A method for irregularity detection within a first transaction between a security-device and a first merchant terminal device in the course of executing a second transaction between said security-device and a second merchant terminal device, wherein said security device is a device of a user, such as a smart card, wherein executing the second transaction involves a backend operator device, said second merchant terminal device and said security-device, wherein the first transaction precedes the second transaction, wherein the first merchant terminal device has a unique first device identifier, wherein the second merchant terminal device has a unique second device identifier and comprises a Secure Access Module configured to prevent the second merchant terminal device from setting or modifying its unique second device identifier, wherein the backend operator device comprises a transaction MAC (message authentication code) key, a first device identifier stored in a first device identifier field, a second device identifier stored in a second device identifier field, a last-reported transaction counter, and a currently-reported transaction counter, and wherein the security-device comprises a unique security-device identifier, a transaction MAC key shared with the backend operator device, a transaction MAC counter, a first device identifier, a second device identifier, and a transaction MAC value; wherein the method comprises, in the course of executing the second transaction: said second merchant terminal device transmits its unique second device identifier to said security-device which stores it as its second device identifier, and exchanges with the security-device data relating to the second transaction; said security-device encrypts its first device identifier with said transaction MAC key shared with the backend operator device, transmits the encrypted first device identifier to said second merchant terminal device, receives a commit transaction message from said second merchant terminal device, and then sets its first device identifier equal to its second device identifier, increments the transaction MAC counter, computes the transaction MAC value based on the transaction MAC key, the transaction MAC counter and the data relating to the second transaction, and transmits its unique security-device identifier, the transaction MAC counter and the transaction MAC value in a message to the second merchant terminal device; said second merchant terminal device aggregates the encrypted first device identifier and said message comprising the unique security-device identifier, the transaction MAC counter and the transaction MAC value into a claim, and transmits this claim to the backend operator device; said backend operator device checks if the claim is complete; said backend operator device stores the transaction MAC counter into the currently-reported transaction MAC counter; the backend operator device decrypts the first device identifier with said transaction MAC key and stores it as first device identifier in the first identifier field; the backend operator device detects an irregularity within the first transaction if the currently-reported transaction counter value differs, by other than an increment, from a last-reported transaction counter value associated with the first transaction; and if the backend operator device detects said irregularity, it identifies, using the first device identifier, the first merchant terminal device as having not reported the first transaction. said second merchant terminal device transmits its unique second device identifier to said security-device which stores it as its second device identifier, and exchanges with the security-device data relating to the second transaction; said security-device encrypts its first device identifier with said transaction MAC key shared with the backend operator device, transmits the encrypted first device identifier to said second merchant terminal device, receives a commit transaction message from said second merchant terminal device, and then sets its first device identifier equal to its second device identifier, increments the transaction MAC counter, computes the transaction MAC value based on the transaction MAC key, the transaction MAC counter and the data relating to the second transaction, and transmits its unique security-device identifier, the transaction MAC counter and the transaction MAC value in a message to the second merchant terminal device; said second merchant terminal device aggregates the encrypted first device identifier and said message comprising the unique security-device identifier, the transaction MAC counter and the transaction MAC value into a claim, and transmits this claim to the backend operator device; said backend operator device checks if the claim is complete; said backend operator device stores the transaction MAC counter into the currently-reported transaction MAC counter; the backend operator device decrypts the first device identifier with said transaction MAC key and stores it as first device identifier in the first identifier field; the backend operator device detects an irregularity within the first transaction if the currently-reported transaction counter value differs, by other than an increment, from a last-reported transaction counter value associated with the first transaction; and if the backend operator device detects said irregularity, it identifies, using the first device identifier, the first merchant terminal device as having not reported the first transaction.