Reader for determining the validity of a connection to a transponder, wherein the reader is designed to measure a response time of a transponder and to authenticate the transponder in two separate steps, wherein at least a part of data used for the authentication is included in communication messages transmitted between the reader and the transponder during the measuring of the response time, wherein: the reader is designed to measure the response time based on a time interval between sending a first command to said transponder and receiving a first random number (RANDOM #1) from said transponder in response to the first command, and the reader is designed to authenticate the transponder based on an evaluation whether an encryption of the first random number (RANDOM #1), received from the transponder after having received the first random number (RANDOM #1) for measuring the response time, coincides with the first random number (RANDOM #1) received for measuring the response time, or the reader is designed to measure the response time based on a time interval between sending a first command together with a second random number (RANDOM #2) to said transponder and receiving a first random number (RANDOM #1) from said transponder in response to the first command and the reader is designed to authenticate the transponder based on an evaluation whether an encryption of the second random number (RANDOM #2) and an encryption of the first random number (RANDOM #1), received from the transponder after having received the first random number (RANDOM #1) for measuring the response time, coincides with the first random number (RANDOM #1) received for measuring the response time and with the second random number (RANDOM #2), and wherein the reader is designed to qualify the connection to the transponder as valid only upon determining that both the time interval is within a predetermined time window and that there is coincidence and that a communication speed at which the transponder operates is the same as what the reader assumes the transponder operates on based on information about the speed at which the transponder and the reader operate contained in a further command transmitted by the reader, wherein the further command also comprises a Message Authentication Code over at least said information and wherein the communication speed at which the transponder operates is deemed to be the same as what the reader assumes the transponder operates on if the transponder accepts further operation upon or after a successful verification of the Message Authentication Code. the reader is designed to measure the response time based on a time interval between sending a first command to said transponder and receiving a first random number (RANDOM #1) from said transponder in response to the first command, and the reader is designed to authenticate the transponder based on an evaluation whether an encryption of the first random number (RANDOM #1), received from the transponder after having received the first random number (RANDOM #1) for measuring the response time, coincides with the first random number (RANDOM #1) received for measuring the response time, or the reader is designed to measure the response time based on a time interval between sending a first command together with a second random number (RANDOM #2) to said transponder and receiving a first random number (RANDOM #1) from said transponder in response to the first command and the reader is designed to authenticate the transponder based on an evaluation whether an encryption of the second random number (RANDOM #2) and an encryption of the first random number (RANDOM #1), received from the transponder after having received the first random number (RANDOM #1) for measuring the response time, coincides with the first random number (RANDOM #1) received for measuring the response time and with the second random number (RANDOM #2),