Threshold Schemes from Isogeny Assumptions

Threshold cryptography and secret sharing are large areas of interest in the cryptographic community since the late 1970s, when Shamir [51] and Blakley [7] published the first secret sharing schemes. In 1989, Desmedt and Frankel [21] constructed a practical threshold cryptosystem based on Shamir’s secret sharing and ElGamal encryption [26].

The goal of a k-out-of-n, or (k, n)-threshold scheme is to split a secret key into multiple shares and distribute them among n parties, each party receiving one share. Then, for a certain threshold \(k\le n\), any k collaborating parties must be able to compute the cryptographic operation, e.g. decrypt or sign, without learning the secret key, while any set of less than k parties must be unable to do so.

After the publication of Desmedt and Frankel’s scheme, several other threshold protocols were proposed; among others, a threshold variant of ElGamal signatures by Harn [34], a threshold DSA scheme by Gennaro et al. [32], and Desmedt and Frankel’s and Shoup’s threshold RSA signature schemes [22, 53]. More recently, applications of threshold schemes in the context of blockchains and cryptocurrencies led to a renewed interest in threshold ECDSA schemes [24, 31].

However, all of these schemes are either based on discrete logarithm or integer factorization problems, and are thus not quantum-resistant, since they fall prey to Shor’s algorithm [52]. Only very recently, Cozzo and Smart [15] reviewed the post-quantum signature schemes that entered the second round of the NIST PQC standardization process [43] for threshold variants. Their main observation is that only the multivariate-based schemes LUOV [5] and Rainbow [23] allow for a natural threshold construction.

Another popular family of post-quantum schemes is provided by isogeny-based cryptography [35, 36]. While this family is not represented in the NIST PQC track for signatures, isogeny-based signatures have recently attracted much attention [4, 17, 19]. In this work we introduce the first isogeny-based threshold encryption and signature schemes, based on Shamir’s secret sharing.

Our schemes are simple adaptations of Desmedt and Frankel’s and related schemes to the Hard Homogeneous Spaces (HHS) framework. This framework was introduced by Couveignes [14], to generalize both discrete logarithm and isogeny-based schemes. Encryption schemes for HHS were first proposed by Couveignes [14] and Rostovtsev and Stolbunov [49], then improved by De Feo, Kieffer and Smith [18], eventually lead to the development of CSIDH by Castryck, Lange, Martindale, Panny, and Renes [11].

The possibility of signature schemes based on HHS was first suggested by Couveignes [14] and Stolbunov [55, 56], although no instantiation was known until recently, when Beullens, Kleinjung, and Vercauteren introduced CSI-FiSh [4]. Before that, an alternative signature scheme based on a weaker notion of HHS, named SeaSign, was presented by De Feo and Galbraith [17].

Our Contributions. We introduce threshold variants of the Couveignes–Rostovtsev–Stolbunov encryption and signature schemes, based on Shamir’s secret sharing. To make the results more easily accessible to non-experts, we first present our schemes in an abstract way, using the language of HHS, and only later we analyze their instantiation using CSIDH/CSI-FiSh.

The encryption scheme is a direct adaptation of [21]; the signature scheme is similar to threshold versions of Schnorr signatures [50]. Both schemes can only be proven secure in a honest-but-curious security model [9]; we skip the easy proof for the encryption scheme, and we focus on the more technical one for the signature scheme, which we prove secure in a static corruptions model, under a generalization of the Decision Diffie-Hellman Group Action (DDHA) assumption of Stolbunov.

We conclude with an analysis of the instantiations of the schemes based on isogeny graphs, in particular on the supersingular isogeny graphs used in CSIDH and CSI-FiSh.

We view this work as an initial step towards practical threshold schemes based on HHS and isogenies. Several technical improvements, such as better security properties and proofs, are necessary before these protocols can be considered truly practical. We discuss these issues at the end of this work.

