Here are the things I tried, which did not help at all: (1) Computing
md5s on the whole files, which is not satisfactory because files are
-often not read entirely, hence the md5s can not be properly computed,
+often not read entirely, hence the md5s cannot be properly computed,
(2) computing XORs of the first 4, 16 and 256 bytes with rejection as
soon as one does not match, (3) reading files in parts of increasing
sizes so that rejection could be done with only a small fraction read