A method for secure registration of a new application with a server system, the new application operating on an electronic device wherein an old application operating on the same electronic device has already been securely registered with the server system by reference to a user-specific credential, the method comprising: (a) initialising a first secure link between the new application and the server system and thereby establishing a first encryption key; (b) communicating first check data from the server system to the new application over the first secure link; (c) passing the first check data from the new application to the old application; (d) receiving an input of the user-specific credential from the user to the old application; (e) initialising a second secure link between the old application and the server system based on a second encryption key, the second encryption key being based on the input of the user-specific credential by the user to the old application; (f) communicating the first check data from the old application to the server system over the second secure link; (g) communicating enciphered second check data from the server system to the old application over the second secure link, the enciphered second check data being encrypted with the first encryption key, the enciphered second check data being further encrypted by the old application using a third encryption key to generate doubly-enciphered check data thereby, wherein the third encryption key is derived from the first check data and the user-specific credential inputted to the old application; (h) passing the doubly-enciphered second check data from the old application to the new application; (i) receiving an input of the user-specific credential from the user to the new application; and (j) communicating a decrypted version of the doubly-enciphered check data from the new application to the server system over the first secure link between the new application and the server system, the decrypted version of the doubly-enciphered check data being generated at the new application by decrypting the doubly-enciphered check data using the first encryption key and a fourth encryption key, wherein the fourth encryption key is generated at the new application based on the first check data and the input of the user-specific credential by the user to the new application. (a) initialising a first secure link between the new application and the server system and thereby establishing a first encryption key; (b) communicating first check data from the server system to the new application over the first secure link; (c) passing the first check data from the new application to the old application; (d) receiving an input of the user-specific credential from the user to the old application; (e) initialising a second secure link between the old application and the server system based on a second encryption key, the second encryption key being based on the input of the user-specific credential by the user to the old application; (f) communicating the first check data from the old application to the server system over the second secure link; (g) communicating enciphered second check data from the server system to the old application over the second secure link, the enciphered second check data being encrypted with the first encryption key, the enciphered second check data being further encrypted by the old application using a third encryption key to generate doubly-enciphered check data thereby, wherein the third encryption key is derived from the first check data and the user-specific credential inputted to the old application; (h) passing the doubly-enciphered second check data from the old application to the new application; (i) receiving an input of the user-specific credential from the user to the new application; and (j) communicating a decrypted version of the doubly-enciphered check data from the new application to the server system over the first secure link between the new application and the server system, the decrypted version of the doubly-enciphered check data being generated at the new application by decrypting the doubly-enciphered check data using the first encryption key and a fourth encryption key, wherein the fourth encryption key is generated at the new application based on the first check data and the input of the user-specific credential by the user to the new application.