A method is disclosed. The method includes, in a client device, acquiring first and second asymmetric cryptographic key pairs for a user, where each key pair includes a public key and a corresponding private key, securing the private key of the second key pair in a cryptographic processor, and splitting the private key of the first key pair into plural private key fragments, so that a sum of the plural private key fragments equals the private key of the first key pair. The method further includes storing at least one of the plural private key fragments on the client device, and registering the user with an identity service not hosted on the client device. Registering the user includes providing to the identity service, for use in securely authenticating the user, the public keys of the first and second key pairs, and the plural private key fragment(s) excluding the at least one private key fragment secured on the client device.