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

[Falko Strenzke <[e-mail]>]

Hi Markkhu,

I understand the hardware perspective, but I don't think that it is 
necessary or even possible that NIST opens up the algorithm interface 
like you propose.

First of all, a hardware implementation is not bound to provide a module 
that has the interface that NIST defines. For instance hardware support 
for ECC and RSA for embedded applications that I know works on the 
arithmetic level. So whatever NIST will specifies as the algorithm 
interface, this does not affect the hardware design.

Second, it will most likely matter for FIPS certifications how NIST 
specifies the algorithm and its interface. Namely, as I understand from 
previous conversations on one of our lists, FIPS conformance will 
require the implementation of the FIPS-algorithm /within the secure 
module/. And that should most likely have be the "closed" algorithm 
interface that I proposed, as otherwise there are too many critical 
operations left to the application using the module. This is why I think 
NIST will not open up the algorithm interface like you propose.

Does that make sense to you?

- Falko

Am 21.11.23 um 13:46 schrieb Markku-Juhani O. Saarinen:
> Falko,
>
> That seems workable, albeit slightly non-ideal from hardware 
> perspective. One alternative is to align the interface and signature 
> logic with ML-DSA / Dilithium. The module would always be passed "mu" 
> to represent the message to be signed/verified:
>
> mu = H( PK || M )
>
> Where PK = (PK.seed, PK.root) in the case of SPHINCS+, and maps to 
> "tr" in Dilithium, albeit without the hash (in Dilithium tr=H(PK)). In 
> both cases mu has at least "double" (collision-resistant) length -- 
> perhaps always 512 bits as in Dilithium.
>
> Changes in Alg. 18, slh_sign():
> Line 7: R <- PRF_msg(SK.prf, opt_rand, mu)
> Line 10: digest <- H_msg(R, mu)
>
> Changes in Alg. 19, slh_verify():
> Line 9: digest <- H_msg(R, mu)
>
> This corresponds to the BUFF transform (See Fig 6 
> https://ia.cr/2020/1525 ), which Dilithium already implements.
>
> Note the mapping; R = rho' in Dilithium: SK.prf = "K" in Dilithium, 
> opt_rand = "rnd" in Dilithium.
>
> Cheers,
> -markku
>
> On Tue, Nov 21, 2023 at 12:25 PM Falko Strenzke 
> <[e-mail]> wrote:
>
>     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
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>>
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>
>     *MTG AG*
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