SOLVED. YA/childrens, codes, key on cover [s]

[Elizabeth]

Not sure why this popped in my head but it's been bugging me. I read it in fifth grade, back around 2004-2005, newer book at that time I think as I'm pretty sure I got it at the book fair. It was a series, there were codes throughout the series although I don't remember what for exactly. I think the cover was black with a key on the front. Thanks in advance!

[Juels]

Could it be the 39 Clues series?




The 39 Clues: Agent Handbook
Into the Gauntlet
One False Note
The Sword Thief
The Maze of Bones

[Elizabeth]

It's not 39 clues. That's too new.

[Juels]

Can you remember anything else about the series?

[Elizabeth]

Not really, it's been so long since I read them. And I just randomly thought about it the other day.

[Elizabeth]

I'm pretty sure there was no supernatural element, just smart kids who figured out the codes.

[Dramapuppy]

The Code

Published 2004 by Scholastic Paperbacks

[Elizabeth]

That's it! Thanks!

[Dramapuppy]

No problem!