Decryption wrong if format() is less than firstFactor

[sparecycles]

The calculation of left in decryption can result in a negative number when modulu is less than 0,

node-fe1-fpe/index.js

Line 63 in ceae15b

left = (modulu > 0) ? firstFactor - modulu : modulu;

this leads to a failure to recover the original value.

The value produced should be based on - modulu in all cases (to ensure a positive result):

 left = (modulu > 0) ? firstFactor - modulu : - modulu; 

You can confirm this change is an improvement by

• first removing all expectedEV assertions from the unit tests (tests should still pass with less assertions)
• modifying the format() function to return ... % 100n to force the modulu <= 0 case to occur, (tests now fail).
• then updating the code as above. (tests pass again, because the code was fixed).

Then remember to

• restore the current format() function.
• restore the current test code (all original tests will still pass with all assertions).