On Tue, Mar 15, 2016 at 12:51 PM, Bill Cox <waywardg...@google.com> wrote:
> I think it is worth documenting what we feel would be the simplest secure > 0-RTT mode that is safe for any company to use. I think we owe the users a > link to such a document in the TLS 1.3 spec. Here is my attempt at > creating the simplest possible TLS 1.3 compatible safe 0-RTT scheme. It > uses a traditional server-side session-cache. The protocol looks like this: > > During the initial 1-RTT handshake: > - create an unencrypted ticket containing only the session ID and a > "resumption count" set to 0 > - send the ticket to the client > > During a 0-RTT resumption handshake: > - Get the session ID from the ticket, check for a cache hit, and drop to > 1-RTT if not found > - compare the resumption counts in the session state and ticket, and fall > back to 1-RTT if they do not match > - increment the resumption count in the ticket and session state > - send the updated ticket to the client > > With this protocol, POST requests are allowed in 0-RTT application data, > just like TLS 1.2. Client certs are allowed only on the initial 1-RTT > handshake, since the 0-RTT connection cannot complete proofs of possession > before the 0-RTT application requests are executed by the server. > > To see that this provides roughly TLS 1.2 security, note that so long as > the protocol never has to drop to 1-RTT, it is basically equivalent to TLS > 1.2 session-ID based resumption. When we do drop down to 1-RTT, we will > replay 0-RTT application data, opening up new replay attacks. However, > after this paper > <http://blog.valverde.me/2015/12/07/bad-life-advice/#.VucOsJMrIxN>, I do > not believe there are any significant new replay attacks against HTTPS, > ssh, or any real protocol I can think of. There may be weaknesses in TLS > 1.3 resumption caused by the client forgetting the original connection's > security parameters, but if any are found, a client-side cache can be added. > > Can this protocol be simplified by dropping the server cache? No. While > we can play with various server-side stateful schemes, such as strike > registers, it turns out that state on the server is _required_ for TLS 1.2 > levels of security when using 0-RTT resumption. Without server-side state, > 0-RTT application data can be infinitely replayed, strong client > authentication stops working, and full PFS becomes impossible. Note that > the above protocol does not deliver full PFS, but I claim that the modified > version described below does. > > That's all I have for my attempt at "Simple, secure 0-RTT for the > masses". Below I just talk about extending the protocol to provide full > PFS, which AFAIK, the world has never implemented. > > . . . > > It turns out that providing full PFS over 0-RTT resumptions requires > server-side state. > > Proof: Assume a server Sam has no server-side remembered session state, > and uses the same ticket decryption key to decrypt session tickets from > both Alice and Bob. Different decryption keys are not possible because > that would require remembering which session ticket used which key. Eve > records Alice’s resumed session, and later when Bob resumes, a server > compromise reveals Sam’s ticket decryption key to Eve. Now Eve can decrypt > Alice’s previous resumed session since the ticket is sent from Alice to Bob > before encryption is enabled. Clearly, the ticket contains all the > information needed to resume the encrypted session, since Sam does so > without access to any other server-side state. Also, Alice is sending > 0-RTT application data before any new ephemeral keys can be established. > Eve simply simulates resuming Alice's session and decrypts the 0-RTT > application data, proving that Alice's original session lacked forward > secrecy. > > So, stateless ticket resumption does not, and never will provide full > PFS. On the other hand, AFAIK, no one has ever provided full PFS using > session caching either. Is there a solution? > > Yes, there are multiple solutions. For example, if TLS 1.3 were enhanced > to allow the session secrets to be re-keyed before saving the session state > to the cache, that would stop a server compromise from leaking session > secrets from past connections. > Can you expand on this point? The session secret that is derived from connection N (the RMS) cannot be used to compute the traffic keys for connection N. Is that not the property you are looking for? -Ekr Given the current TLS 1.3 protocol as I understand it, this is not > possible, so my next-best solution is to have the clients hold the server's > session-cache decryption keys, which are generated uniquely for each 0-RTT > resumption. > > In this scheme, the server stores a session-cache decryption key in the > ticket, and the ticket is not encrypted. The protocol looks like this: > > During the initial 1-RTT handshake: > - create a ticket containing the session ID, "resumption count" set to 0, > and a new session encryption key > - encrypt the session state in the cache with the session encryption key > - send the ticket to the client, and delete the session encryption key. > Note that the ticket is sent after encryption is enabled on the connection. > > During a 0-RTT resumption handshake: > - check for a cache hit, and drop to 1-RTT if not found > - decrypt the cached session state with the encryption key from the ticket > - compare the resumption counts in the session-cache entry and ticket, and > fall back to 1-RTT if they do not match > - increment the resumption count > - create a new session encryption key in the ticket > - encrypt the updated session cache entry with the new session encryption > key > - send the updated ticket to the client > > Did I miss anything? Can it be simplified? > > This time, when Sam's server is compromised, Eve cannot decrypt the 0-RTT > application data from Alice's previous session, because the session state > in the cache is encrypted with keys held only by the clients. These 0-RTT > connections enjoy full PFS. > > Bill > > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls > >
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