Recall the spam reduction method discussed in Section 5.8.2. a. What property or properties of a secure hash function does this scheme rely on to reduce spam? b. In Section 5.8.2, it is stated that...

Recall the spam reduction method discussed in Section 5.8.2.

a. What property or properties of a secure hash function does this scheme rely on to reduce spam?

b. In Section 5.8.2, it is stated that "The message M includes the sender's and intended recipient's email addresses, but does not include any additional addresses." Suppose we relax this so that we only require that the message M includes the intended recipient's email address. Find an attack on this modified spam reduction system, that is, show that a spammer could still send spam without doing a large amount of work.