[UB1-831] FPE Decrypt Fix
When decrypting the key number the algorithm needed fixed.
Calling int(s[0])
caused an invalid literal for int() with base 10: '&'
error.
Updated the function based on the Java library's implementation decode_keynum
.
Produced successful round trip with multiple strings, including the one that started this issue!
Sharai Attenburrow => &-|:`l -B^%Nk]P$Z\ => Sharai Attenburrow
after 3 key rotations
Sharai Attenburrow => @&2?.w >QyN:Lc/42= => Sharai Attenburrow
Edited by Ian Freshwater