Getting reliable checksums

Question

Update: A version of transmission that contains malware has been distributed very recently. Quoting this ars technica article:

It appears that somehow the Transmission website may have been compromised as it was served via HTTP rather than the primary HTTPS Transmission

Original question

I want to download the transmission bittorrent application for mac, from here. There is a sha1 checksum on that page if you mouseover the link to the .dmg file link, but as the page is not served over https, I cannot tell if this is authentic. My main concern is a man in the middle attack (to corrupt the data). The fact that https is not used seems silly, because another page on their domain can establish a secure connection with a signed certificate.

I wonder if I am missing something, though this Q&A seems to agree with me. I would also like to know if there is a method to still find out if a file is authentic in a scenario like this, or in my particular case.

Answer 1

Getting reliable checksums

The checksum itself is reliable for what it should be used. But your expectation is wrong.

The purpose of the given hash if not to check against attacks but for inadvertent file corruption (bit flips, missing ends...). It can not be used for authenticity since both the hash and the file are served by the same site and a hacker could simply replace both when hacking the site. For checking authenticity you would need a digital signature signed by the author and not a simple hash.

Answer 2

The SHA-1 hash is probably not there for integrity, but because in BitTorrent files are indexed by hashes. I suppose that people who want to download that BitTorrent client may be already using a competing implementation and might like to get the new one through BitTorrent.

If you try to connect to the download page through HTTPS, it redirect you to the HTTP download page. The same happens if you use the direct URL to the file (with an https:// prefix). This looks like a misconfiguration to me. Maybe they intend to support SSL, but fumbled. Maybe they fear the "obvious high cost" of SSL (without having actually measured it). Maybe they consider that if you are going to use BitTorrent you must already be prepared to download and use shady things.

You could use the hash value in search requests on Google, to see if you can locate a "trusted" location that lists the hash and guarantees in some way that it is the right one. In this case, it does not yield much interesting results, but, generally speaking, it is something to try.

Personally, I would fear less the possibility of a MitM that modifies the hash and the file on the fly, than the idea that that compiled binary may contain malware (how could you know if the source servers were not hacked into ?). SSL would protect only against alteration in transit. (This is a generic comment that I make about the dangers of downloading things; I do not want to make disparaging assertions about that specific piece of software or its authors.)