Outline. Section 2 recalls basic facts on secret sharing, threshold cryptography, and HHS. Section 3 then introduces threshold encryption and signature schemes based on HHS, and reviews their security features. In Sect. 4, we give details about the instantiation of these threshold schemes using isogeny graphs. We conclude by summarizing open problems towards practical applications of our schemes.

We briefly recall here two fundamental constructions in group-theoretic cryptography. The first, Shamir’s secret sharing [51], lets a dealer split a secret s into n shares, so that any k shares are sufficient to reconstruct s; it is a basic primitive upon which several threshold protocols can be built.

The second, Couveignes’ Hard Homogeneous Spaces (HHS) [14], is a general framework that abstracts some isogeny protocols, and that eventually inspired CSIDH [11]. Although most popular isogeny-based primitives are not, strictly speaking, instances of HHS, the protocols introduced in this work require an instance of an HHS in the strictest sense, and will thus be presented using that formalism.

2.1 Shamir’s Secret Sharing and Threshold Cryptosystems

Shamir’s scheme relies on polynomial interpolation to construct a k-out-of-n threshold secret sharing, for any pair of integers \(k\le n\).

Concretely, a prime \(q>n\) is chosen, and the secret s is sampled from \(\mathbb {Z}/q\mathbb {Z}\). To break the secret into shares, the dealer samples random coefficients \(c_1,\ldots ,c_{k-1}\in \mathbb {Z}/q\mathbb {Z}\) and forms the polynomial

then they form the shares \(s_1=f(1), \ldots , s_n=f(n)\) and distribute them to the n participants, denoted by \(\mathcal {P}_1, \dots , \mathcal {P}_n\). We shall call i the identifier of a participant \(\mathcal {P}_i\), and \(s_i\) his share.

Any k participants, but no less, can reconstruct f using Lagrange’s interpolation formula, and then recover s by evaluating f at 0. Explicitly, a set of participants \(\mathcal {P}_i\), with indices taken from a set \(S\subset \{1, \dots , n\}\) of cardinality at least k, can recover the secret s in a single step through the formula

Shamir’s secret sharing enjoys perfect or information theoretic security, meaning that less than k shares provide no information on the secret. Indeed, assuming that \(k-1\) participants, w.l.o.g. \(\mathcal {P}_1, \dots , \mathcal {P}_{k-1}\), put their shares together, the map

is, by Lagrange’s formula, an isomorphism of \((\mathbb {Z}/q\mathbb {Z})\)-vector spaces; hence, each tuple \(\bigl (s=f(0),f(1), \dots , f(k-1)\bigr )\) is equally likely to occur.

Threshold Schemes. A major step towards practical threshold schemes based on Shamir’s secret sharing was Desmedt and Frankel’s threshold variant of ElGamal decryption [21]; a similar approach to design threshold signatures was proposed by Harn [34]. Many other threshold protocols follow a similar pattern, colloquially referred to as secret sharing in the exponents, that we are now going to briefly recall.

Let the secret \(s\in \mathbb {Z}/q\mathbb {Z}\) and the shares \(s_i\) be distributed as above. Let G be a cyclic group of order q, and let g be a generator. Assuming that discrete logarithms are hard in G, the participants’ goal is to compute the shared key \(g^s\) without letting anyone learn the secret s. We can again use Lagrange interpolation, but this time in the exponent:

To make this idea into a protocol, each party computes \(g^{s_i}\) from its share \(s_i\), and sends it to all other parties. Given at least k shares \(s_i\) of the key with \(i\in S\) and \(\#S\ge k\), any party can then compute the shared key as

where the exponents

can be precomputed from public information.

If broadcasting the shares \(g^{s_i}\) to all participants is too expensive, an alternative is to send them to a central combiner, who is then in charge of computing \(g^s\) and finalizing the protocol. As we shall see later, this flexibility will be lost in our setting.

