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

[Falko Strenzke <[e-mail]>]

What I would propose is just to request the features of 
Ed25519ctx/Ed25519ph in RFC 8032 for the PQC signature algorithms, i.e. 
at the moment for ML-DSA. The comment to NIST could be as follows:

===>

/We propose to in build the following two features into ML-DSA:/

/1. Introduction of a flag in the algorithm's interface that gives 
domain separation between the two uses direct-signing and 
sign-pre-hashed-message.
2. Introduction of a context string into the algorithm's interface of a 
length of 0 between 255 octets that allows the protocol or application 
to define a custom domain separation string./

/This proposal is meant to follow the construction of 
Ed25519ctx/Ed25519ph in RFC 8032. /

/The reason for the first point is the need to avoid any ambiguity with 
respect to what is the signed message given the alternate options of 
//direct-signing and sign-pre-hashed-message. When lacking this domain 
separation feature in ML-DSA, protocols that allow both variants and 
cannot provide authentic information about the variant during signature 
verification will be subjected to signature forgery attacks./

/The reason for the second point is that currently there are plans to 
design composite signature schemes that fulfil non-separability notions, 
i.e. make it impossible that a signature is stripped from the set of 
signatures in a composite signature and the remaining signature(s) 
appear as (a) valid signature(s) of the message. Existing proposals for 
protocols that cannot provide for authentic information about the nature 
(composite/standalone) of the signature algorithm during verification 
can achieve non-separability with a means of domain separation in the 
signature algorithm. The protocol can thus use the context string to 
ensure domain separation between composite and standalone use and 
possibly further aspects such as domain separation between applications.
/

/<===
/

Actually then, at least with Ed25519ctx/Ed25519ph, we can achieve secure 
non-separable composite signatures with ML-DSA, as both support a 
context string for domain separation. Whether that will happen in the 
LAMPS solution I cannot predict, but in any case, it might serve other 
protocols as well. It seems RFC 8032 has been written with a lot of 
foresight and is a good template.

- Falko

Am 21.11.23 um 12:47 schrieb Markku-Juhani O. Saarinen:
> Hi All,
>
> I'd also encourage sending out a public comment, perhaps on behalf of 
> "LAMPS WG". The public comment period on FIPS 205 will actually close 
> November 22, 2023 (Weds.) The date and the e-mail address for comments 
> can be found at https://csrc.nist.gov/pubs/fips/205/ipd
>
> I agree that the discussion in Section 9.4 ("Prehash SLH-DSA") seems 
> to leave it to the application whether M is the message itself, or 
> generated via on of the approved functions. There is no built-in 
> domain separation, which is a potential issue. So if someone is 
> sending a public comment, feel free to add my name to it.
>
> /_Quote_: "For some use cases, these issues may be addressed 
> by signing a digest of the message rather than signing the message 
> directly. In order to maintain the same level of security strength, 
> the digest that is signed needs to be generated using an approved hash 
> function or extendable-output function (XOF) (e.g., from FIPS 180-4 
> [12] or FIPS 202 [10]) that provides at least 8n bits of classical 
> security strength against both collision and second preimage attacks 
> [10, Table 4]."/
>
> Hence, an adversary can seemingly just state to the verification 
> function whether a (say) 512-bit blob is a message M itself, or hash 
> from SHAKE256(M1), SHA2-512(M2), or SHA3-512(M3) -- all are 
> authenticated by the same signature. EUF-CMA -- the stated security 
> goal of FIPS signature standards -- is indeed violated if the same 
> signature that was queried for M1/M2/M3 is also a signature for M 
> itself (different message).
>
> 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.)
>
> 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*
>     Dr. Falko Strenzke
>     Executive System Architect
>
>     Phone: [phone]
>     E-Mail: [e-mail]
>     Web: mtg.de <https://www.mtg.de>
>
>     <https://www.linkedin.com/search/results/all/?fetchDeterministicClustersOnly=true&heroEntityKey=urn%3Ali%3Aorganization%3A13983133&keywords=mtg%20ag&origin=RICH_QUERY_SUGGESTION&position=0&searchId=d5bc71c3-97f7-4cae-83e7-e9e16d497dc2&sid=3S5&spellCorrectionEnabled=false>
>     Follow us
>
>     ------------------------------------------------------------------------
>
>     <https://www.mtg.de/de/aktuelles/MTG-AG-erhaelt-Innovationspreis-des-Bundesverbands-IT-Sicherheit-e.V-00001.-TeleTrust/><https://www.itsa365.de/de-de/companies/m/mtg-ag>
>
>     MTG AG - Dolivostr. 11 - 64293 Darmstadt, Germany
>     Commercial register: HRB 8901
>     Register Court: Amtsgericht Darmstadt
>     Management Board: Jürgen Ruf (CEO), Tamer Kemeröz
>     Chairman of the Supervisory Board: Dr. Thomas Milde
>
>     This email may contain confidential and/or privileged information.
>     If you are not the correct recipient or have received this email
>     in error,
>     please inform the sender immediately and delete this
>     email.Unauthorised copying or distribution of this email is not
>     permitted.
>
>     Data protection information: Privacy policy
>     <https://www.mtg.de/en/privacy-policy>
>
>     _______________________________________________
>     Spasm mailing list
>     [e-mail]
>     https://www.ietf.org/mailman/listinfo/spasm
>
-- 

*MTG AG*
Dr. Falko Strenzke
Executive System Architect

Phone: [phone]
E-Mail: [e-mail]
Web: mtg.de <https://www.mtg.de>

<https://www.linkedin.com/search/results/all/?fetchDeterministicClustersOnly=true&heroEntityKey=urn%3Ali%3Aorganization%3A13983133&keywords=mtg%20ag&origin=RICH_QUERY_SUGGESTION&position=0&searchId=d5bc71c3-97f7-4cae-83e7-e9e16d497dc2&sid=3S5&spellCorrectionEnabled=false>
Follow us
------------------------------------------------------------------------
<https://www.mtg.de/de/aktuelles/MTG-AG-erhaelt-Innovationspreis-des-Bundesverbands-IT-Sicherheit-e.V-00001.-TeleTrust/> 
<https://www.itsa365.de/de-de/companies/m/mtg-ag>

MTG AG - Dolivostr. 11 - 64293 Darmstadt, Germany
Commercial register: HRB 8901
Register Court: Amtsgericht Darmstadt
Management Board: Jürgen Ruf (CEO), Tamer Kemeröz
Chairman of the Supervisory Board: Dr. Thomas Milde

This email may contain confidential and/or privileged information. If 
you are not the correct recipient or have received this email in error,
please inform the sender immediately and delete this email.Unauthorised 
copying or distribution of this email is not permitted.

Data protection information: Privacy policy 
<https://www.mtg.de/en/privacy-policy>