Communication device, communication method, and communication system

A communication device comprising:

• a communication circuit which performs proximity communication with a reader/writer;
• a memory which stores data and a sequence number where a value is updated regularly each time data is written into the memory;
• and
• a control device which controls the writing of the data to the memory in accordance with a command from the reader/writer,
• wherein the communication circuit sends the sequence number stored in the memory to the reader/writer and receives first encryption data, which is generated by the reader/writer using a value based on the sequence number and writing target data which is target data which is written into the memory, and the writing target data from the reader/writer, and
• the control device generates second encryption data using the value based on the sequence number and the writing target data which is received by the communication circuit, writes the writing target data which is received by the communication circuit into the memory, and updates the sequence number stored in the memory in a case where the first encryption data and the second encryption data match,
• wherein the reader/writer generates a random number and sends the random number to the communication device,
• the communication circuit receives the random number sent from the reader/writer,
• the control device generates a session key used in the session with the reader/writer using an individual key which is unique to the communication device which is generated using distinguishing information which distinguishes the communication device and the random number received by the communication circuit, and generates third encryption data using the distinguishing information and the session key,
• the communication circuit sends the distinguishing information and the third encryption information to the reader/writer, and
• the reader/writer generates the individual key using the distinguishing information sent from the communication circuit, generates the session key using the individual key and the random number, generates fourth encryption data using the distinguishing information sent from the communication circuit and the session key, performs one-side authentication which authenticates the communication device by comparing the third encryption data and the fourth encryption data, and sends the first encryption data and the writing target data to the communication device in a case where the one-side authentication is successful.