Re: [TLS] Refactoring client auth/re-key
Karthikeyan Bhargavan <karthikeyan.bhargavan@inria.fr> Sun, 19 October 2014 06:57 UTC
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From: Karthikeyan Bhargavan <karthikeyan.bhargavan@inria.fr>
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Date: Sun, 19 Oct 2014 08:57:25 +0200
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To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] Refactoring client auth/re-key
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> So I seem to remember DJB telling me that once you have a properly > made channel via Diffie-Hellman, that authenticating requires just > signing a value derived from the key. (Sadly I forgot/don't know the > proof, so someone will have to check this or tell me I'm completely > wrong) So we don't need the session hash to include the certificates, > and so don't need to keep the running hash going past the first > exchange. At least one form of this construction (as used in IKEv2) is well-explained and proved in the original SIGMA paper: http://www.iacr.org/cryptodb/data/paper.php?pubkey=1495 > Trying to think through this a bit, is this a generic comment on the > session-hash proposal even for TLS 1.2 where they are careful to > sign everything up to CertificateVerify? See: As Ilari hints, it is not enough to hash the client-server key shares into the master secret. In a negotiated protocol like TLS, we need to add the DH parameters, the algorithms used to derive the keys, and the algorithms that the resulting key will be used with. (The crypto proofs typically fix these values and so they don’t necessarily appear in the academic protocols.) For session hash in TLS 1.2, the log up to ClientKeyExchange contains all this data, and we need to hash at least the Hellos and the KeyExchanges. So even if the client and server are anonymous the session hash context binds the master secret. One may argue that the client and server authenticated identities (Certificates) do not need to be included in the session hash. This would be true for the initial handshake, but not for resumption. One use of the TLS 1.2 master secret is as a key for session resumption, and the Triple Handshake attack relies on the master secret not being bound to the peer’s identity. For TLS 1.3, I need to stare about the Update protocol a bit more, but I think authentication after an initial DH handshake could be made safe (a la SSH and IKEv2). That is, the first master secret hashes the keyshares, and the second master secret/connection key hashes in the identities. The question that bothers me is which key/master secret would we use for session resumption? If it is the initial (unauthenticated) master secret, I think there will be attacks. If we only allow the last (authenticated) master secret to be resumed, we would probably recover the guarantees of session hash. > > https://tools.ietf.org/html/draft-ietf-tls-session-hash-02#section-4.1 > > Perhaps one of the authors of that document can weigh in here. > > Best, > -Ekr > > > > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls
- [TLS] Refactoring client auth/re-key Eric Rescorla
- Re: [TLS] Refactoring client auth/re-key Watson Ladd
- Re: [TLS] Refactoring client auth/re-key Eric Rescorla
- Re: [TLS] Refactoring client auth/re-key Ilari Liusvaara
- Re: [TLS] Refactoring client auth/re-key Manuel Pégourié-Gonnard
- Re: [TLS] Refactoring client auth/re-key Eric Rescorla
- Re: [TLS] Refactoring client auth/re-key Watson Ladd
- Re: [TLS] Refactoring client auth/re-key Karthikeyan Bhargavan
- Re: [TLS] Refactoring client auth/re-key Ilari Liusvaara
- Re: [TLS] Refactoring client auth/re-key Karthikeyan Bhargavan
- Re: [TLS] Refactoring client auth/re-key Martin Thomson
- Re: [TLS] Refactoring client auth/re-key Martin Thomson