"It's written in some kind of code!"

[RavenBlackthorne]

Could I throw a suggestion? If your players are that kind of people, why not give them a handout in code?
My players are definitely of this ilk. I gave them a very hard code at the beginning of this "chapter" of my campaign and they're still trying to figure it out and they love it.
You could just take some runes of the internets and do a simple replacement cipher. You could combine this with the skill challenge after for a little bit of fun?
Just my 2p and of course if your players aren't the type then ignore me

 

[Primitive Screwhead]

One of the oldest, and easiest ciphers has been too write the message vertically while the paper is wrapped around a dowel of a specfic diameter, then obscure the message by writing boring bland stuff to mask it.

This means that the PCs would have to discover the proper diameter to wrap the message around.... with kings it is often the scepter they tend to carry, but it could also be something unique like the corner pedestal in thier chamber... anything that the message can be wrapped around.

In a magic world, the other option is to have the encryption done by ritual, similar to the public key concept in modern encryption. You need to have the right decoding ritual and the right keyword.
This means a skill challenge to determine the both of these items, achievable by a number of means. Perhaps a mission into the enemies lair to search through desks or bribe a med-level official in the bad guys organization.

I would prefer that the deciphering of the code lead to adventures instead of just a stale series of skill checks, so perhaps key information or tools need to be acquired from somewhere... or provide links down the road, perhaps the helpful scribe they pay to unravel the mystery is a local Guild Thief and you end up owing a favor to him...

My 2cp.. if you want this more detailed, do some googling and come up with a real-world cipher method for them to break.
The Emerald claw is probably a paranoid enough organization to code thier coded message, so once they break the 'code', the resulting message may not make too much sense, leading to more investigative adventures.
After all, losing an encrypted message can have disasterous effects on a secret organization.