Kinda pseudocode test describing the use of beforeEach to create input data for a test.

mock.test.js

/* eslint-env node, mocha */
const should = require('should');

describe('Some encrypt/decrypt test', function(){
  const passphrase = 'good passphrases are long';
  const passphraseIV = Buffer.from('AAAAAAAAAAAAAAAAAAAAAA==', 'base64');
  let fileBlob = 'start';
  let ciphertext = null;
  let nock = require('nock');

  // 'after' does not need to respect top-down ordering
  after( function(){
    nock.enableNetConnect();
  })

  beforeEach( function(){
    return new Promise( async function(accept){
      // expand fileBlob size
      while( fileBlob.length < 5 * 16 * 1024 ){
        fileBlob += '0';
      }
      fileBlob += 'end';

      nock.cleanAll();
      nock.disableNetConnect();

      // generate key from passphrase
      let key = await keyDerivationFunction(passphrase, passphraseIV);
      let ciphertext = await encryptFunction(key, fileBlob);
    });
  }); // end before

  it('should decrypt a blob in memory', async function(done){
    let plaintext = await decryptFunction(key, ciphertext)
    .catch( (err) => {
      done(err);
    });

    plaintext.should.equal(fileBlob);
    done();
  });
  
  // disabled test
  xit('should decrypt a blob to stream', function(done){
  });

});