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Is it not true that SSL can be decrypted by simply having listened to the network activity of a PC on a network? (for example, from the first time a random PC connects to a coffee shop, and keep listening until the user stops browsing and you no longer care for further data) Aren't all the keys there and the algorithms published so that it can be decrypted without any brute force?

(This is a yes or no question, but citing a source would be nice for furthur reading)

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This is almost a duplicate: security.stackexchange.com/questions/5/… –  Bruno Jul 11 '11 at 18:54

4 Answers 4

up vote 8 down vote accepted

The short answer to your question is, in the general case*, no.

The longer answer is that the reason that this is not possible is largely to do with the use of Public-Private key cryptography in the establishment of the SSL session which means that the session key is not transmitted over the network in the clear at any time during the session.

*There's a variety of sets of circumstances which may result in someone being able to decrypt an SSL encrypted communication but they're unlikely to apply to the general coffee shop scenario.

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The private key of the server is never transmitted. This means that the client can send information encrypted with the public key of the server and only the server can decrypt it. This information will result in a session key that is only known to the client and the server and will be used to encrypt the traffic.

Note: This is an extremely simplified summary. There is a lot more "magic" going on to ensure that your are using the public key of the right server and the traffic cannot be replayed.

Wikipedia has a good article able Public/private key cryptography

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You can create a common shared secret without transferring it over the network. Read on RSA and Diffie-Hellman key exchange in Wikipedia or other sources to get an intuition on how this works. You can even do a simple test computation by yourself (but it is only secure for extremely large numbers and several minor modifications).

Once you understand this, you may want to have a look at the (equally simple) Man-In-The-Middle attacks against these algorithms (probably also on wikipedia). To prevent these, you need mutual authentication, e.g., by signing the DH/RSA public keys and letting the peer verify the signature.

This authentication in turn requires some common knowledge from the "signature issuer"(certification authority), but that information is not required to be secret. This final ingredient("root certificate") is thus simply shipped with each browser and there are extensive procedures for creating and accepting it.

So, as long as you trust these authorities whose root certificate your browser vendor put into your browser to issue correct authentication information(certificates), you will be able to create encrypted+authenticated connections. (encryption is close to useless if you don't know where you're connected to!)

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+1 for talking about server authentication. However, there are other ways than using CAs (and PKI). You can have more explicit (or out-of-bands) trust mechanisms using self-signed certs or PSK cipher suites. There are also PGP and Kerberos-based cipher suites. –  Bruno Jul 11 '11 at 18:29
    
Self-signed+out-of-band verification is, with some additional client support, the same as web-of-trust, i.e. PGP. It is an alternative to the centralized PKIX model, but fundamentally you can convert one scheme into the other just by using it differently. Symmetric authentication is the other big part but I did not want to go into it here. I am not sure if ZK-protocols should be counted in as the third generic authentication method. They can be used to authenticate with a weak "PSK" (PAKE protocols) or based on asymmetric commitments/signatures (DAA). Some ZK-expert care to comment? –  pepe Jul 11 '11 at 20:01

SSL cannot be decrypted by the method you mentioned.

However, what you described is similar to an attack on a wireless network key such as WEP.

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