A waiter spotted a happy customer which, after finishing his meal, lit a cigar, ignoring the big sign "No smoking allowed."
Approaching the smoker, the waiter asked politely in a soft and friendly voice:
:
"Excuse me, Sir, didn't you happen to notice the sign displayed just in front of you?"
Exhaling a perfect ring of smoke, the jolly fellow quipped:
..." I did, but M'P MDM DPDBMMD AHDPP."

Rem: the puzzle's encryption was not by a simple substitution code , but rather via N(CODE)=N(TEXT)mod K + L or similar (like F(N)instead of N), where N is the ordinal value of a letter in English ABC and K, L integers to be found.

(In reply to thoughts by Charlie)

One more remark re your comment.

k cannot be 3 or 5, since it would produce only 3 or 5 distinct letters, which is not the case.

K should be over 20, or in the high "teens" to minimize ambiguity, so when you see ALOUU, ALOUD, ALODU, and ALODD, you choose the right one. 

Posted by Ady TZIDON on 2014-10-12 14:27:53