[openssl-users] [openssl-dev] Replacing RFC2712 (was Re: Kerberos)

Nico Williams nico at cryptonector.com
Wed May 13 15:46:42 UTC 2015

On Wed, May 13, 2015 at 12:03:33PM +0200, Jakob Bohm wrote:
> On 12/05/2015 21:45, Nico Williams wrote:
> >On Tue, May 12, 2015 at 08:23:34PM +0200, Jakob Bohm wrote:
> >>How about the following simplifications for the new
> >>extension, lets call  it "GSS-2" (at least in this e-mail).
> >>
> >>1. GSS (including SASL/GS2) is always done via the SPNego
> >>GSS mechanism, which provides standard handling of
> >>mechanism negotiation (including round-trip optimizations),
> >>and is already its own standard (complete with workarounds
> >>for historic bugs in the dominant implementation...).
> >SASL/GS2 and SPNEGO are incompatible.
> How?  I thought SPNEGO encapsulated and negotiated
> arbitrary GSS mechanisms.

The problem is that negotiating twice is bad (for various reasons), and
SASL has non-GSS mechanisms, so negotiating SASL mechanisms, then GSS is
a two-level negotiation that is fraught with peril, therefore forbidden.

> To me the key benefit of SPNEGO is the existence of
> already battle tested negotiation code readily available
> i many/most current GSS implementation.  It is one less
> thing to design and implement wrong.

It's quite complex owing to having been underspecified in the first
place then having grown a number of bug workarounds over the years.

It'd be much easier to send a list of mechanism OIDs in the ClientHello,
have the server announce a choice in its response, and have the first
GSS token sent as early application data in the same flight as the
client's Finished message (assuming traditional TLS handshakes here),
with GSS channel binding.  When the client knows what mechanism they
want they could send the initial context token in the ClientHello (if
it's not too large) and use MIC tokens for channel binding.

> >The ALPN approach is to do the mechanism negotiation via ALPN.  This is
> >much better than SPNEGO in general.
> However I strongly suspect that using ALPN will cause
> practical conflicts with early HTTP/2 implementations
> and early ALPN implementations, as such early
> implementations are likely to only cater to that
> single use of ALPN.

Perhaps so.  I would prefer to optimize the GSS flights as well too.

> >>3. The TLS server (if it supports and allows the extension)
> >>responds with a 0 byte TLS extension "GSS-2" to confirm
> >>support.
> >Well, presumably the first response GSS token should go here.
> No, see below.

In your protocol the client already sent a SPNEGO initial security
context token.  A response is required, as GSS context establishment
token exchanges are strictly synchronous.

> >>5. In the last legs, the GSS mechanism is told to (mutually
> >>if possible) authenticate some already defined hash of the
> >>TLS handshake, thereby protecting the key exchange.Other
> >>than the round trip saving for the first 2 legs, this is
> >>what distinguishes GSS-2 from simply doing application level
> >>GSS over a TLS connection.  Any GSS negotiated keys are not
> >>used beyond this authentication of the TLS key exchange.
> >
> >This is the MIC exchange I mention above.
> Yep, however as this entails extra round trips, it is
> not the only option.

With PROT_READY there should be no need for an extra round-trip.

> >>6. If the GSS mechanism preferred by the client requires the
> >>authenticated hash value to be known before sending the
> >>first GSS leg, then the client shall simply abstain from
> >>including that first leg in the first leg SPNego message
> >>if sent in the client hello extension.
> >
> >If we're doing a MIC exchange then we don't need to know the channel
> >binding a initial security context token production time.
> However the early channel binding might save a leg.

You mean late.  Your idea seems to be to exposed knowledge of when is
the latest that a mechanism can begin to use the channel binding so as
to delay giving it the channel binding until we know it.  That would be
a significant change to GSS, and often it won't help (e.g., Kerberos,
the mechanism of interest in this thread).

> >>7. If the client wants encryption of the first GSS leg, it
> >>can either abstain from including that leg in the first
> >>SPNego GSS leg, or it can send a 0-byte first leg and then
> >>send the real first SPNego leg in the first encrypted client
> >>o server record, with the server responding with the second
> >>leg in the first encrypted server to client record as before
> >>(but no longer in the same round trip as the second half of
> >>the TLS handshake).
> >
> >With the ALPN approach this is a given.
> However if the first leg need not be encrypted and
> need not know thechannel binding, it can be sent a
> round earlier. This can (I hope) be decided on a per
> mechanism basis, thus if a GSS mechanism need not know
> its channel binding until the second leg,
> implementations that can provide the binding to the
> GSS layer later can take advantage of it.

No, this can't be decided on a per-mechanism basis, not without first
modifying GSS significantly.

> >>9. When the "GSS-2" extension is negotiated, TLS
> >>implementations SHOULD allow anonymous (unauthenticated)
> >>cipher suites even if they would not otherwise do so,
> >>however they MUST be able to combine the "GSS-2" extension
> >>with any and all of the cipher suites and TLS versions they
> >>otherwise implement.  For instance, if an implementation of
> >>the "GSS-2" extension is somehow bolted on to a fully
> >>patched OpenSSL 1.0.0 library (via generic extension
> >>mechanisms), then that combination would support it with
> >>TLS 1.0 only, and TLS 1.3 capable implementations would be
> >>negotiating TLS 1.0 when doing "GSS-2" with such an
> >>implementation.
> >
> >If only GSS mechanisms that provide integrity protection or better as
> >used, then this is fine.
> And why would this not work with any other form of
> channel binding?  Key requirement is that the TLS
> key exchange is bound to the specific authentication
> in some way not susceptible to man in the middle.

I was referring to GSS per-message tokens.  You're proposing changes to
GSS -- I hadn't understood that then.

> >You're missing a status message for authorization (GSS authentication
> >might complete, but authorization fail), though this is not strictly
> >necessary: the server can simply close the connection, including sending
> >an alert about this (or not) just before closing the connection.
> For security it is best if successful authentication
> of an unauthorized account (thinkroot) is
> indistinguishable from unsuccessful authentication of
> that account.

With GSS that's not really true.  The mechanism token exchange is a
black box.  It may yield failure with error tokens.  The application can
detect this and elect not to send error tokens, but if a final token was
expected in the success case then the peer will know what happened
anyways.  It's very difficult to generically ensure what you propose,
and not that valuable.


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