Re: [lamps] [EXTERNAL] pre-hashing the OID in draft-ounsworth-pq-composite-sigs-10

[Falko Strenzke <[e-mail]>]

Hi Markkhu,

Am 21.11.23 um 12:47 schrieb Markku-Juhani O. Saarinen:
> Hi All,
>
>
> Apart from existential forgery technicalities, here's a little 
> scenario where this matters: Imagine a remote firmware update that is 
> authenticated with a signature of M = SHA3-512(firmware). An adversary 
> modifies the metadata to state that, oh, here actually M=firmware 
> itself (the update just happens to be only 512 bits long.) Since the 
> metadata is not authenticated, the device replaces its firmware with 
> that garbage. As a result, the device is permanently bricked (perhaps 
> by remote control.)

This is a good example but actually any signing of unstructured data or 
a parser lacking in robustness may be affected.

- Falko

>
> Cheers,
> -markku
>
> Dr. Markku-Juhani O. Saarinen
>
> Staff Cryptography Architect
> PQShield Ltd
>
> M:            [phone]
>
> E:[e-mail]
>
> W:www.pqshield.com <http://www.pqshield.com/>
>
>
> On Tue, Nov 21, 2023 at 10:43 AM JOHNSON Darren 
> <darren.johnson=[e-mail]> wrote:
>
>     THALES GROUP LIMITED DISTRIBUTION to email recipients
>
>     Hi,
>
>     Did you pushed this concern to NIST?  They were accepting public
>     comments on their drafts of ML-DSA up until November 20’th.  I’m
>     sure they will accept a comment one day late.
>
>     Given that they loosely defined pre-hashing in FIPS 204, it sounds
>     like something that should be addressed by the base signature
>     algorithm, addressed or justified as not required..
>
>     In fact, this should be a base requirement that should be
>     addressed by any new signature algorithm considered; either
>     addressed, or justified as not required.
>
>     -Darren
>
>     *From:* Spasm <[e-mail]> *On Behalf Of * Falko Strenzke
>     *Sent:* Tuesday, November 21, 2023 4:29 AM
>     *To:* Seo Suchan <[e-mail]>; LAMPS WG <[e-mail]>
>     *Subject:* Re: [lamps] [EXTERNAL] pre-hashing the OID in
>     draft-ounsworth-pq-composite-sigs-10
>
>     This is why using raw RSA is insecure and isn't used in any protocol.
>
>     - Falko
>
>     Am 21.11.23 um 10:26 schrieb Seo Suchan:
>
>         not sure we care about existential forgery: RSA have one
>         natively( sign for product of two message is product of sign
>         of those two message) and
>
>         On 2023년 11월 21일 오후 6시 12분 54초 GMT+09:00, Falko Strenzke
>         <[e-mail]> <mailto:[e-mail]> 작성함:
>
>             Hi David,
>
>             Am 21.11.23 um 01:33 schrieb David A. Cooper:
>
>                 Hello Falko,
>
>                 I am unclear about the concern you are raising and the
>                 proposed solution. Is the concern specific to the
>                 proposed composite signature scheme or would it apply
>                 whenever a message could be signed using either a
>                 "pure" or "prehash" option?
>
>                 If I understand correctly, you're concern is that if
>                 message M could be signed as either:
>
>                      -- sig = sigalg(K, M); or
>
>                      -- sig = sigalg(K, OID || Hash(M)); or
>
>                 then an attacker could obtain a "pure" signature for
>                 "OID || Hash(M)" by requesting a "prehash" signature
>                 for "M," and that this is a concern regardless of how
>                 the "prehash" input to the signing function is
>                 constructed, e.g., "OID || Hash(M)", " 'prehash sig'
>                 || OID || Hash(M) ", etc. Is this correct?
>
>             Yes, it applies whenever there is an ambiguity with
>             respect to what is signed in the sense that anyone can
>             come up with one or more further validly signed messages
>             given a validly signed message from the signer. This is
>             called an existential signature forgery vulnerability
>             (signing M yields M' ≠ M which can be verified with the
>             same signature (in a different context)). That this is
>             referred to as an existential signature forgery is a fact,
>             not my opinion or interpretation. And I think also that a
>             protocol being vulnerable to such forgeries is considered
>             flawed is common sense.
>
>                 If so, isn't this already an issue with CMS? Content
>                 can be signed using CMS by either signing the content
>                 itself or by signing a set of signed attributes, one
>                 of which is the hash of the content. An attacker could
>                 present some content to the signed using CMS, with the
>                 CMS message containing signedAttrs. The attacker could
>                 then construct a new CMS message with no signed
>                 attributes for which the signedAttrs from the original
>                 message was the content.
>
>             Yes, true! I discovered that last week, too, and wrote the
>             attached paper about it, which I was even planning to make
>             subject to responsible disclosure to give library
>             maintainers some time to implement the countermeasures.
>             Now that you are pointing out the vulnerability yourself I
>             publish it right away. (I hope you believe me and don't
>             assume I wrote it now after your revelation ;-) Actually I
>             sent it to Russ already yesterday before your message to
>             hear his opinion.)
>
>             I think this kind of principal weakness is a concern and
>             so far I met no one who was aware of that problem. And I
>             can well imagine that some vendors are now going to review
>             or test their systems to preclude that they are vulnerable
>             – no matter how unlikely a harmful result may be assumed.
>
>                 This also seems related to general concerns about
>                 cross-application attacks, where an attacker attempts
>                 to obtain a signature in one context to exploit in
>                 another context. (For example, if an attacker could
>                 obtain a signature generated for TLS server
>                 authentication and somehow use that signature to
>                 perform TLS client authentication, and thus
>                 impersonate the server.)
>
>             Yes, very related. And this should show how serious the
>             concerns about signature forgery vulnerabilities should be
>             taken. Who had thought that TLS application layer
