Barrabi mined Bitcoin years ago that is now worth millions of dollars. He encrypted the password but can't remember the key he used to encrypt it$.$ Upon employing the services of Barnum, a skilled security professional, he was eventually able to determine the key. Which of the following best explains what Barnum did or used to determine the key?

a$.$ He used a reverse hashing algorithmm. b$.$ He performed a ciphertexT attack

c$.$ He performed a collision attack.

d$.$ He used a quantum computer.