Now let´s assume that there is Mallory (the man in the middle), who receives the message from Alice, and forwards only the following sequence (he only sends the first package of every serial number) to Bob:
| 1 | 1 | Hi Larry | 523105 |
| 3 | 2 | Meet me at | 782290 |
| 5 | 3 | 6PM | 891231 |
| 7 | 4 | Yours-Susan | 553419 |
Then Mallory waits, which serial numbers are rerequested. Bob will be able to identify package 3 as authentic, so he rerequests the serial numbers 1, 3 and 4. Mallory catches the rerequests, and gains the knowledge, that the serial numbers, which were not rerequested are authentic. In this case only the number 2 was not rerequested, so package 3 has to be correct.
Now there are 2 possibilities: With the serial number 1 and 3, Mallory can assume that the packages left (2 and 6) are grain. But that isn´t necessarily the case. They could be wrong as well. Think a moment about the case when there are two mallory´s, who withhold the information each other. Then the remaining packages do not necessarily need to be authentic. Let´s assume that Mallory believes that they are authentic. So Mallory decides to send the next package from the serial number, where he doesn´t know the answer yet:
| 8 | 4 | Love-Alice | 312265 |
Bob ``acknowledges'' this packet by rerequesting serial numbers 1 and 3, which tells Mallory that the package 8 is correct. So Mallory knows the following: Packages 3 and 8 are authentic; Packages 2 and 6 can be guessed to be authentic. Mallory can terminate the connection to Bob now! But look at the knowledge of Bob: Bob has acknowledged the Packages 3 and 8. Bob has no authenticated information about the serial numbers 1 and 3!