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Re: [TLS] Last Call: draft-ietf-tls-renegotiation (Transport Layer Secur

Subject: Re: [TLS] Last Call: draft-ietf-tls-renegotiation (Transport Layer Security (TLS) Renegotiation Indication Extension) to Proposed Standard
From: Bodo Moeller
Date: Mon, 30 Nov 2009 22:10:02 +0100
On Nov 30, 2009, at 4:37 PM, The IESG wrote:

The IESG has received a request from the Transport Layer Security WG
(tls) to consider the following document:

- 'Transport Layer Security (TLS) Renegotiation Indication Extension '
  <draft-ietf-tls-renegotiation-01.txt> as a Proposed Standard

The IESG plans to make a decision in the next few weeks, and solicits
final comments on this action. Please send substantive comments to the
[email protected] mailing lists by 2009-12-14.
I support the move, contingent to two clean-ups to specified behavior:


1. Section 3 specifies "For ClientHellos which are renegotiating, this field contains the verify_data from the Finished message sent by the client on the immediately previous handshake", and "For ServerHellos which are renegotiating, this field contains the concatenation of the verify_data values sent by the client and the server (in that order) on the immediately previous handshake."
There is no good reason for this asymmetry of having just the client's
verify_data in the client's message, but both the client's and the
server's verify_data in the server's message -- the latter is pure
overhead, both in the specification and in the on-the-wire protocol.
To remove the overhead from both, the second sentence should be
saying, "For ServerHellos which are renegotiating, this field contains
the verify_data sent by the server on the immediately previous
handshake" (according changes to dependent parts of the specification
then should be made).
For earlier Internet Drafts in the series preceding draft-ietf-tls-
renegotiation-01.txt (draft-rescorla-tls-renegotiation.00.txt, etc.),
a reason to keep the current text would have been to maintain
compatibility with early implementations of those Internet Drafts.
However, draft-ietf-tls-renegotiation-01 has a newer
TLS_RENEGO_PROTECTION_REQUEST cipher suite and thus breaks
compatibility with those early implementations anyway.
(This means that now, changing the ServerHello extension definition as
suggested above gives the beneficial side-effect of exposing premature
implementations, besides merely avoiding overhead. Those incomplete
implementations will be more obviously incompatible because they'd
still be using the protocol variant with overhead, instead of just
differing subtly in the behavior if faced with
TLS_RENEGO_PROTECTION_REQUEST.)

2. The Security Considerations section of draft-ietf-tls- renegotiation-01.txt (section 7) includes language that unnecessarily restricts the flexibility that the TLS protocol specifically provides for, regulating areas that the TLS standard intentionally does not specify (RFC 5246 section 1) -- the current Internet Draft says:
"By default, TLS implementations conforming to this document MUST
verify that once the peer has been identified and authenticated within
the TLS handshake, the identity does not change on subsequent
renegotiations. For certificate based cipher suites, this means
bitwise equality of the end-entity certificate. If the other end
attempts to authenticate with a different identity, the renegotiation
MUST fail. If the server_name extension is used, it MUST NOT change
when doing renegotiation."
There is no security reason for this restriction. This sounds like a
bad and incomplete workaround for certain problems with TLS that the
updated protocol does not need a workaround for at all, because the
protocol update is all about properly *solving* those problems.
Keeping this language in the specification would have the weird
implication that applications that *need* the flexibility provided by
the TLS protocol (e.g., to allow renegotiation handshakes to switch to
a different ciphersuite, which may require a different certificate)
would have to to *skip* implementing the protocol update, and thus
stay vulnerable.
The new restriction that draft-ietf-tls-renegotiation-01.txt tries to
sneak in thus seems harmful both for interoperability and for
security. I haven't seen any signs of working group consensus on
including this new restriction.

Bodo




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