Method, device, server arrangement, system and computer program products for securely storing data in a portable device

A piece of confidential data (C) is stored in encrypted form in a memory (102) of a portable device (101). A decrypting key needed for decrypting the piece of confidential data (C) is stored (115) in a service provider's system (111). A piece of encrypted digital data (KSIGN) is also stored in the memory (102) of the portable device (101), and all decrypted forms of both said piece of confidential data and said piece of encrypted digital are erased from the portable device (101). A piece of digital data that has a cryptographic one-to-one correspondence with said piece of encrypted digital data exists (114) in the service provider's system. When a user gives a passphrase, the encrypted digital data (KSIGN) is decrypted and used to cryptographically process a request message, which causes the service provider's system (111) deliver said decrypting key to the portable device.