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From: "Bellebaum, Thomas" <thomas.bellebaum@aisec.fraunhofer.de>
To: "draft-irtf-cfrg-bbs-blind-signatures@ietf.org"
	<draft-irtf-cfrg-bbs-blind-signatures@ietf.org>
Thread-Topic: BBS Blind Signature Generators
Thread-Index: AQHcWv7+iX4zvXbp/UCc5CmLdlqY9Q==
Date: Fri, 21 Nov 2025 15:53:34 +0000
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Dear authors of draft-irtf-cfrg-bbs-blind-signatures,

I have a question about the generators used in the scheme.
I understand that Q_2 is required to make the pedersen commitment C hiding.=
 What I do not understand is why the other blind generators (J_i) are chose=
n to be separate from the usual message generators (H_i).

Intuitively and as mentioned in section 6.1, this effectively reveals to a =
verifier which messages were disclosed to the signer beforehand. While the =
section points at the main BBS draft for some discussion, I believe the dis=
cussion in the main draft is too limited to cover the implications of this =
addition.
As an example, a signature over a credential might consider its structure p=
ublic, so that the total number of signed messages and mapping of indices t=
o credential keys is considered public (this I understand is discussed in t=
he main document).
However, assume that some provers are now forced to reveal certain messages=
 to the signer, while others are not (A contrived example: A notorious tax =
evader is forced to temporarily reveal their blindly issued coins to their =
bank). The problem here is that every verifier can now see which provers we=
re forced to do so.

It also makes the draft more complicated IMHO, because you have to generate=
 two sets of generators with different api_id's. And it prevents easily mov=
ing from non-blind signatures to blind signatures.

In case the disclosure was intentional in some cases, note that trust in th=
e signer implies that the signer can just have a public policy over which m=
essages they allow/require to be revealed to them. Alternatively, one can e=
ncode this information as a message and disclose it to verifiers on demand.

Would it be possible to make the blind draft more similar to the main BBS d=
raft, such that blind issuance becomes a simple extension of the capabiliti=
es of the main scheme?
This would entail one or more of the following:

1. Rather than having two sets of message generators (J_i and H_i), have th=
e prover communicate to the signer which indices it intends to keep secret =
and split the H_i generators according to this information. The commitment =
C and associated linear proof should be calculated over the exponents of Q_=
2 and the H_i corresponding to undisclosed indices.
2. Allow non-blindly signed credentials to used with the blind proving inte=
rface by setting secret_prover_blind=3D0 for such credentials (which need n=
ot even be disclosed). Then, in the main draft, designate a generator as Q_=
2 with exponent 0 (or any fixed value) unless the blind-draft is used (just=
 so that it appears as part of the domain calculation). This way, you could=
 move almost all discussion about the proving mechanism to the main documen=
t, apart from one sentence to never disclose secret_prover_blind when using=
 the normal proving mechanism (which is irrelevant for the main scheme).

Best,

-- TBB

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