GiTxHextech-Challenge-Repo/challenges/crypto/Messenger/writeup.md

The message that was found in the letter has been given 964lCv5sFqsdip50KeyCkg== where it has been encrypted by a block cipher symmetric-key algorithm that has a key length size of 56 bits. The cipher that has a 56 bits is DES (Data Encryption Standard)

The clue for what the key is actually in the statement which is treasure.

Use any type DES decoder online to decode the message

The flag is CH3CK_CH3ST