int CG21_KEY_RESHARE_ROUND1_T1()

in src/cg21/cg21_reshare.c [108:187]

• 52 lines of code
• 6 McCabe index (conditional complexity)

int CG21_KEY_RESHARE_ROUND1_T1(csprng *RNG, const CG21_SSID *ssid, int ID, CG21_RESHARE_SETTING setting,
                               const SSS_shares *myShare, CG21_RESHARE_ROUND1_STORE_SECRET_T1 *storeSecret,
                               CG21_RESHARE_ROUND1_STORE_PUB_T1 *storePub, CG21_RESHARE_ROUND1_OUT *pubOut){

    // check ID is in T1 (T1 is the set of t1 players' IDs )
    bool check = false;
    for (int i=0; i<setting.t1; i++){
        if (ID == *(setting.T1 + i)){
            check = true;
        }
    }
    if (check==false){
        return CG21_ID_IS_INVALID;
    }

    if (setting.t1 < 2){
        return CG21_RESHARE_t1_IS_SMALL;
    }

    /* converts SSS shares to additive */
    BIG_256_56 w;
    BIG_256_56 q;
    BIG_256_56 s;
    ECP_SECP256K1 G;

    char x_[setting.t1 - 1][EGS_SECP256K1];
    octet X[setting.t1 - 1];
    init_octets((char *) x_, X, EGS_SECP256K1, setting.t1 - 1);

    // convert array of integers T1 to array of octets X
    CG21_lagrange_index_to_octet(setting.t1, setting.T1, ID, X);

    // convert SSS shared to additive
    SSS_shamir_to_additive(setting.t1, myShare->X, myShare->Y, X, storeSecret->a);

    // computes public Key associated with the additive share
    ECP_SECP256K1_generator(&G);
    BIG_256_56_fromBytesLen(w, storeSecret->a->val, storeSecret->a->len);
    ECP_SECP256K1_mul(&G, w);
    ECP_SECP256K1_toOctet(storePub->Xi, &G, true);
    BIG_256_56_zero(w); // clean up the secret

    char cc[setting.t2][EFS_SECP256K1 + 1];
    octet CC[setting.t2];
    init_octets((char *)cc,   CC,   EFS_SECP256K1 + 1, setting.t2);

    // apply VSS on the additive shares to get shares and the corresponding checks
    VSS_make_shares(setting.t2, setting.n2, RNG, &storeSecret->shares, CC, storeSecret->a);

    // pack the checks into one octet (storePub->checks)
    CG21_pack_vss_checks(CC,setting.t2,storePub->checks);

    // sample rho_i
    BIG_256_56_rcopy(q, CURVE_Order_SECP256K1);
    BIG_256_56_randomnum(s, q, RNG);
    storePub->rho->len=EGS_SECP256K1;
    BIG_256_56_toBytes(storePub->rho->val,s);
    BIG_256_56_zero(s);

    // sample u_i
    BIG_256_56_randomnum(s, q, RNG);
    storePub->u->len=EGS_SECP256K1;
    BIG_256_56_toBytes(storePub->u->val,s);
    BIG_256_56_zero(s);

    // i component of (ssid,i,V)
    *pubOut->i = ID;
    *storePub->i = ID;

    // commit to random r
    SCHNORR_commit(RNG, storeSecret->r, storePub->A);

    //compute V_i
    int rc = CG21_KEY_RESHARE_GEN_V_T1(ssid, storePub, pubOut, setting);
    if (rc!=CG21_OK){
        return rc;
    }

    return CG21_OK;
}