On Sun, Dec 13, 2020 at 1:03 PM Iván Arce (Quarkslab) <iarce@xxxxxxxxxxxxx> wrote:
Hello Eric, Fernando
On 12/13/20 5:38 AM, Fernando Gont wrote:
> Hello, Eric,
>
> Thanks for your comments! In-line....
>
>
> On 12/12/20 18:44, Eric Rescorla wrote:
..
>>
>> At a high level, many of the attacks listed here (especially in TCP)
>> result from the reliance (potentially accidental) on unpredictable
>> identifiers as a countermeasure against various forms of attack. For
>> instance, TCP is subject to a variety of off-path injection attacks
>> which are partly mitigated by randomizing sequence numbers and port
>> numbers. However, the more modern practice is to cryptographically
>> authenticate datagrams, thus preventing injection attacks even if the
>> port and sequence number are predictable.
>
> I don't think the two are mutually-exclusive. Nobody is arguing that
> randomizing numeric IDs is an alternative to cryptographic authentication.
Indeed. I would further argue that use of authenticated encryption is
not warranted on all protocol designs so dismissing the need for
security and privacy considerations for transient IDs on the basis that
"we encrypt and authenticate the packets anyway" is not a good stance in
general.
Fortunately, that's not the position I am taking. Rather, I am saying that authenticated encryption is an increasingly common practice and that we should not publish a set of recommendations in this area which does not engage with that properly.
>> Of course, it might be the case that these defenses are insufficient
>> (that would be useful to know!) but this document does not provide
>> much assistance in making that evaluation.
The intend of the document is not to provide cryptoanalysis guidance to
evaluate specific protocol designs but to provide general guidance on
how to deal with use of transient numeric identifiers in protocol design.
Indeed, the two protocols that you singled out do not follow our
guidance (they hardly could as the document we are reviewing is not yet
officially published) and assessing if the lack of precision for the
generation of transient IDs weakens the protocols would be a matter for
the respective authors to deal with.
This is an odd argument, as the authors of these documents could certainly
have read your documents and adopted your recommendations. But my
point here is different. As I said to Fernando, we are about to publish these
protocols as PS and yet they appear to violate the guidance here, which seems
like a bad situation if we are about to publish this guidance as BCP. As above,
I think the problem is that the guidance is not well suited to this type of protocol,
which means that this document ought to be adjusted. However, if you
have an argument that it is these protocols that should be revised, that would
be very good to know.
>> Connection IDs MUST NOT contain any information that can be used by
>> an external observer (that is, one that does not cooperate with the
>> issuer) to correlate them with other connection IDs for the same
>> connection. As a trivial example, this means the same connection ID
>> MUST NOT be issued more than once on the same connection.
>
> I believe that not recommending a good/safe choice for generating IDs
> has been proved to be problematic.
Indeed, that has been the case even in documents that describe the use
of cryptographic algorithms in protocols.
See, for example, the case of "RFC 25288 - AES Galois Counter Mode (GCM)
Cipher Suites for TLS" which in section 3 states:
(https://tools.ietf.org/html/rfc5288#section-3)
The nonce_explicit is the "explicit" part of the nonce. It is chosen
by the sender and is carried in each TLS record in the
GenericAEADCipher.nonce_explicit field. The nonce_explicit length
(SecurityParameters.record_iv_length) is 8 octets.
Each value of the nonce_explicit MUST be distinct for each distinct
invocation of the GCM encrypt function for any fixed key. Failure to
meet this uniqueness requirement can significantly degrade security.
The nonce_explicit MAY be the 64-bit sequence number.
The case below is quite similar and even more specific (it hints at a
particular way of implementing it) than the QUIC example and yet it lead
to discovery 8 years later of at least 4 vulnerable implementations
deployed on the Internet, as described by Bock at. al. in
"Nonce-Disrespecting Adversaries: Practical Forgery Attacks on GCM in TLS"
(https://eprint.iacr.org/2016/475.pdf)
Note that in the AES-GCM in TLS case the need for a counter was
warranted but not explicitly called for and such underspec'd text lead
to flawed implementations. In other cases, a random nonce/ID is better
than a numeric sequential value, we argue that authors should assess
what is appropriate in their particular case and write their spec and
rationale for it accordingly.
Well I'm certainly not arguing that it's not important to describe good practices for cryptographic protocols, but: it seems to me that this RFC did precisely what your document recommends in S 5.
- It specified the interop requirements (none)
- It provided the security and privacy analysis (they must be unique and it's bad if they're not)
- It recommended an algorithm (the sequence number)
I would note that the eventual outcome in this case in TLS 1.3 was to remove the explicit nonce entirely and to replace it with a value generated from the sequence number.
>
>> I'm not saying that cryptographic protocols don't need to take care in
>> identifer selection, but the guidance in this document does not seem
>> very helpful in that respect.
I'd like to dig deeper into that feedback. In which way do you think it
does not seem helpful. What would you expect it to say to be helpful?
See above for a specific example.
Baring a more detailed proposal I propose to include text that
explicitly calls out that in cases where protocols require cryptographic
algorithms to provide confidentiality and integrity (ie. authenticated
encryption) of the transient identifier fields some of the inherent
weaknesses in transient ID generation may be mitigated.
Does that sound like a sensible caveat?
Not really, no. I believe substantial rework is needed to address the role that identifiers work in cryptographic protocols.
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