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I2P is an alternative anonymity system, with a very different design - it focuses more on intra-I2P communication, rather than communication with the wider internet. Unlike Tor, every user of I2P is also the equivalent of a Tor relay.

I'm interested in how their threat model differs from the one assumed by Tor.

  • What kind of adversary does Tor protect against, that I2P doesn't?
  • What kind of adversary does I2P protect against, that Tor doesn't?
  • Are there any adversaries that neither project protects against?
  • When you’re asking about “adversaries that neither project protects against”, what sort of answer are you looking for? Neither one protects against people looking over your shoulder. – Ry- Sep 25 '13 at 20:10
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    I'm thinking more along the lines of global adverseries. Nefarious network administrators. Malicious relay/node operators. I personally would take Looking-over-your-shoulder as an implicit attack, as there's nothing that an anonymity network can do to prevent it. – Megan Walker Sep 25 '13 at 20:12
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Tor does not have a single threat model. It provides a various degrees of protection against different adversaries: local observers, malicious node operators, observers of the underlying Internet, etc. That said, the usual stated adversary in security analyses is one that owns a fraction c/n of the relays, as well as some clients and destinations. With all of these the adversary can do anything feasible (sometimes computationally, sometimes practically), generate traffic, drop traffic, violate protocols, etc. The adversary is usually assumed to also observe some, but definitely not all, of the Internet connections between relays, clients, and destinations from observers at ASes, IXPs, ISPs, etc. This Tor-network-link adversary is either passive or may drop, replay, or induce timing signatures and traffic passing over it. Adversaries for i2p are similar. For both systems there is no adversary against which one is completely secure and the other completely broken. It's all a matter of degree.

There are many specifics, for example, attacks on the different approaches to propagating network information. (Tor currently distributes information about all the nodes in the network to each client before it begins communicating using about a dozen directory authorities and a system of mirrors. i2p uses a modified DHT.) A complete answer would discuss all of these as well as destination fingerprinting, latency attacks, etc. It would be long and require frequent updating depending what new attacks have been discovered and what countermeasures have been designed and deployed. For similar reasons, which of Tor and i2p is more secure for each of these would be similarly complicated and dynamic. In the interest of giving a meaningful answer to the question I thus limit comparative points to the most significant and fundamental.

Because it is a low latency system, Tor is effectively broken against an adversary that can watch both the client and destination end of a connection. This is true whether the adversary is at a Tor relay, the destination, or is an observer on the Internet. The same is essentially true of i2p and all low latency systems although the details vary. (For example, i2p uses variable length tunnels which make this slightly less certain, but they are typically 3 hops like Tor and do not vary much. Latency and intersection across a few connections can reduce the uncertainty.) Tor gets its security against such an adversary by being large enough that it becomes expensive for an adversary to have the resources to observe much of the network. The latest most detailed analysis of this can be found in http://www.freehaven.net/anonbib/cache/ccs2013-usersrouted.pdf

The heart of the difference comes to how design differences and other factors affect the usability of the two. Tor is primarily for clients to make TCP connections to arbitrary Internet destinations and lets users run clients without needing to carry traffic for others. i2p is (with limited exceptions) a closed p2p system both in its destinations and in how traffic is carried. The induced motivational differences mean that Tor is orders of magnitude larger than i2p and is likely to remain such regardless of how either can scale in principle. The resources (IP addresses, bandwidth, IXPs etc.) necessary to observe and deanonymize most communication on the largest i2p network are thus much less than those needed to observe and deanonymize most communication on the Tor network. If the adversary is local or distributed but small, both systems will provide reasonable protection as long as the client and destination are not local to each other. Tor is likely to retain better protection as the adversary grows, although against a strong enough adversary both are effectively broken---like any actual system designed to protect security.

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    That seems like a pretty thorough answer! – Megan Walker Oct 2 '13 at 17:41
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Quote from http://www.reddit.com/r/i2p/comments/13kd3t/an_overview_of_the_differences_between_and_the/ :

I think it would be a good idea to have an overview of the differences between and the comparative strengths and weaknesses of I2P and Tor hidden services. That's what this post is for.

Strengths of I2P compared to Tor

I think I2P is by nature of its design more secure than the Tor network. The strengths of I2P over Tor hidden services (that means not using Tor to access regular (clearnet) resources such as https://duckduckgo.com) are:

Heavily decentralized. Tor has a user:relay ratio of 165:1 (excluding non-public bridge relays; see metrics) while I2P has a user:relay ratio of 0.99:1 (a very limited amount of users don't route traffic for others because they are, for example, in a hostile country with a limited number of I2P users). This means that you would need a a lot more resources to have a chance of deanonymizing users by observing network traffic over malicious nodes (meaning a set of relays that are all observed by a hostile entity) for I2P than for Tor.