Secret Sharing in Rings. The proof of perfect security of Shamir’s secret sharing scheme fundamentally relies on \(\mathbb {Z}/q\mathbb {Z}\) being a field. For reasons that will become apparent later, we shall need to adapt the scheme to non-prime q, and thus to general rings of modular integers. This presents two problems: ensuring that no impossible inversions happen when computing the coefficients \(L_{l,i}^S\) in Eq. (1), and proving security in the more general setting. These obstacles are not difficult to overcome, as already highlighted in, e.g., RSA-based threshold schemes [53]; we briefly explain how this is done.

Impossible inversions arise during the reconstruction of the shared secret whenever one of the denominators \((j-i)\) in Lagrange’s formula is not coprime to q. If \(q_1\) is the smallest prime factor of q, then there can be at most \(q_1\) distinct values modulo \(q_1\); however, any identifier i congruent to 0 modulo \(q_1\) must be prescribed, since otherwise \(f(i) \bmod q_1\) would leak information on \(s\bmod q_1\). Hence, at most \(q_1-1\) participants can take part to Shamir’s scheme in \(\mathbb {Z}/q\mathbb {Z}\); for example, using \(1,2,\dots ,q_1-1\) as identifiers ensures that no difference of two of them shares a common factor with q.

Perfect security of the scheme is also achieved by restricting the identifiers to \(1,2,\dots ,q_1-1\), or any other set of integers distinct and non-zero modulo all divisors of q, thus restricting the number of participants to \(n < q_1\). We formally prove this below.

Proposition 1

Let q be an integer with prime factorization \(q=\prod q_i^{e_i}\). Assume \(q_1\) is the smallest of the prime factors, let \(k\le n<q_1\), and sample \(s,c_1,\dots ,c_{k-1}\in \mathbb {Z}/q\mathbb {Z}\) uniformly at random. Let

and let \(x_1,\dots x_{k-1}\in \mathbb {Z}/q\mathbb {Z}\) be distinct and non-zero modulo all \(q_i\). Associate a random variable S to s, and random variables \(Y_i\) to each \(f(x_i)\).

The random variables \(S, Y_1,\ldots Y_{k-1}\) are independent; in particular Shamir’s (k, n)-secret sharing scheme over \(\mathbb {Z}/q\mathbb {Z}\) is perfectly secure, in the sense that, given the shares \(f(x_1),\dots ,f(x_{k-1})\), every secret s is equally likely to have originated them.

Proof

Consider the map

since all \(x_i\mod q_j\) are distinct and non-zero, its reduction modulo \(q_j\) is an isomorphism of \(\mathbb {Z}/q_j\mathbb {Z}\)-vector spaces; thus, by the Chinese Remainder Theorem, \(\rho \) is an isomorphism of \(\mathbb {Z}/q\mathbb {Z}\)-modules.

Introducing random variables \(Y_0\) for f(0) and \(C_i\) for the \(c_i\)’s, we have that

from which we deduce that \(P\{Y_i=f(x_i)\}=q^{-1}\). In particular, since \(s=f(0)\),

for any \(s,f(x_1),\dots ,f(x_{k-1})\), implying that S and the \(Y_i\)’s are independent.    \(\square \)

2.2 Hard Homogeneous Spaces

Hard Homogeneous Spaces (HHS) were introduced by Couveignes in [14] as a generalization of Diffie-Hellman schemes. A principal homogeneous space, or \(\mathcal {G}\)-torsor is a set \(\mathcal {E}\) endowed with a faithful and transitive group action by a group \(\mathcal {G}\).^{Footnote 1} In other words, it is defined by a mapping

satisfying the following properties:

• Compatibility: \(\mathfrak {g}'*(\mathfrak {g}*E) = (\mathfrak {g}'\mathfrak {g})*E\) for any \(\mathfrak {g},\mathfrak {g}'\in \mathcal {G}\) and \(E\in \mathcal {E}\);

• Identity: \(\mathfrak {e}*E = E\) if and only if \(\mathfrak {e}\in \mathcal {G}\) is the identity element;

• Transitivity: for any \(E,E'\in \mathcal {E}\) there exists a unique \(\mathfrak {g}\in \mathcal {G}\) such that \(\mathfrak {g}*E=E'\);

In particular, if \(\mathcal {G}\) is finite, these axioms imply that \(\#\mathcal {G}=\#\mathcal {E}\).

