Re: [Unbearable] Possible attack on Token Binding with RSA key exchange

[Subodh Iyengar <[e-mail]>]

Having said that, even if we only support forward secure ciphers, I do not think we can reasonably claim that we are secure against the threat of stolen private keys.

I like the the carrot opportunity this brings to encourage adoption of forward secure cipher suites.


Subodh

________________________________
From: Subodh Iyengar
Sent: Friday, September 1, 2017 3:24:25 PM
To: Nick Harper; IETF Tokbind WG
Subject: Re: [Unbearable] Possible attack on Token Binding with RSA key exchange


+1 to requiring that token binding only be used with forward secure cipher suites.

I believe the extended master secret requirement of token binding prevents the attack on forward secure suites, i.e. an attacker cannot change the DH share advertised by the real server and thus needs to get the private key for it.

Subodh

________________________________
From: Unbearable <[e-mail]> on behalf of Nick Harper <[e-mail]>
Sent: Friday, September 1, 2017 3:03:37 PM
To: IETF Tokbind WG
Subject: [Unbearable] Possible attack on Token Binding with RSA key exchange

I came across an attack on Token Binding today, which I think is worth
addressing in TBPROTO in some fashion (likely another paragraph in
Security Considerations). This attack involves an adversary with the
private key of a server it wishes to impersonate, and was mentioned in
draft-balfanz-tls-channelid in the last paragraph of its Security
Considerations.

In brief, if an attacker has possession of a server's private key, it
can hijack a TLS connection between client and server if the
connection uses RSA key exchange instead of (EC)DHE key exchange,
which allows the attacker to exercise the bound token without
possession of the Token Binding private key.

I realize that we're past WGLC at this point, but I think this should
be addressed. On one end, we could require forward-secret key exchange
modes with Token Binding. We could also describe this specific attack
in the Security Considerations, or we could expand the Security
Considerations to describe what attacks are and aren't in the Token
Binding threat model, to say that attacks where the adversary has the
server's private key are out of scope.

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