Update encryption-hashing.md

Author: kamilmysliwiec

content/security/encryption-hashing.md

@@ -12,9 +12,15 @@ As an example, let's use AES (Advanced Encryption System) `'aes-256-ctr'` algori
 
 ```typescript
 import { createCipheriv, randomBytes } from 'crypto';
+import { promisify } from 'util';
 
 const iv = randomBytes(16);
-const cipher = createCipheriv('aes-256-ctr', 'secretKey', iv);
+const password = 'Password used to generate key';
+
+// The key length is dependent on the algorithm.
+// In this case for aes256, it is 32 bytes.
+const key = (await promisify(scrypt)(password, 'salt', 32)) as Buffer;
+const cipher = createCipheriv('aes-256-ctr', key, iv);
 
 const textToEncrypt = 'Nest';
 const encryptedText = Buffer.concat([
@@ -28,7 +34,7 @@ Now to decrypt `encryptedText` value:
 ```typescript
 import { createDecipheriv } from 'crypto';
 
-const decipher = createDecipheriv('aes-256-ctr', 'secretKey', iv);
+const decipher = createDecipheriv('aes-256-ctr', key, iv);
 const decryptedText = Buffer.concat([
   decipher.update(encryptedText),
   decipher.final(),