DDOC is an extension specific to files associated with the DigiDoc Client application. DDOC is a file used to store data and certificates with which this data was signed. Technically speaking, a DDOC file is a data container that can store multiple files. DDOC is a format based on the ETSI TS 101 903 standard called XML Advanced Electronic Signatures (XAdES).

DigiDoc's digital data signing system is a common data encryption solution in Estonia and Finland.