DigiDoc4 is an application for interacting with the Estonian ID system. Using an ID-card, Mobile-ID, or Smart-ID, it can sign, encrypt, and decrypt documents and files, and check the validity of digital signatures. It can also view information stored on ID cards and change the PIN1, PIN2 and PUK codes.

Digitally signed containers use .ddoc, .bdoc or .asice extensions, and encrypted/decrypted files use the .cdoc extension.