When looking for key files, we could use isdigit rather than checking
if the character is within the range [0-9].
Use (unsigned char) cast to ensure the value is representable in the
unsigned char type (as suggested by the isdigit manpage).
Change " & 0xff" occurrences to the recommended (unsigned char) type
cast.
(cherry picked from commit 1998ad6c776a9c17c27788b17765dee90d9e25df)
72deed194d