ECCRYPTO_STATUS decode()

in FourQ_32bit/crypto_util.c [46:133]
77 lines of code
8 McCabe index (conditional complexity)

ECCRYPTO_STATUS decode(const unsigned char* Pencoded, point_t P)
{ // Decode point P
  // SECURITY NOTE: this function does not run in constant time.
    velm_t r, t, t0, t1, t2, t3, t4;
    v2elm_t u, v, one = {0};
    digit_t sign_dec;
    vpoint_extproj_t R;
    vpoint_t VP;
    unsigned int i, sign;

    one[0] = 1;
    memmove((unsigned char*)P->y, Pencoded, 32);   // Decoding y-coordinate and sign
    sign = (unsigned int)(Pencoded[31] >> 7);
    P->y[1][NWORDS_FIELD-1] &= mask7fff;
    from_std_to_ext(P->y, VP->y);

    v2sqr1271(VP->y, u);
    v2mul1271(u, (digit_t*)&PARAMETER_d, v);
    v2sub1271(u, one, u);
    v2add1271(v, one, v);

    vsqr1271(&v[0], t0);                              // t0 = v0^2
    vsqr1271(&v[VWORDS_FIELD], t1);                   // t1 = v1^2
    vadd1271(t0, t1, t0);                             // t0 = t0+t1
    vmul1271(&u[0], &v[0], t1);                       // t1 = u0*v0
    vmul1271(&u[VWORDS_FIELD], &v[VWORDS_FIELD], t2); // t2 = u1*v1 
    vadd1271(t1, t2, t1);                             // t1 = t1+t2 
    vmul1271(&u[VWORDS_FIELD], &v[0], t2);            // t2 = u1*v0
    vmul1271(&u[0], &v[VWORDS_FIELD], t3);            // t3 = u0*v1
    vsub1271(t2, t3, t2);                             // t2 = t2-t3 
    vsqr1271(t1, t3);                                 // t3 = t1^2    
    vsqr1271(t2, t4);                                 // t4 = t2^2                   
    vadd1271(t3, t4, t3);                             // t3 = t3+t4
    for (i = 0; i < 125; i++) {                       // t3 = t3^(2^125)
        vsqr1271(t3, t3);
    }

    vadd1271(t1, t3, t);                           // t = t1+t3
    vmod1271(t, t);
    if (is_zero_ct(t, VWORDS_FIELD) == true) {
        vsub1271(t1, t3, t);                       // t = t1-t3
    }
    vadd1271(t, t, t);                             // t = 2*t 
    vsqr1271(t0, t3);                              // t3 = t0^2      
    vmul1271(t0, t3, t3);                          // t3 = t3*t0   
    vmul1271(t, t3, t3);                           // t3 = t3*t
    vexp1251(t3, r);                               // r = t3^(2^125-1)  
    vmul1271(t0, r, t3);                           // t3 = t0*r          
    vmul1271(t, t3, &VP->x[0]);                    // x0 = t*t3 
    vsqr1271(&VP->x[0], t1);
    vmul1271(t0, t1, t1);                          // t1 = t0*x0^2 
    vdiv1271(&VP->x[0]);                           // x0 = x0/2
    vmul1271(t2, t3, &VP->x[VWORDS_FIELD]);        // x1 = t3*t2  

    vsub1271(t, t1, t);
    vmod1271(t, t);
    if (is_zero_ct(t, VWORDS_FIELD) == false) {        // If t != t1 then swap x0 and x1       
        memmove((unsigned char*)t0, (unsigned char*)&VP->x[0], 20);
        memmove((unsigned char*)&VP->x[0], (unsigned char*)&VP->x[VWORDS_FIELD], 20);
        memmove((unsigned char*)&VP->x[VWORDS_FIELD], (unsigned char*)t0, 20);
    }

    v2mod1271(VP->x, VP->x);
    if (is_zero_ct(VP->x, VWORDS_FIELD) == true) {
        sign_dec = VP->x[2*VWORDS_FIELD-1] >> 22;
    } else {
        sign_dec = VP->x[VWORDS_FIELD-1] >> 22;
    }

    if (sign != (unsigned int)sign_dec) {           // If sign of x-coordinate decoded != input sign bit, then negate x-coordinate
        v2neg1271(VP->x);         
    }

    v2mod1271(VP->x, R->x);
    v2mod1271(VP->y, R->y);
    if (ecc_point_validate(R) == false) {
        vneg1271(&R->x[VWORDS_FIELD]);
        if (ecc_point_validate(R) == false) {       // Final point validation
            return ECCRYPTO_ERROR;
        }
    }

    v2mod1271(R->x, R->x);
    from_ext_to_std(R->x, P->x);
    from_ext_to_std(R->y, P->y);

    return ECCRYPTO_SUCCESS;
}