After you return from the bathroom with Sofie on the first level, you'll be required to decrypte a message:
OH LAME SAINT _ _ _ _ _ _ _ _ _ _ _
This jumbled up message forms the phrase :: The Mona Lisa
Now, after a while of progressing through the level, you'll need to decrypte the message containing a large amount of symbols, after examining the Mona Lisa painting. The message will form the phrase ::
Da Vinci Painted Those That Knew Between Bacchus and Uriel Lies the Clue
Keep a pen and piece of paper nearby while you play The Da Vinci Code... They prove useful when trying to decrypte a code. You can use them in various ways, such as copying down codes (An example being the phone number on the first level 454) and other uses.
It's a good idea to keep a dictionary around at all times... They help greatly for cracking codes. For example: You know the first, second and last letter of a 5 letter word... You can now scale it down and decrypte the code using the dictionary... That should make sense.