With the approval of the President, the Secretary of the Treasury may issue savings bonds and savings certificates of the United States Government and may buy, redeem, and make refunds under section 3111 of this title. Proceeds from the bonds and certificates shall be used for expenditures authorized by law. Savings bonds and certificates may be issued on an interest-bearing basis, on a discount basis, or on an interest-bearing and discount basis. Savings bonds shall mature not more than 20 years from the date of issue. Savings certificates shall mature not more than 10 years from the date of issue. The difference between the price paid and the amount received on redeeming a savings bond or certificate is interest under the Internal Revenue Code of 1986 (26 U.S.C. 1 et seq.).