PSA: How 2 Factor Authentication works, and how Blockchain.info's doesn't work the way you expect it too.

There's been some confusion in this subreddit regarding what 2 Factor Authentication (2FA) is, and how it works in conjunction with services such as Coinbase.com and Blockchain.info. I'm going to explain how they work in each context, and some misconceptions regarding them.

The concept of 2FA is quite simple, it's designed to prevent users from being affected when their usename and password is compromised or when they are a victim of a phishing attempt. As well as needing something you know (the password) you must also need something you have (your phone or other 2FA token generator). As someone who has phished you doesn't have the physical device their attack against you is useless.

For coinbase.com this is perfectly acceptable no actions can be performed against your account without the token being presented, as they're a trusted party they can ensure that the attack surface is absolutely minimal. This is different for blockchain.info, which has designed their wallet to work in a less trusting way. In a sense the coinbase system is stronger against compromise, but weaker in that there's a lot more trust required of the company by the end user.

The normal flow of a blockchain.info wallet login is that a user presents an identifier (the unique URL) and the server returns an encrypted wallet for the client to decrypt in their browser using javascript. If the user has 2FA enabled, they are asked for a 2FA token before they are allowed to see the encrypted file. It works up until this point, but once the encrypted wallet is exposed there's no takebacks. If you log into your wallet on a malicious shared computer, or leave the encrypted file in your dropbox account, or or leave your wallet in your email address, there's no place for blockchain.info to present you with the 2FA prompt. If somebody gets their hands on this file, they can crack it offline with no speed restrictions.

The encryption doesn't matter either. To their credit they have made it stronger, but it's really no match for the GPU based crackers that are already made public. It's hardly like this community is in a lack of available GPUs to attack passwords with. If you want to be protected against this you must have an incredibly strong, long, non dictionary password.

Also know that once your wallet has been made public with a weak password, changing the password of your wallet after the fact will make no difference. The attacker already has your weakly encrypted one.

So should you not use blockchain.info?.

I'm not saying that. They have a nice interface and I know a lot of people have used it flawlessly for years. Just know the limitations of their 2FA, and protect your password and backups accordingly. There's no recourse generally if you are compromised. ronger in that the 2FA can never be bypassed, but it requires trust in them as a company a

^{This text is unlicensed. Print it, modify it, sell it.}

submitted by explainschange

[link] [6 commentaires]



from Bitcoin http://ift.tt/1efHWva