A method of establishing secure communication between a host computer and a server by using a protocol that authenticates a subscriber identity module, SIM, of the host computer such as a cellular phone to a mobile infrastructure over an untrusted network such as the internet, wherein the untrusted network is independent from a mobile operator system's radio network, wherein the SIM is in physical proximity to the host computer and connected thereto via wireless or wired connection, the method performed by the host computer in a distributed computing system and the method comprising: creating a request security token response in order to request a session key, wherein the request security token response includes a computed challenge response from the SIM of the host computer attempting to establish a secure communication with the server, wherein the challenge response is used to authenticate the SIM to the mobile infrastructure that holds billing status information thereof, and wherein a shared key, shared between the SIM and the mobile infrastructure and known only to the SIM and the mobile infrastructure is used by the SIM for generating the challenge response and wherein the shared key indicates that the SIM appropriately authenticated to the mobile infrastructure using the challenge response and wherein the challenge response is generated upon receiving a challenge generated and sent to the SIM from the mobile infra structure; sending the request security token response to the server, which has a trusted relationship with the mobile infrastructure and sends the request security token response to the mobile infrastructure, wherein sending the request security token response is performed over the untrusted network; receiving from the mobile infrastructure a network security token as response to the request security token response, wherein the network security token includes the session key and is encrypted by the mobile infrastructure using the shared key; passing the session key to the SIM for validation and decryption using the shared key, by establishing a tunneled communication between the SIM and the mobile infrastructure; and upon validation of the session key, allowing the host computer to use the decrypted session key returned from the SIM for secure communicating with the server over the untrusted network, wherein the server received the session key from the mobile infrastructure. creating a request security token response in order to request a session key, wherein the request security token response includes a computed challenge response from the SIM of the host computer attempting to establish a secure communication with the server, wherein the challenge response is used to authenticate the SIM to the mobile infrastructure that holds billing status information thereof, and wherein a shared key, shared between the SIM and the mobile infrastructure and known only to the SIM and the mobile infrastructure is used by the SIM for generating the challenge response and wherein the shared key indicates that the SIM appropriately authenticated to the mobile infrastructure using the challenge response and wherein the challenge response is generated upon receiving a challenge generated and sent to the SIM from the mobile infra structure; sending the request security token response to the server, which has a trusted relationship with the mobile infrastructure and sends the request security token response to the mobile infrastructure, wherein sending the request security token response is performed over the untrusted network; receiving from the mobile infrastructure a network security token as response to the request security token response, wherein the network security token includes the session key and is encrypted by the mobile infrastructure using the shared key; passing the session key to the SIM for validation and decryption using the shared key, by establishing a tunneled communication between the SIM and the mobile infrastructure; and upon validation of the session key, allowing the host computer to use the decrypted session key returned from the SIM for secure communicating with the server over the untrusted network, wherein the server received the session key from the mobile infrastructure.