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

[Falko Strenzke <[e-mail]>]

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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