Use key generation logic from optimized implementation

[wultra]

Use key generation logic from optimized implementation

[romanstrobl]

See also key generation from the Circl library: https://github.com/cloudflare/circl/blob/master/dh/sidh/sidh.go#L188

[romanstrobl]

The round 3 package contains a simplified implementation of key generation, let's use it:

void random_mod_order_A(unsigned char* random_digits)
{  // Generation of Alice's secret key  
   // Outputs random value in [0, 2^eA - 1]

    randombytes(random_digits, SECRETKEY_A_BYTES);
    random_digits[SECRETKEY_A_BYTES-1] &= MASK_ALICE;    // Masking last byte 
}


void random_mod_order_B(unsigned char* random_digits)
{  // Generation of Bob's secret key  
   // Outputs random value in [0, 2^Floor(Log(2, oB)) - 1]

    randombytes(random_digits, SECRETKEY_B_BYTES);
    random_digits[SECRETKEY_B_BYTES-1] &= MASK_BOB;     // Masking last byte 
}