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Note that unlike what TFA claims, symmetric keys are safe. They're, in fact, SAFER than public key crypto for the SAME key size, as public key crypto depends on
by zobzu 14y ago
Note that unlike what TFA claims, symmetric keys are safe. They're, in fact, SAFER than public key crypto for the SAME key size, as public key crypto depends on prime numbers. That's why you'd see 256bit symmetric keys, and 2048bit or more public/secret keys.
It's just that the key exchange for symmetric keys is inherently unsafe online.
But if you share the keys in a safe manner (ie, physically, secret location, no camera, etc.), its all good.
A long time ago, well, not so long in fact, before public key cryptography was invented, symmetric crypto was used by businesses. They had people carrying those keys and delivering them. That was a pain.
it also worked that way for a couple of thousand years, with weaker algorithms, but the same concept.
- xyzzyz 14y agoIt's just that the key exchange for symmetric keys is inherently unsafe online. I don't understand what you mean. There are plenty of ways to exchange symmetric keys online, for instance by Diffie-Hellman scheme, or even using public key cryptography. I even recall that SSL/TLS uses symmetric key for encryption and asymmetric crypto is used only for key exchange.
- pi18n 14y agoDiffie-Hellman is fine if there is no one between you and Alice, otherwise if you read the protocol you will see it is easy for Eve to do the protocol with both ends and just act as a conduit for messages after reading/altering them. SSL/TLS relies on trusting a number of certificate authorities to verify that, yes, the other end is Bob. Note that for this to work securely you must already have their public key info. There are problems with this also but it's the best we have got right now.
- yason 14y agoThere's the interlock protocol that ensures that you'll only be talking with one other party instead of one other and one in the middle but still you don't know who you're talking to. But it's impossible to identify someone out of nowhere. Even in physical life we need external information of a person or an organization we don't know to identify him. So, we will always need some leads with which to cross-check the other party's identity. And given that, there are plenty of protocols to exchange symmetric keys securely.
- pi18n 14y agoBut that's exactly the problem, that there is no way to securely verify over an insecure connection that the keys you've associated with A are actually A's keys.
- yason 14y agoThat's really a non-problem since how could you ever know? The only way to verify that A's keys are legit is to use a secure channel first. Then you can leverage that over any number of insecure channels later to reconnect with A.
- keeperofdakeys 14y agoIn the end, security all comes down to trust. It is rather hard to get 100% trust without establishing it outside the system (PGP key exchange, SSL/TLS certificate authorities).
- nhaehnle 14y ago> That's why you'd see 256bit symmetric keys, and 2048bit or more public/secret keys. This is true for RSA based schemes, but keep in mind that elliptic curve based asymmetric schemes are believed to be secure even with much shorter key lengths.
- josephlord 14y agoYou also need to understand whether it is safe to reuse the keys and under what conditions that is safe. Information may leak if there are repeated patterns. Things can also be problematic if there are conditions under which you will encrypt data provided by you adversary as they could trick you into decoding the message for them. Finally opponents may be able to tamper with the message even if they can't decode it unless there is a MAC to protect it.