Re: [TLS] New drafts: adding input to the TLS master secret
Paul Hoffman <paul.hoffman@vpnc.org> Wed, 03 February 2010 04:47 UTC
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Date: Tue, 02 Feb 2010 20:47:51 -0800
To: Marsh Ray <marsh@extendedsubset.com>, "tls@ietf.org" <tls@ietf.org>
From: Paul Hoffman <paul.hoffman@vpnc.org>
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Subject: Re: [TLS] New drafts: adding input to the TLS master secret
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At 9:26 PM -0600 2/2/10, Marsh Ray wrote: >Let's say the server has a perfect RNG and the client is broken Debian Etch. > >The client provides 15 bits of usable entropy in the Client Hello, the >server provides 224. > >RSA key exchange is negotiated. > >The client generates the 48-byte premaster secret with an effective >entropy of less than 15 bits (his first handshake since power on). > >Game over, right? Wrong. The master secret (the only one that matters for channel security) gets the advantage of all the randomness added by the client *and* the server. Without this extension, that is still probably acceptable for some uses; with the extension (if the server contributes more randomness), for all uses. Look again at section 8.1 in RFC 5246. >I think similar effects apply with DHE key exchange. Indeed. You will get half the expected randomness in the master secret. > >> How big were you planning to make those symmetric keys anyway? >> >> 48 bytes, as shown in the document. > >So at best there are 384 bits of entropy in play? Yes. >Assuming the server supplies his expected 224, our Debian Etch client >only has to pull together 160 usable BoE for them to hit that limit. Or the server can contribute all that is needed, if you are using the proposed extension. >I do appreciate a healthy safety margin, but there is some complexity >overhead and potential security risk. The complexity overhead is only paid for in systems that want to use these kinds of extensions. (Did I already say that?) >Your point about "potentially would allow an attacker who had partially >compromised the inputs to the master secret calculation greater scope >for influencing the output" is significant. So does the sentence that follows it: "Hash-based PRFs like the one used in TLS master secret calculations are designed to be fairly indifferent to the input size." >Some kinds of hash attacks >let the attacker mess with the bytes earlier in the handshake by placing >colliding blocks in the Hello Extensions. Are you saying that that affects the HMAC-based PRF calculation in TLS 1.2? If so, this is a pretty significant flaw in TLS that no one else has noticed. Note that there have been papers by well-known cryptographers saying that the HMAC construction does not have the weakness you ascribe to it here. >It also requires clients and servers to buffer the stuff and later >implement a sorting algorithm based on type order. By "later" you mean "still within the same handshake sequence" and by "sorting algorithm" you mean "ascending order of two-byte numbers", yes? >Lots could go wrong >with that. Pathological sort order DoS attacks, etc. Please explain further. The TLS spec says that you can only have one instance of an extension in a handshake. Thus, all TLS implementations today already should be looking for multiple instances of the same extension. What is left is a relatively small number of extension values, each of which has a unique two-byte number associated with it. Where does "pathological sort order DoS attacks" or "etc" come in? --Paul Hoffman, Director --VPN Consortium
- Re: [TLS] New drafts: adding input to the TLS mas… Stefan Santesson
- [TLS] New drafts: adding input to the TLS master … Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Dean Anderson
- Re: [TLS] New drafts: adding input to the TLS mas… Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Simon Josefsson
- Re: [TLS] New drafts: adding input to the TLS mas… Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Simon Josefsson
- Re: [TLS] New drafts: adding input to the TLS mas… Brian Smith
- Re: [TLS] New drafts: adding input to the TLS mas… Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Brian Smith
- Re: [TLS] New drafts: adding input to the TLS mas… Martin Rex
- Re: [TLS] New drafts: adding input to the TLS mas… Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Marsh Ray
- Re: [TLS] New drafts: adding input to the TLS mas… Martin Rex
- Re: [TLS] New drafts: adding input to the TLS mas… Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Marsh Ray
- Re: [TLS] New drafts: adding input to the TLS mas… Martin Rex
- Re: [TLS] New drafts: adding input to the TLS mas… Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Marsh Ray
- Re: [TLS] New drafts: adding input to the TLS mas… Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Bill Frantz
- Re: [TLS] New drafts: adding input to the TLS mas… Marsh Ray
- Re: [TLS] New drafts: adding input to the TLS mas… Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Marsh Ray
- Re: [TLS] New drafts: adding input to the TLS mas… Paul Hoffman
- Re: [TLS] New drafts: adding input to the TLS mas… Eric Rescorla
- Re: [TLS] New drafts: adding input to the TLS mas… Dean Anderson
- Re: [TLS] New drafts: adding input to the TLS mas… Eric Rescorla
- Re: [TLS] New drafts: adding input to the TLS mas… Dean Anderson
- Re: [TLS] New drafts: adding input to the TLS mas… Jeffrey A. Williams
- Re: [TLS] New drafts: adding input to the TLS mas… Eric Rescorla
- Re: [TLS] New drafts: adding input to the TLS mas… Eric Rescorla
- Re: [TLS] New drafts: adding input to the TLS mas… Martin Rex
- Re: [TLS] New drafts: adding input to the TLS mas… Dean Anderson
- Re: [TLS] New drafts: adding input to the TLS mas… Dean Anderson
- Re: [TLS] New drafts: adding input to the TLS mas… Marsh Ray
- Re: [TLS] New drafts: adding input to the TLS mas… Eric Rescorla
- Re: [TLS] New drafts: adding input to the TLS mas… Eric Rescorla
- Re: [TLS] New drafts: adding input to the TLS mas… Eric Rescorla
- Re: [TLS] New drafts: adding input to the TLS mas… Martin Rex
- Re: [TLS] New drafts: adding input to the TLS mas… Eric Rescorla
- Re: [TLS] New drafts: adding input to the TLS mas… Marsh Ray
- Re: [TLS] New drafts: adding input to the TLS mas… Dean Anderson
- Re: [TLS] New drafts: adding input to the TLS mas… Marsh Ray
- Re: [TLS] New drafts: adding input to the TLS mas… Dean Anderson
- Re: [TLS] New drafts: adding input to the TLS mas… Martin Rex
- Re: [TLS] New drafts: adding input to the TLS mas… James Manger