No central point of failure for building tunnels. Tor has directory servers that form a catalog of (public) Tor relays. A user asks these directory servers for (a copy of the entire list of) Tor relays (or just part of them?) including their properties (such as Exit Node, Guard Node, Fast Node, etc.) If (a number of?) these directory servers are compromised, they could manipulate the information that they are supplying to the users that use those compromised directory servers. The Tor directory servers can also be attacked, making it impossible for users to form tunnels because they lack the required information. I2P uses DHT which allows all I2P relays to inform other I2P relays of relays that they known. There is no central (set of) point(s) that can be attacked to make building of tunnels impossible (except attacking all I2P relays).

Asymmetric tunnels. I'll use an analogy to explain this. This analogy is wrong and inaccurate in some regards because the contents of the traffic that is sent through Tor and through I2P is encrypted and cannot be read. The amount of intermediary countries used also doesn't match. The purpose of the analogy is to make you understand the difference. With Tor, you send a letter from US to Canada through France, Germany and Brazil (in that order). The letter reads "Please send me the combination of our granddad's bank vault now that he has deceased.". The letter that is sent in reply from your friend in Canada (reading "19502118") is sent to your address in the US through Brazil, Germany and France (in that order). With I2P, the first letter (from US to Canada) is sent through France, Germany and Brazil (in the order), but the second letter is sent through Paraguay, Norway and Ukraine (in that order). Suppose the postal services in France, Germany, Brazil and Paraguay are compromised. In that case, those postal services can figure out that 19502118 is the combination for your granddad's bank vault, if you were using Tor to send the both of those letters. If you used I2P, they would not be able to figure out what the combination for the vault is, although they do know that you have requested the combination for the vault. A version of the above scenario that is more true to the nature of Tor and I2P would include letters sent in an unbreakable envelopes (the encrypted data). If that was the case, the compromised postal services would be able to confirm that a letter was sent from a person in the US to a person in Canada in both the case of Tor and I2P, but only in the case of Tor would they be able to also confirm that a letter was sent from that person in Canada to that person in the US. (They would also be able to guess that it was probably a letter in reply to the US -> Canada letter because of the rapid response time).

Short-lived tunnels. Adapting the analogy above, this means that communications between the US resident and the Canadian resident are only shortly passed through Brazil, Germany and France + Paraguay, Norway and Ukraine. Much sooner than is the case with Tor will I2P change the intermediary nodes that the communications are using (to, for example, Peru, Mexico and Australia + Greece, Nicaragua and Russia). This is useful because if a tunnel is compromised, you will only send data using that tunnel for a short amount of time, thus limiting the amount of data that is compromised (though the data is encrypted, so unless the server you are connecting to is also compromised, the adversary cannot inspect the unencrypted data).

Some protection against human errors. Tor simply relays TCP/IP packets while I2P is able to modify or trim those packets for some tunnels (such as the default IRC tunnel) to prevent human errors. Once again, an analogy is useful, though not accurate. Suppose you want to anonymously leak a document to a newspaper. You decide to use the (analog) Tor network to prevent your identity from being compromised. You send the letter through Bolivia, Colombia and Japan and then finally to the US HQ of a newspaper. Unfortunately though, you have forgotten to remove some identifying remarks from your letter (your data). Let's for the sake of clarity say that you have left fingerprints on your letter (a digital equivalent would be HTTP headers that indicate the local server time). You can then be deanonymized even though the delivery of the letter was securely anonymous.

BitTorrent functionality. Unlike Tor, I2P has been designed with BitTorrent support in mind (can someone verify this?). Tor isn't supportive of the Tor network being used for clearnet BitTorrent activity and, unlike I2P, it doesn't have its own internal BitTorrent functionality.

Weaknesses of I2P compared to Tor

Technical

No family flag for relays. This means that if one entity controls a bunch of relays, he can add this information to his relays so that the anonymization software will never choose more than 1 relay from the same family to build a tunnel. I'm not sure if I2P is actually missing this feature!

Non-technical / social

Lower amount of users (though more relays).

No extensive documentation and noob-friendly start-up tutorials (though there has been some progress as of late).

No extensive academic peer reviewing.

No noob-friendly user interface.

No noob-proof out-of-the-box solutions like the Tor Browser Bundle.

No (charismatic) public representative like Jacob Appelbaum is for the Tor Project.

Here you have also information from Wikipedia about Tor and I2P - you can make a comparation - there's written about weaknesses, benefits, etc.

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    When you quote (or copy) the source, try to hone in on the relevant portions. Dont just copy and paste. Also, are you aware of Reddit's policy on licensing content? Does it allow copying entire threads? – asheeshr Sep 29 '13 at 17:19
  • @AsheeshR Thank you for your edit on my answer. I have just read Reddit's license policy (reddit.com/wiki/licensing) and I found here that texts are shared on CC license, so my qoute is legal. – TN888 Sep 29 '13 at 18:46
  • I'm not seeing anything on that page about user-submitted content being licensed under Creative Commons. I'm seeing a lot about use of the Reddit trademark though. reddit.com/help/useragreement seems to tell me that we can't copy-paste content wholesale. – Megan Walker Oct 1 '13 at 19:16

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