Authorized_keys file is required for setting up destination when trying to obtain data from Data Feeds or Data Warehouse via SFTP.
Authorized_keys file for both Data Feed and Data Warehouse can be downloaded in either RSA or DSA formats within the Data Feeds Manager in Adobe Analytics UI. Data Feeds Manager can be accessed from Admin tab in Analytics UI.
The keys can be downloaded while trying to create a new feed. You would need to Click the + Add button.