I found myself in the strange situation where I had an encrypted data bag and the secret key but no way to decrypt it without my friendly operations coworkers.

This script solved all my issues, writing the decrypted data to stdout.

require 'chef/encrypted_data_bag_item'
require 'json'

keyfile = "./secret_environment.key"
encrypted_path = "./my-secret-file.json"

secret = Chef::EncryptedDataBagItem.load_secret(keyfile)
encrypted_data = JSON.parse(File.read(encrypted_path))
plain_data = Chef::EncryptedDataBagItem.new(encrypted_data, secret).to_hash
puts JSON.generate(plain_data)

Make sure to change keyfile and encrypted_path to match your files.

Invoke the script using Chef's built in Ruby to make sure that the Chef gem is available

/opt/chefdk/embedded/bin/ruby script.rb > decrypted.json