Which of the following is NOT a common technique to make passwords harder to crack?

Encryption of password plain text using symmetric encryption is not a common technique for making passwords harder to crack due to the fundamental issue of recovering the original password. When passwords are stored, they should generally not be encrypted and then decrypted to validate against user input. Instead of using symmetric encryption, which allows for decryption back to the original plain text, the industry standard is to hash passwords. Hashing applies a one-way function to the password, making it impossible to retrieve the original text.

In contrast, using password policies, salting passwords, and creating complex passwords with various characters are all effective strategies for strengthening password protection. Password policies establish guidelines for users to create stronger passwords, increasing overall security. Salting adds random data to passwords before hashing to prevent the use of precomputed hash tables (rainbow tables), effectively addressing vulnerabilities associated with common passwords. Lastly, incorporating complex characters (such as uppercase letters, symbols, and numbers) into passwords enhances their complexity, making them more resistant to brute-force attacks.