A system for communication between devices, the system comprising: a server configured to: generate a sequential connection relationship among first hash values by repeatedly hashing a key token as many times as a first maximum count; transmit the first hash values, the sequential connection relationship among the first hash values, and information about the first maximum count to a first device; and transmit information about the key token and information about a second maximum count to a second device, wherein the information about the key token includes the key token or a value obtained by hashing the key token a predetermined number of times; the first device configured to: store the sequential connection relationship among the first hash values, and the first maximum count, transmit a first hash value corresponding to the first maximum count among the first hash values, and decrease the first maximum count by one each time the first hash value is transmitted; and the second device configured to: store information about the key token, and the second maximum count, receive the first hash value from the first device, compare the first hash value with each of second hash values generated by repeatedly hashing the information about the key token a number of times up to the second maximum count until a second hash value identical to the first hash value appears, and decrease the second maximum count based on the second hash value that is identical to the first hash value. a server configured to: generate a sequential connection relationship among first hash values by repeatedly hashing a key token as many times as a first maximum count; transmit the first hash values, the sequential connection relationship among the first hash values, and information about the first maximum count to a first device; and transmit information about the key token and information about a second maximum count to a second device, wherein the information about the key token includes the key token or a value obtained by hashing the key token a predetermined number of times; generate a sequential connection relationship among first hash values by repeatedly hashing a key token as many times as a first maximum count; transmit the first hash values, the sequential connection relationship among the first hash values, and information about the first maximum count to a first device; and transmit information about the key token and information about a second maximum count to a second device, wherein the information about the key token includes the key token or a value obtained by hashing the key token a predetermined number of times; the first device configured to: store the sequential connection relationship among the first hash values, and the first maximum count, transmit a first hash value corresponding to the first maximum count among the first hash values, and decrease the first maximum count by one each time the first hash value is transmitted; and store the sequential connection relationship among the first hash values, and the first maximum count, transmit a first hash value corresponding to the first maximum count among the first hash values, and decrease the first maximum count by one each time the first hash value is transmitted; and the second device configured to: store information about the key token, and the second maximum count, receive the first hash value from the first device, compare the first hash value with each of second hash values generated by repeatedly hashing the information about the key token a number of times up to the second maximum count until a second hash value identical to the first hash value appears, and decrease the second maximum count based on the second hash value that is identical to the first hash value. store information about the key token, and the second maximum count, receive the first hash value from the first device, compare the first hash value with each of second hash values generated by repeatedly hashing the information about the key token a number of times up to the second maximum count until a second hash value identical to the first hash value appears, and decrease the second maximum count based on the second hash value that is identical to the first hash value.