From HEXONET Wiki
An authorization code (also referred to as an Auth Code, AuthInfo Code, EPP Code, or authKEY) provides an extra level of security for domain name transfers. The authorization code is unique for each domain name and is assigned by the sponsoring registrar at the time the domain name is being registered. You can obtain the authorization code from the sponsoring registrar. According to the ICANN Guidelines, the sponsoring registrar is obliged to provide the authorization code to the registrant of a domain name. Normally you should find the authorization code in the user interface of the sponsoring registrar. If not please contact the sponsoring registrar and ask for the authorization code.