The commission acting for and on behalf of the state may issue from time to time bonds in such principal amounts as shall be necessary to provide sufficient funds to defray any or all of the cost of construction of highways. The principal amount of the bonds so authorized to be issued shall not exceed, in the aggregate, the total amount authorized by the Legislature from time to time for such purpose. The proceeds from the sale of any bonds issued under the Nebraska Highway Bond Act shall be deposited in the state treasury to the credit of the Highway Cash Fund and shall be used only to finance or to refinance through the issuance of refunding bonds the construction of highways in this state as authorized by law. The commission is hereby granted all powers necessary or convenient to carry out the purposes and exercise the powers granted by such act.