Last Saturday (2018-12-08), the Brazilian police caught those letters written by the bosses of a big Brazilian criminal organization in jail intended to be delivered to their men in the streets. The police decoded them and the local press published the decoded plaintext. It says something about killing prosecutors and other people from Brazil's judiciary system with the intend to avoid prisoners' transfers.
Note 1: Differently from most puzzles posted here, this is a real letter from a real police case. The decoded text and even the encryption and decryption scheme and keys were already published by the local press.
Note 2: If the police could decode it quickly, surely people from this community would also be able to do so. This should be a fun puzzle for this community nevertheless. So, you should explain how you solved it instead of just googling for the solution or for the keys. The police solved it without googling for the decoded text nor the encryption key, so should you.
Note 3:vOriginally, when I posted this (see the edition history, if interested), the local press had not yet published the decoded text, although it already claimed that the police solved it. And I didn't knew yet at the time what was the solution nor how to reach it (now I do).
The encryption and decryption is simple and can be done by hand using only pen and paper.
The plaintext is in Portuguese (and feature several typos and slangs), so don't rely in methods that assume that the plaintext message would be English.
The decoded text is significantly smaller that the encoded text.
The cryptography is symmetric and is based in a letter substitution scheme.
Being both symetric and based on a letter substitution scheme doesn't imply a 1-to-1 mapping. The reason is simple: To encode a letter (say 'A'), you might replace all the 'A's in the plain-text for 'XYZ' in the encoded text.
Click the image to see it in full resolution.