Re: [TLS] Simple, secure 0-RTT for the masses

Eric Rescorla <ekr@rtfm.com> Tue, 15 March 2016 20:22 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Tue, 15 Mar 2016 13:21:42 -0700
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To: Bill Cox <waywardgeek@google.com>
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Subject: Re: [TLS] Simple, secure 0-RTT for the masses
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On Tue, Mar 15, 2016 at 12:51 PM, Bill Cox <waywardgeek@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
>
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