Magnitude in field implementation #240

1. 
   jhoenicke commented at 12:59 pm on April 15, 2015: none

   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

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

   while in field_5x52_impl.h the function is

   0    r &= (a->magnitude >= 0);
   1    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.

2. 
   real-or-random commented at 9:59 pm on February 8, 2022: contributor

   The first part is tracked in #1060, the second is now answered by comments in the code.
3. real-or-random closed this on Feb 8, 2022

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