18. 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.