Magnitude in field implementation

[jhoenicke]

Hello,

I started to dig deeper into the implementation and stumbled over a small issue with the magnitudes.

From field_10x26_impl.h, the function secp256k1_fe_verify

    r &= (a->magnitude >= 0);
    r &= (a->magnitude <= 32);

while in field_5x52_impl.h the function is

    r &= (a->magnitude >= 0);
    r &= (a->magnitude <= 2048);

This means that multiplying a number with the constant value 100 is totally fine on a 64-bit machine, but will lead to an error on 32-bit machines, which IMHO is bad. The two implementation should have the same external interface.

Finally, is there a good reason for the 2 * in the second line of secp256k1_fe_verify? Why is the magnitude given in multiple of 2's? Doesn't the code work with odd magnitudes? This also means that one cannot express that magnitude should be 1 without having a fully normalized number.

[real-or-random]

The first part is tracked in #1060, the second is now answered by comments in the code.