Patent EP-4071642-A1 (EP)
Title: Method for concealing a subscription identifier at a user equipment of a mobile communication network, and corresponding system and computer program product
Inventor: CASERTA FRANCESCO
IPC: H04L9/14
Filed: 2022-03-18
Abstract: Method for concealing a subscription identifier (SI), in particular Subscription Permanent Identifier (SUPI), at a user equipment (11) of a mobile communication network, comprising a mobile equipment (11a) and an integrated circuit card (12) which stores subscription data for accessing said mobile communication network including said subscription identifier (PI), 
said method comprising, upon receiving at said user equipment (11) a corresponding request (RQT) by a server (13) to provide a corresponding subscription identifier (PI, CI), performing an Elliptical Curve encryption of said subscription identifier (PI) generating a concealed subscription identifier (CI), said concealing operation (100) comprising that said mobile equipment (11a) of the user equipment (11) sends an identity retrieve command, in particular a GET IDENTITY command (GI), to an integrated circuit card (12) in the mobile equipment (11a), 
said Elliptical Curve encryption including performing at the integrated circuit card (12) the operations of: 
generating an ephemeral key pair (ephPrK, ephPuK) comprising an ephemeral private key (ephPrK) and ephemeral public key (ephPuK), performing a first scalar multiplication (Eq. 1) of the ephemeral private key (ephPrK) by a generator value (G) to obtain said ephemeral public key (ephPuK), 
generating a Shared secret key (ShS) performing a second scalar multiplication (Eq. 2) of the Ephemeral Private key (ephPrK) by a server public key (srvPuK); 
using said shared secret key (ShS) to derive keys to encrypt the subscription identifier (SI), which is to be sent to said server (13) as concealed subscription identifier (CI) as response of said identity retrieve command (GI), 
 
said scalar multiplication being performed iteratively performing and iteration (i) comprising a set of operations for each bit of the Ephemeral Private key (ephPrK), 
said method comprising performing, before receiving said identity retrieve command (GI) at the card (12), a pre-calculation of said ephemeral key pair (ephPrK, ephPuK) comprising an ephemeral private key (ephPrK) and ephemeral public key (ephPuK) and said shared secret key (ShS), 
said pre-calculation including 
performing an interruptible calculation (100) of said first (Eq. 1) and second (Eq. 2) scalar multiplication during the execution time of given periodic commands, in particular APDU STATUS commands, sent by the mobile equipment (11) to the card (12), storing a respective state of completion (EPuKState, ShSState) of said calculation (100), 
said interruptible calculation (100) including checking (110, 130) at the beginning of each of said execution time said respective state of completion (EPuKState, ShSState), 
if said respective state of completion (EPuKState, ShSState) indicates that completion of the computation of a valid ephemeral key pair or shared secret (ShS), 
storing the corresponding values of ephemeral private key (ephPrK), ephemeral public key (ephPuK) and shared secret (ShS) in a table in a memory, in particular a flash memory, of the integrated circuit card (12) at the user equipment (11).