Re: [TLS] MITM Attacks on Client Authentication after Resumption
mrex@sap.com (Martin Rex) Wed, 05 March 2014 12:56 UTC
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Subject: Re: [TLS] MITM Attacks on Client Authentication after Resumption
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I've just started wondering: Is the real problem acutally worse than described, and will changing the Master Secret computation actually _not_ fix the renegotiation problem? I wrote: > > The original renegotiation problem and the newly describe session resumption > problem are very similar: flawed assumptions about a cryptographic binding > that doesn't exist. But until you write down the exact properties and > think about it, you may not notice the problem. As soon as you write it > down, the problem becomes obvious. > > The abbreviated TLS handshake authenticates only the MasterSecret. > But the MasterSecret is derived exclusively from ClientHello.random, > ServerHello.random and the keyExchange transform, there is no cryptographic > binding to any identities that are authenticated during the TLS handshake. > > > So the fix of the abbreviated TLS handshake needs to account for the > (re-)authentication of the identities of the original full handshake, > either by modifying the MasterSecret computation or by authenticating > the original full handshake during the abbreviated TLS handshake > through a TLS extension similar to how the renegotiation_info extension > does this in the renegotiation handshake. The only "non-static" data covered by the Finished handshake messages of the abbbreviated TLS handshake (i.e. that goes into the renegotiation_info of the resumed TLS handshake are): ClientHello.random ServerHello.random ClientHello.session_id == ServerHello.session_id In particular, there is *NO* dependency on the MasterSecret, so the attack by an MitM should be possible, even if the MasterSecrets differ between the two sessions that are merged (I never had a requirement in my attack scenario that the MasterSecrets are the same; in fact I wasn't even aware that it was possible to create two SSL sessions with identical MasterSecrets & traffic keys when I described my attack). The MitM will only have to decrypt and re-encrypt the data during the original two sessions. The handshake_message hash does not "see" the encryption, it operates on the plaintext only. So it seems to me that changing the MasterSecret computation will not help. We need a TLS extension on the abbreviated TLS handshake that authenticates the original full handshake, such as the renegotiation_info from that handshake. -Martin
- [TLS] MITM Attacks on Client Authentication after… Karthikeyan Bhargavan
- Re: [TLS] MITM Attacks on Client Authentication a… Paterson, Kenny
- Re: [TLS] MITM Attacks on Client Authentication a… Dr Stephen Henson
- Re: [TLS] MITM Attacks on Client Authentication a… Karthikeyan Bhargavan
- Re: [TLS] MITM Attacks on Client Authentication a… Martin Thomson
- Re: [TLS] MITM Attacks on Client Authentication a… Paterson, Kenny
- Re: [TLS] MITM Attacks on Client Authentication a… Adam Langley
- Re: [TLS] MITM Attacks on Client Authentication a… Martin Rex
- Re: [TLS] MITM Attacks on Client Authentication a… Andrei Popov
- Re: [TLS] MITM Attacks on Client Authentication a… Watson Ladd
- Re: [TLS] MITM Attacks on Client Authentication a… Adam Langley
- Re: [TLS] MITM Attacks on Client Authentication a… Martin Rex
- Re: [TLS] MITM Attacks on Client Authentication a… Salz, Rich
- Re: [TLS] MITM Attacks on Client Authentication a… Watson Ladd
- Re: [TLS] MITM Attacks on Client Authentication a… Nico Williams
- Re: [TLS] MITM Attacks on Client Authentication a… Dr Stephen Henson
- Re: [TLS] MITM Attacks on Client Authentication a… Martin Thomson
- Re: [TLS] MITM Attacks on Client Authentication a… Dr Stephen Henson
- Re: [TLS] MITM Attacks on Client Authentication a… Adam Langley
- Re: [TLS] MITM Attacks on Client Authentication a… Martin Rex
- Re: [TLS] MITM Attacks on Client Authentication a… Daniel Kahn Gillmor
- Re: [TLS] MITM Attacks on Client Authentication a… Nico Williams
- Re: [TLS] MITM Attacks on Client Authentication a… Bodo Moeller
- Re: [TLS] MITM Attacks on Client Authentication a… Xuelei Fan
- Re: [TLS] MITM Attacks on Client Authentication a… Martin Rex
- Re: [TLS] MITM Attacks on Client Authentication a… Adam Langley
- Re: [TLS] MITM Attacks on Client Authentication a… Karthik Bhargavan
- Re: [TLS] MITM Attacks on Client Authentication a… Karthik Bhargavan
- Re: [TLS] MITM Attacks on Client Authentication a… Martin Rex
- Re: [TLS] MITM Attacks on Client Authentication a… Xuelei Fan
- Re: [TLS] MITM Attacks on Client Authentication a… Karthik Bhargavan
- Re: [TLS] MITM Attacks on Client Authentication a… Xuelei Fan
- Re: [TLS] MITM Attacks on Client Authentication a… Liz meeks