The TAILS website, in discussing how to guard against man-in-the-middle (MITM), offers the crypto-sig and a link to GnuPG to process it.
If there was an MITM between me and tails.boum.org, wouldn't they be able to also fake the sig and the GPG site?
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Which page of the website are you referring to? Trusting Tails Signing Key?– Richard HorrocksCommented Sep 3, 2015 at 22:16
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No, it was some other page. That one seems reliable, and yet its existence implies that the simpler one is not. What I noticed was that the signature is downloaded from the same page as the ISO. If one is bogus, the other would be as well. Then that page directs us to the PGP, but if we're already on an MITM, we could be getting a compromised version of PGP and so on. The "web of trust" is not available to me, and I'm not sure how to find the "multiple sites" to get the key from. But even that may not be any good if a MITM is messing with my traffic.– WGroleauCommented Sep 3, 2015 at 22:29
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2 Answers
I think this comes down to the level of trust you have - or the amount of confidence you require - in the methods you're using to obtain what you hope is an authentic binary.
Different people will have different thresholds; some will download Tails without even attempting to check the signature, others will feel they need to be more diligent, and follow the further recommendations for trusting the signing key.
Yes, you could have been a victim of MITM when downloading the Tails binary. Yes, you could have been a victim when downloading the signing key. And yes, you could be a victim if you then attempt to follow the additional suggestions for trusting it.
Does the possibly lesser likelihood of being a victim all three times make you more confident that the binary and key are genuine? That's for you to decide. Is there a perfect solution? Probably not.
From Trusting Tails Signing Key:
We will present you three techniques from the easiest to the safest. Again, none of them is a perfect and magic solution.
answered Sep 11, 2015 at 14:42
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The one that I missed before seems good enough. But it isn't a "lesser likelihood of being a victim all three times." If someone is interfering with my traffic, they won't stop after one connection.– WGroleauCommented Sep 20, 2015 at 20:58
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Yep, true, but I think it would depend on how the MITM attack was set up. If you visited those three pages on three separate occasions, then it's possible those visits would take three completely different routes through the internet. (I'm talking about non-Tor routes here, but the same extends to three different Tor circuits if you visited the pages using Tor.) If the Man is your ISP, then you probably can't escape. If the Man was a guy sitting in a cafe eavesdropping on an unencrypted wireless signal when you happened to be there yesterday, then other routes would be safe. Commented Sep 21, 2015 at 13:11
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OK, so if my paranoia is high enough, I should visit tails.org via Tor at a public WiFi in another city with WPA, copy the ISO link, get the sig link the same way somewhere else, go to a third town to actually do the download, … :-)– WGroleauCommented Sep 23, 2015 at 16:53
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Yes :-) Though if your machine has been infected with malware that routes all your traffic through a certain malicious node, then there's basically no escape. Commented Sep 23, 2015 at 19:18
To answer your question, yes. If you were the victim of a MITM attack, the attacker could easily modify anything on the page before it gets to your browser including links to the signing key, iso files, and the key fingerprint.
That being said, there is a way you can verify the identity of a website if it uses an EV (Extended Validation) SSL certificate. If you use either Firefox or Chrome to view the site and you see the special EV treatment in the address bar, you can be confident that the site hasn't been tampered with in transit.
(I tried to include images of "EV Treatment" in Chrome and Firefox but apparently Stack Exchange counts images as additional links and I need "at least 10 reputation to post more than 2 links")
Internet Explorer also shows this special EV treatment but Microsoft deliberately allows EV indications to be forged.
For a more in depth explanation of why EV certificates are "spoof proof" checkout this article by Gibson Research Corporation.
GRC also provides a way to verify sites with non-EV certificates by comparing a site's "certificate fingerprint" with one obtained by GRC's fingerprints page. Of course relying on that tool implies some level of trust in GRC. Namely that they aren't responsible for, nor in support of a MITM attack against you.
Let's assume for a moment that GRC were victim to a MITM attack themselves. The attacker could theoretically return the same bogus certificate to GRC that they displayed to you. While this would only be possible for a global actor to accomplish it doesn't really matter because GRC uses an EV certificate so if they were victim to a MITM attack, you could detect it before using their fingerprint tool.
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EV sign can be faked too - just by injecting a fake CA certificate and issuing just-like-true-one EV cert. It's hard, but possible. See Blackhat Project Convergence video for an example of real exploitation.– Alexey Vesnin ♦Commented Jan 14, 2016 at 2:32