How does SSL work?

http://security.stackexchange.com/questions/20803/how-does-ssl-work/20833

by lucb1e • 14 years ago

152 44 14 years ago

security.stackexchange.com

AndrewNCarr• 14 years ago

It doesn't, according to Moxie Marlinspike and others.

http://www.theregister.co.uk/2011/04/11/state_of_ssl_analysi...

If you haven't seen any of Marlinspikes presentations, you are missing out on some fascinating stuff.

Moxie @ Defcon 19: http://www.youtube.com/watch?v=xIiklPyS8MU

tptacek9• 14 years ago

That's a misstatement of Moxie's position, which is unfortunate because Moxie's position is important, probably correct, and needs all the credibility it can get.

What's broken about SSL/TLS is the current CA model. Since SSL/TLS was introduced, we've been running with almost exactly the same trust "UX": a hidden browser config panel listing a series of complicated-sounding trusted root CAs, each with the authority to sell or transfer their business to some other entity, or even to delegate the authority to sign certificates to other organizations.

That's absurd; it's a security model that clearly can't work in the real world --- and, more, demonstrably hasn't worked. SSL CA's have been caught red-handing selling their authority for dubious reasons. For instance, Trustwave sold a CA=YES certificate to an undisclosed third-party corporation solely for the purpose of making it easier (not "possible" but merely easier) for that corporation to monitor their own users.

We need a radical rethinking of the UI/UX and trust model behind SSL/TLS, and Moxie's idea of decentralizing that trust --- so that, say, the ACLU could operate a sort of CA root that would vouch for Verisign's signatures on core ecommerce sites but not accept a crazy delegation from Iran.

The protocol, on the other hand, is for all its warts the best-tested crypto protocol in possibly the history of computing. Baby & bathwater, and all that.

StavrosK5• 14 years ago

I remember seeing an interview where someone asked a Chrome team member if they had plans to support Moxie's idea. They said, roughly, that if Chrome supported it, Google would have to run some notary servers, and people would rest assured that Google is running them and not be bothered to run their own, leaving Google with having to support this for the entire internet.

It's too bad, it sounds like a very good idea.

tptacek9• 14 years ago

Roughly:

1. C & S exchange messages to agree on versions, ciphersuites, and nonces.

2. S->C certificate, which includes an RSA public key.

3. C verifies certificate against its local cache of CA roots.

4. C->S random secret encrypted under the RSA key (the "pre-master secret").

5. C & S derive (the same) set of MAC keys, crypto keys, and crypto parameters.

6. C & S verify every message of the handshake with the MAC keys.

Other useful things to know:

* SSL/TLS operates over a "record layer" of TLV-style messages. The TLS record layer itself supports fragmentation, which is a little crazy.

* The server can ask the client to send a certificate too; this is common on backend connections and unfortunately not common with browsers, because the UI is terrible.

* Less commonly, C & S can opt for a "DHE" (ephemeral Diffie-Hellman) key exchange, in which the RSA key from the certificate is used to sign a DH key exchange (DH allows both sides to use random public keys instead of long-term fixed keys, but suffers from exposure to MITM attacks --- the RSA key from the cert "breaks the tie" in a MITM situation, making the exchange secure). This has the advantage of ensuring that even if an attacker has been recording all your traffic for years, she can't compromise a server's private key and then decrypt older connections. This is called "forward secrecy".

* The two common cipher suites used on most connections are AES in CBC mode and RC4. AES-CBC chunks plaintext into 16 byte blocks, padding the last block if there are insufficient bytes to fill it. Until TLS 1.1, TLS ran CBC in a continuous stream over the whole connection, using the last block of the most recent message as the IV for the next, which gave rise to the BEAST flaw. RC4 is a stream cipher that encrypts byte-at-a-time --- but nobody trusts RC4 much.

* TLS 1.2 (IIRC) introduces AES-CTR, which runs AES as a stream cipher.

lucb1e4• 14 years ago

How about posting that on the site? Quite concise, and looks like a good answer to me.

tptacek9• 14 years ago

I agree with the commenters on SE that say "Thomas Pornin will have the best answer". While I absolutely understand the utility of SE's competition to produce the best message, I find it destroys any interest I have in contributing to the site.

zanny3• 14 years ago

I've always wondered if / what is stopping someone from eves dropping or duping the initial handshakes before the communications are encrypted. If you get the cipher and understand the schema used you should be able to decode the otherwise secure traffic.

andrewcooke3• 14 years ago

the magic of public (asymmetric) key crypto is what stops you. http://en.wikipedia.org/wiki/Public-key_cryptography

in step 4 above the client sends data to the server that is encrypted with the server's public key. you don't have the server's private key, so you cannot decrypt that data. but the server can. so you cannot duplicate things, even if you are watching.

[edited to swap client/server roles]

cheald• 14 years ago

This video explains it in very easy-to-grasp terms:

http://www.youtube.com/watch?v=3QnD2c4Xovk

tptacek9• 14 years ago

The magic is the cache of CA root public keys that comes preloaded with your browser or OS, up to which all presented certificates need to verify.

mgummelt4• 14 years ago

I'm surprised you're pro-client certs given the privacy implications.

tptacek9• 14 years ago

If the UI was better, there wouldn't need to be any; each app could be its own CA.

mitchellh• 14 years ago

I just finished reading this book: "SSL and TLS: Designing and Building Secure Systems"[1] and I now have a _much_ better understanding of how SSL works, what security it provides, how it provides it, and when and where to use it.

It is a thorough read and not the easiest, but you can always pick and choose what chapters you're interested in. I highly recommend it. You can buy it from Amazon here (not an affiliate link):

http://www.amazon.com/gp/product/0201615983/

[1] http://www.amazon.com/gp/product/0201615983/

philhippus• 14 years ago

>(not an affiliate link)

Heaven forbid anyone but Amazon gains financially from your recommendation.

k-mcgrady• 14 years ago

People like to point this out to make it clear they are actually recommending the product because it is useful and not for financial gain.

RegEx• 14 years ago

Wow. 8 bucks used AND recommended by an SSL v3 architect. I keep telling myself to stop buying books but...I must...

ta12121• 14 years ago

I will never understand why random articles describing well documented commonplace things make it to the front page.

mgummelt4• 14 years ago

Well documented -\-> Easily understandable. I'd rather read this than an RFC. Can you point me to a better summary of SSL?

lucb1e4• 14 years ago

Thanks by the way, HN moderators, for expanding the short URL generated by the site to share. Now it didn't keep track of how many people visited the URL, and I missed the Announcer badge. May I ask why?

madsushi• 14 years ago

Because the purpose of linking something on HN should be to share it, not to personally gain from it.

autocracy• 14 years ago

If it's all about sharing, why do we attribute anything here? Might as well make the submission and all the comments anonymous here as well.

Reputation is mark of making _good_ contributions, and is something that every site has utility for, including HN.

lucb1e4• 14 years ago

I don't see links to sites with ads being removed, even when submitted by the author itself.

tptacek9• 14 years ago

http://www.hnsearch.com/search#request/all&q=shortener+b...

lucb1e4• 14 years ago

I didn't use an url shortener, stackexchange just gives me a short link when I click the share button. And if they're banned, how could I have posted this...

ryanwhitney• 14 years ago

Aza Raskin posted this on twitter a while back:

The private-public key encryption concept explained through mixing paint: http://www.youtube.com/watch?v=6NcDVERzMGw#t=147s (Starts at about 2:20.)

How does SSL work?

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