A method for authenticating a reader to a radio tag includes the following steps, which are implemented by the tag: receiving a piece of information ciphered by a reversible operation, which is parameterized by an authentication coupon for the reader, and a data item allowing determination of an index for identifying the authentication coupon of the reader; calculating a current authentication coupon for the reader as a function of the index; deciphering the received piece of information ciphered by the reversible operation, which is parameterized by the calculated current authentication coupon; and checking the deciphered piece of information that is intended to verify that the calculated current coupon corresponds to the authentication coupon used by the reader in order to parameterize the reversible operation.