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30 September 2013 Why the Web of Trust Sucks
Date: Mon, 30 Sep 2013 15:07:49 +0200 ----- Forwarded message from Mike Perry <mikeperry[at]torproject.org> -----
Date: Sat, 28 Sep 2013 23:43:27 -0700
> Hi Mike, The web of trust has three main problems: 1. It leaks information. This includes social graph metadata, time and place of interaction, and in some cases government/slave-name identity. This issue has been discussed at length, of course. 2. It has many single points of failure. Because by default GPG uses shortest-weighted paths to establish trust in a key, and moreover because nothing authenticates the *entire* Web of Trust graph, each and every member of the "Strong Set" essentially functions as a CA, especially for keys only weakly connected to the Strong Set. If you compromise just one of those keys, you get to use that key to certify arbitrary keys for any name you like. To understand how and why this is a problem, let's walk through a typical Web of Trust workflow. Let's say we have a GPG user named Edward who wants to send an encrypted email about the extreme level of corruption at his workplace to a journalist that he has never met. Let's call that journalist Glenn. For the sake of argument, let's say that both individuals are active participants in the Web of Trust. Edward also knows that the network systems administrators at his workplace are very sophisticated, and intercept all encrypted communications for purposes of active MITM attacks to obtain the communications content. So Edward decides to download Glenn's key from subkeys.pgp.net, and requests that his gpg client provide him with a trust value for Glenn's key. Now, the network systems administrators at Edward's workplace have anticipated this. They have a compromised HTTPS CA cert, as well as have compromised a couple of highly trusted keys from the Web of Trust. Let's call one of these GPG keys Roger. When Edward goes to download a key to use for Glenn, the network systems administrator gives him a new fake key that they generate on the spot. The network systems administrator also attaches a fully trusted signature using Roger's compromised key. They also block the actual key for Glenn from reaching Edward. Edward's GPG client has trust in a couple keys. It turns out that one of his trusted keys, Bruce, has full trust in Roger's key (the compromised key). Edward's GPG client then computes a fully trusted path from Bruce to Roger to the fake Glenn, and Edward then sends an encrypted email to fake Glenn that is then subsequently read by the network systems administrator. Game over for Edward :/. This scenario is possible against arbitrary keys using any of the high degree keys in the Strong Set. They effectively function as single point of failure CAs for the Web of Trust, which destroy its utility as an independent key authentication mechanism. 3. It doesn't scale very well to the global population. The amount of storage to maintain the Web of Trust for the whole world would be immense. For the level of authentication it provides, it just doesn't make sense to have this much storage involved. So what should we do instead? Well, I think it is important to take a step back and think about what the Web of Trust is trying to accomplish. Aside from being a global popularity contest and some kind of weird quasi-religious hacker ritual, it is an authentication mechanism for the keys that you retrieve. It turns out there are lots of ways to authenticate keys using multipath authentication that do not suffer from the Web of Trust's downsides**. Here's a few examples: 1. Every time GPG downloads a new key, re-download it several times via multiple Tor circuits to ensure you always get the same key. 2. Every time I verify a signature from a key sent to an email address that is not mine (like a mailinglist), my mail client adds a tiny amount of trust to that key (since each new public email+signature downloaded represents an observation of the key via a potentially distinct network path that should also be observed by multiple people, including the sender). 3. Every time I am about to encrypt mail to a key, check the key servers for that email address, download the key, and make sure it is still the same (SSH/TOFU-style). 4. When downloading a key, GPG could verify that the same email to key mapping exists on multiple key servers, with each key server authenticated by an independent TLS key that is stored in the GPG source code or packaging itself. (Perspectives/notary-style cryptographic multipath authentication). ** The Web of Trust is technically capable of multipath authentication by itself, but only if you are aware of all of the multiple paths that *should* exist. Unfortunately, nothing authenticates the whole Web of Trust in its entirety, so it is impossible to use it to reliably verify that multiple paths to a key do actually exist and are valid. -- Mike Perry --
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