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

[Falko Strenzke <[e-mail]>]

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]> 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

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