The Iranian Rial currently is the official currency. For the time being, it can only be exchanged inside the country. You will therefore have to take your entire money in cash (Euros or US Dollars) wirh you into the country, and it will be exchanged after arriving in Iran. This is standard procedure and has not caused any problems in the past. 
It is NOT yet possible do withdraw money or pay with debit or credit cards, or any other international means of financial transaction.