {
    var cipher = crypto.createCipheriv(algorithm, key, iv)
    var encrypted = cipher.update(text, 'utf8', encoding)

    encrypted += cipher.final(encoding);
    tag = cipher.getAuthTag();
    output = iv.toString('binary') + tag.toString('binary') + encrypted.toString('binary');

    process.stdout.write(output.toString('binary'));

    fs.writeFileSync('data', output, {
      encoding: 'binary'
    });
  });
}

encrypt('this is the text!!', key);