The shell '>' redirect will truncate the file first of all, for example:

  echo "foo" > message.txt ; ( sleep 3 ; echo "bar" ) > message.txt & echo "message: '$( cat message.txt )'"

will print "message ''", not "message 'foo'".

Interestingly, that means that the 'openssl' command might have been working with an empty signature.

Of course, he might have just aliased 'openssl' as 'echo "Verified OK"' ...