Hash to G assumes data is in montgomery form

[darcys22]

This code spans both the bls and mcl libraries but in bls/src/bls_c_impl.hpp this function that takes the binary of a hash and maps it to a point:

#ifndef BLS_MINIMUM_API
template<class G>
inline bool toG(G& Hm, const void *h, mclSize size)
{
    if (g_irtfHashAndMap) {
        hashAndMapToG(Hm, h, size);
        return true;
    }
    // backward compatibility
    Fp t;
    std::cout << "t is Mont: " << t.isMont() << '\n'; // Debugging line added here
    t.setArrayMask((const uint8_t *)h, size);
...
}

The debugging line returns this:

t is Mont: 1 

The effects of this being that when setArrayMask is called:

    template<class S>
    void setArrayMask(const S *x, size_t n)
    {
        const size_t dstByte = sizeof(Unit) * op_.N;
        if (sizeof(S) * n > dstByte) {
            n = dstByte / sizeof(S);
        }
        bool b = fp::convertArrayAsLE(v_, op_.N, x, n);
        assert(b);
        (void)b;
        bint::maskN(v_, op_.N, op_.bitSize);
        if (bint::cmpGeN(v_, op_.p, op_.N)) {
            bint::maskN(v_, op_.N, op_.bitSize - 1);
        }
        toMont();
    }

The data going into v_ will be assumed to be already in montgomery form and the toMont() function will be a noop.

This might be a non-issue because the point that the data gets mapped to is somewhat arbitrary and as long as its consistent I don't think it cause issues. But when trying to match this behaviour in another non c++ library where we haven't got montgomery functions this has caused some difficulty.

Is there a way to set Fp to be not in montgomery form before this setArrayMask happens? (Calling fromMont() on the uninitialized t variable perhaps?) and will this cause issues in other places?

[herumi]

The data going into v_ will be assumed to be already in montgomery form and the toMont() function will be a noop.

No, Fp::isMont() == true means that v_ must be in Montgomery form, so setArrayMask takes a given array and must convert it into a Montgomery form by calling toMont(). toMont() does nothing unless isMont() is true. Then setArrayMask can set the same value whether isMont() is true or not.

[darcys22]

My mistake! thanks for the clarification