A system for signing transactions, the system comprising: - a first module comprising: - a communication interface to a public network; - a controller configured to handle a transaction with a Blockchain network or a transaction server accessible at the public network; and - a data interface for communicating with the controller; - a second module comprising: - a random number generator for generating random sequences; - a secure controller comprising a flash memory and configured to generate seed words and private keys based on the random sequences generated by the random number generator and to store the seed words and the private keys in said flash memory and to sign a transaction request using said private keys by generating a signed transaction; and - a data interface (206, 207) for communicating with the secure controller; - a bridge module comprising: - a controller; - a data interface for communicating with the controller of the bridge module; and - a switch configured to selectively connect the data interface of the bridge module to either the data interface of the first module or the data interface (206, 207) of the second module such that the data interface of the first module is never connected with the data interface (206, 207) of the second module, wherein the second module is never connected to the public network; - wherein the controller of the bridge module is configured to receive a transaction request from the first module, pass the transaction request to the second module, receive the signed transaction from the second module and pass the received signed transaction to the first module. - a first module comprising: - a communication interface to a public network; - a controller configured to handle a transaction with a Blockchain network or a transaction server accessible at the public network; and - a data interface for communicating with the controller; - a communication interface to a public network; - a controller configured to handle a transaction with a Blockchain network or a transaction server accessible at the public network; and - a data interface for communicating with the controller; - a second module comprising: - a random number generator for generating random sequences; - a secure controller comprising a flash memory and configured to generate seed words and private keys based on the random sequences generated by the random number generator and to store the seed words and the private keys in said flash memory and to sign a transaction request using said private keys by generating a signed transaction; and - a data interface (206, 207) for communicating with the secure controller; - a random number generator for generating random sequences; - a secure controller comprising a flash memory and configured to generate seed words and private keys based on the random sequences generated by the random number generator and to store the seed words and the private keys in said flash memory and to sign a transaction request using said private keys by generating a signed transaction; and - a data interface (206, 207) for communicating with the secure controller; - a bridge module comprising: - a controller; - a data interface for communicating with the controller of the bridge module; and - a switch configured to selectively connect the data interface of the bridge module to either the data interface of the first module or the data interface (206, 207) of the second module such that the data interface of the first module is never connected with the data interface (206, 207) of the second module, wherein the second module is never connected to the public network; - wherein the controller of the bridge module is configured to receive a transaction request from the first module, pass the transaction request to the second module, receive the signed transaction from the second module and pass the received signed transaction to the first module. - a controller; - a data interface for communicating with the controller of the bridge module; and - a switch configured to selectively connect the data interface of the bridge module to either the data interface of the first module or the data interface (206, 207) of the second module such that the data interface of the first module is never connected with the data interface (206, 207) of the second module, wherein the second module is never connected to the public network; - wherein the controller of the bridge module is configured to receive a transaction request from the first module, pass the transaction request to the second module, receive the signed transaction from the second module and pass the received signed transaction to the first module.