A method and apparatus are provided for mutually authenticating a reader and a radio tag. The method includes: a step of the tag sending data to the reader, the data enabling an index to be determined for identifying an authentication coupon of the reader, the index being selected by the tag; a step of the reader obtaining the coupon from the determined index and of the reader sending the coupon to the tag; a step of the tag obtaining an authentication value from the selected index and of the tag verifying that the coupon received from the reader corresponds to the authentication value that is obtained, if the verification is positive, then a step of finalizing the authentication of the tag by the reader; and if the verification is negative, interrupting the mutual authentication method.