Table 1.

Notations and definitions used.

Notations	Definition
$\lambda$	Security parameter
$s$	System master secret key
$P_{pub}$	System public key
$(p{k}_{TA},s{k}_{TA})$	TA’s public and private key pair
$(p{k}_{RSU},s{k}_{RSU})$	RSU’s public and private key pair
$V_i$	The $i$-th vehicle
$K$	Group key
$(\beta ,D_{pub})$	Group public key
$(X_i,Y_i)$	$\mathrm{Vehicle} V_i$’s full public key
$(x_i,y_i)$	$\mathrm{Vehicle} V_i$’s full private key
$s{k}_i$	$\mathrm{Vehicle} V_i$’s secret key
$RI{D}_i$	$\mathrm{Vehicle} V_i$’s real identity
$PI{D}_i=(PI{D}_{i,1},PI{D}_{i,2})$	An pseudo-identity of vehicle $V_i$
$T_i$	Current timestamp
$T_a$	Arrival time
$\Delta T$	The validity period of the pseudo-identity
$T_K$	The validity period of the group key
$H_1,H_2,H_3,H_4,H_5$	Five one-way hash functions
$sg{k}_i$	$\mathrm{The} \mathrm{signature} \mathrm{key} \mathrm{for} \mathrm{vehicle} V_i$
$\left|\right|$	Concatenation operation
$SIG$	Signature algorithm
$W_i^t$	$\mathrm{The} \mathrm{local} \mathrm{model} \mathrm{parameters} \mathrm{of} \mathrm{vehicle} V_i$ in round $t$
$W_{RS{U}_j}^t$	$\mathrm{The} \mathrm{local} \mathrm{model} \mathrm{parameters} \mathrm{aggregated} \mathrm{by} RS{U}_j$ in round $t$
$W_{global}^{t+1}$	$\mathrm{The} \mathrm{global} \mathrm{model} \mathrm{for} \mathrm{round} t+1$