You can verify the integrity of downloaded files against checksums provided in
SHA512SUMS files on Debian mirrors. You can find them in the same places as the installation images itself. Visit the following locations:
To compute the checksum of a downloaded installation file, use
and then compare the shown checksum against the corresponding one in the
The Debian CD FAQ has more useful information on this topic (such as the script
check_debian_iso, to semi-automate above procedure), as well as instructions, how to verify the integrity of the above checksum files themselves.