>             confusion attacks would be possible until
>             https://alpaca-attack.com/ ? I don't think it is good idea
>             to design something with unsound crypto and then wait
>             until a team from Ruhr-Uni Bochum (it's just a fact, most
>             of the real world vulnerable broken crypto research comes
>             from there) takes a closer look at real world applications
>             using the flawed protocol. Efail is of course another
>             perfect example – until then, no one saw a reason for
>             updating the long known vulnerable CBC and CFB encryption
>             in S/MIME and OpenPGP.
>
>                 Could you explain more about how using a different
>                 hash function addresses the problem? Are you
>                 envisioning that the signature algorithm could be used
>                 as a black box and one would preform the pre-hashing
>                 using a different hash algorithm or are you suggesting
>                 the use of a different hash algorithm inside the
>                 signing algorithm? Unless I am misunderstanding your
>                 concern, it seems that it can not be addressed if the
>                 signing algorithm is treated as a black box.
>
>             I wouldn't really say that I am envisioning one or the
>             other, because I am not convinced those separability
>             defences are ultimately necessary. But my initial proposal
>             (when I first mentioned it in a reply to John Gray) was
>             indeed to treat the signature algorithm as a black box
>             with the implication that one then is committed to
>             hash-and-sign. That means a composite signature would be
>             computed as
>
>             S_i = Sign_i(pubKey_i, comp_hash(M))
>
>             where comp_hash(M) = cSHAKE(M,
>             "pkix-composite-signature-hash-domain-separation-label)
>             /// whatever order of the arguments to cSHAKE makes sense
>             here, I haven't given that any attention here/
>
>             Then S_i brought into the standalone scheme context would
>             not be verifiable, because in the standalone context
>             comp_hash() does not exist and thus no preimage can be
>             found for the hash.
>
>             Surely, it might be worth exploring what is possible when
>             opening up the black box of the signature scheme. Then we
>             wouldn't have to commit to hash-and-sign. But then the
>             traditional algorithm would have to be redesigned, too,
>             because non-separability would require both algorithms to
>             be "protected". This topic comes up again under 1. below.
>
>             Two further notes:
>
>             1. There is also generally a problem if a protocol that
>             doesn't fulfil what I describe under 2. wants to enable
>             for instance SLH-DSA in two the variants direct-signing
>             and with pre-hashing. Then again the signature of the
>             pre-hash scheme would be valid for the message that
>             amounts to the value of the pre-hash in the direct-signing
>             scheme. This can be solved in two ways:
>
>                 1a) by a protocol with the features described under 2.
>                 1b) by ensuring domain separation for the two variants
>             direct-signing/pre-hash in the signature algorithm like it
>             is done in RFC 8032
>             <https://datatracker.ietf.org/doc/html/rfc8032#section-5.1>.
>             This is what we mean by "opening up the black box". Thus,
>             to facilitate the support of the variants
>             direct-signing/pre-hash, NIST could consider to include a
>             domain separation flag like in RFC 8032. (However, this
>             cannot be so easily used to address the non-separability
>             concerns since for that we would need to open up the black
>             box of the existing traditional schemes, which is probably
>             undesired, as mentioned above already.)
>
>             2. A protocol that *already* feeds the signature algorithm
>             (and ideally, further context information) in a
>             non-ambiguous way to the message digest computation as
>             context information doesn't face any of these problems
>             (neither separability nor direct-signing/pre-hashing),
>             because it can simply assign different algorithm
>             identifiers in each case and thus automatically achieves
>             domain separation. This is for instance the case for
>             OpenPGP signatures
>             <https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-computing-signatures>.
>             But introducing the context into a protocol that as of yet
>             doesn't feed any context information would again require
>             some "cut-off" measure like domain separation through the
>             hash in order to prevent the downgrade and thus signature
>             forgery.
>
>             - Falko
>
>                 Thanks,
>
>                 David
>
>                 On 11/19/23 9:53 PM, Falko Strenzke wrote:
>
>                     The point I was making is that for instance with a
>                     feasible computational effort of 2⁶³ hash
>                     evaluations on average, the attacker can control 8
>                     bytes in the digest, if he can control a large
>                     enough portion of the signed message and predict
>                     the whole message.
>
>                     Anyway, this is an irrelevant and misleading
>                     discussion, a signature forgery is a signature
>                     forgery and the protocol is broken.
>
>                     - Falko
>
>                     Am 19.11.23 um 01:04 schrieb Scott Fluhrer (sfluhrer):
>
>                         (Falko argued that it could be under total
>                         control if the attacker had sufficient
>                         computational power; however if the attacker
>                         had sufficient computational power, he could
>                         just break the public keys).
>
>                     -- 
>
>                     *MTG AG*
>                     Dr. Falko Strenzke
>                     Executive System Architect
>
>     -- 
>
>     *MTG AG*
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>
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