rs-rpgmvp is a commandline tool(CLI) for decrypting rpg maker(.rpgmvp) files.

Install the latest rust toolset from rustup:
rustup.rs

Run the below commands:

cargo install --git https://github.com/Lonestar137/rs-rpgmvp.git

Ensure that the Cargo bin(~/.cargo/bin on Linux) is on your users PATH.

CLI tool for decrypting RPGMVP tools

Usage: rs-mpgv [OPTIONS] --decryption-key <DECRYPTION_KEY> <--basepath <BASEPATH>|--files <FILES>>

Options:
  -b, --basepath <BASEPATH>
          Directory to glob rpgmvp files from
  -f, --files <FILES>
          Takes a list of files to decrypt
  -e, --extension <EXTENSION>
          File extension of files to decrypt [default: rpgmvp]
  -o, --output <OUTPUT>
          Folder to output processed files into
  -d, --decryption-key <DECRYPTION_KEY>
          Key used to decrypt files. Note: Found in www/data/System.json
  -h, --help
          Print help
  -V, --version
          Print version
  

Decrypt an entire directory of files:
rs-rpgmvp --decryption-key {key from www/data/System.json} --basepath {folder/with/rpgmvp/files}

Decrypt specific files:
rs-rpgmvp --decryption-key {key from www/data/System.json} --files {path/file1} {path/file2}

Save decrypted files to a custom location(Preserves folder structure):
rs-rpgmvp --decryption-key {key from www/data/System.json} --basepath {folder/with/rpgmvp/files} --output /tmp/DecryptedFilesFolder/

If you have an idea of how the tool can be better feel free to raise an issue or pull request!

For providing a working implementation of the spec:
Petschko RPGMVP Decrypter