Dungeon Mastery: Coded Messages from the BBEG

The BBEG says Hi!

"Another one." Rothgar knelt down and lifted the coded note from the corpse and held it aloft. "Why can't your magic break the codes Akanus?"
"There is not much I can do." The Wizard snatched the note from Rothgar's hand and examined it. "It's neither magical, nor a Language."

Your PCs are investigating a series of crimes, each more brutal than the last, and the methodology cannot be tied down to a single style other than the appearance of coded letters left to confuse the investigators, and terrify the masses. The Streets and alleys are abandoned as people huddle in their homes in fear.

Character Intelligence Vs Player Intelligence

In the end its up to you as DM, but as this is about the stats of individual characters, a simple Dice Check (1d20) vs a difficulty level should suffice. Otherwise you might prefer to give the PC the dice check, and the player gets the opportunity to then break the code. Rather than provide the player a single option decoder and allow them to solve the message, you might prefer to hand them a set of potential decoders and allow them to have a number of tries at cracking the code.

I though I recognized it...

The BBEG is a genius level evil with an 18 intelligence. He or she creates a Code to communicate with minions or the PCs or both. At this point the code can be broken with a read language spell. But to avoid that the villain uses the code to disguise numbers. The numbers can then describe the shapes of letters on a grid that spell out the message. A single letter will be described by a collection of Numbers.
If divine intervention is preventing code-breaking, Then the PC might recieve instruction to hunt down a duplicate artefact that the code was created with.