If I understand correctly, a Fujisaki commitment is as follows: $g^m \cdot h^r $ mod $n$, where $m$ is a message, $r$ is a random number, there exists $a$ such that $h^a = g$, and $n$ is an RSA modulus.

Usually, when I read discussions about this scheme, the holder of the commitment does not know the prime factorization of $n$.

Is it a requirement that the order of the group be unknown to the creator of the commitment? (Note added assumption below.)

EDIT: Note that the message can be replaced with $m+k\cdot \lambda(n)$, where $\lambda(n)$ is the order of $n$. Though this is a threat to the commitment, I am more concerned with commitments of small values ($m << \lambda(n)$), so opening the commitment like this would be rejected. So I will rephrase:

Assuming $m << \lambda(n)$, is a Fujisaki Commitment binding if the prime factorization of $n$ is known?

  • $\begingroup$ What happens if you claim to reveal $m + k\lambda(n)$ for any $k$? Can the verifier tell that you've done so instead of returning a message $m < \lambda(n)$? $\endgroup$ – Squeamish Ossifrage Jun 20 '19 at 16:50
  • $\begingroup$ I didn't consider that because, in my use case, I have a range of possible expected values enforced by a range proof. $m + k \lambda(n)$ would be far outside this range. But, in general, this is correct. Especially considering that the other party probably doesn't know the order of $n$. $\endgroup$ – Zarquan Jun 20 '19 at 17:20

Yes! Note that in the original scheme there are two secrets the committer does not know:

  1. The factorization of the modulus: $N=pq$
  2. The value $a$ such that $h^a=g$.

Therefore, the commitment scheme is still binding, even if the factorization is known. Let's assume committer wants to open the commitment $c=g^m h^r \mod N$ to a different $(m^{*},r^{*})$. It means that she needs to find values $(m^{*},r^{*})$ such that:

$$g^m h^r=g^{m^{*}}h^{r^{*}} \mod N $$

$$\iff g^{m-m^{*}}h^{r-r^{*}}=(h^a)^{m-m^{*}}h^{r-r^{*}}=1 \mod N $$ Which equivalently gives the following equality in the exponents: $$\iff a(m-m^{*})+(r-r^{*})=0 \mod \phi(N) $$

Even if committer knows $\phi(N)$, she does not know $a$, therefore she cannot break the binding property, unless she additionally breaks the Discrete-Log in $\mathbb{Z}^{*}_p$ or $\mathbb{Z}^{*}_q$.

In most definitions of the Fujisaki commitment scheme, $N$ is chosen to be a product of safe primes, so commiter cannot even use Pohlig–Hellman algorithm efficiently, to calculate discrete logs modulo primes.

Trivially, if also $a$ is known to committer, then she can decommit to any $m^{*}$ by choosing an appropriate $r^{*}$ through solving the last equation in the exponents.

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