Couveignes defines a HHS as a finite principal homogeneous space with some additional algorithmic properties. He requires that the following problems can be solved efficiently (e.g., in polynomial time):

• Group operations: decide whether a string \(\mathfrak {g}\) represents an element of \(\mathcal {G}\), decide whether \(\mathfrak {g}=\mathfrak {g}'\), compute \(\mathfrak {g}^{-1}\) and \(\mathfrak {g}\mathfrak {g}'\);

• Sampling: sample uniformly random elements from \(\mathcal {G}\);

• Membership: decide whether a string E represents an element of \(\mathcal {E}\), decide whether \(E=E'\);

• Action: Given \(\mathfrak {g}\) and E, compute \(\mathfrak {g}*E\).

Furthermore, the following problems should be hard (e.g., not known to be solvable in polynomial time):

• Vectorization: Given \(E,E'\in \mathcal {E}\), find \(\mathfrak {g}\in \mathcal {G}\) such that \(\mathfrak {g}*E =E'\);

• Parallelization: Given \(E,E',F\in \mathcal {E}\), such that \(E'= \mathfrak {g}*E\), find \(F'=\mathfrak {g}*F\).

As a simple example, let \(\mathcal {E}\) be a group of prime order q, then \(\mathcal {G}=(\mathbb {Z}/q\mathbb {Z})^\times \) acts on \(\mathcal {E}\setminus \{1\}\) by \(a*g=g^a\). In this case, the Vectorization problem is the discrete logarithm problem in \(\mathcal {E}\), and the Parallelization problem is the Computational Diffie–Hellman problem. Hence any discrete logarithm group is also a HHS.

Couveignes’ original proposal used as HHS sets of ordinary elliptic curves over finite fields, with complex multiplication by a quadratic imaginary order \(\mathcal {O}\); indeed, these are torsors for the class group \(\mathrm {cl}(\mathcal {O})\), and the Vectorization and Parallelization problems are not known to be easily solvable. Based on this HHS, he defined key exchange as a straightforward generalization of the Diffie–Hellman protocol, and he also sketched an interactive identification scheme.

However, Couveignes’ proposal presents several difficulties, as neither the group action nor random sampling are known to be easily computable. Independently from Couveignes, Rostovtsev and Stolbunov [49, 55] proposed a key-exchange scheme based on the same group action, but with a different representation of elements of \(\mathrm {cl}(\mathcal {O})\). This proposal had the benefit of making key-exchange feasible, if not practical, and subsequent research [18] eventually led to the development of CSIDH [11], an efficient key exchange scheme based on the action of a quadratic class group on a set of supersingular curves.

Nevertheless, none of these constructions satisfies exactly the axioms of a HHS, since, for example, the cost of evaluating \(\mathfrak {g}*E\) in CSIDH is in the worst case exponential in the size of \(\mathfrak {g}\). While every group element has an equivalent representation that permits to efficiently evaluate the action, computing such representation is difficult in general. This is not a problem for key-exchange schemes based on CSIDH, but, for example, it makes identification and signature schemes more involved and less efficient than what Couveignes had originally envisioned [17, 19].

The roadblock in all these constructions is the fact that the structure of the class group \(\mathrm {cl}(\mathcal {O})\) is unknown, and it is thus impossible to have a unique representation for its elements. The best algorithm for computing the class group structure runs in sub-exponential time, and is thus neither practical nor scalable; nevertheless the application to isogeny-based signatures motivated Beullens, Kleinjung and Vercauteren [4] to run an intensive computation for the CSIDH-512 parameter set, which allowed them to construct CSI-FiSh, the most efficient isogeny-based signature to date.

Currently, CSI-FiSh is the only known instance of HHS based on isogenies: group elements have unique representation, the group action can be evaluated efficiently, and the Vectorization and Parallelization problems are believed to be hard, both classically and quantumly. Unfortunately, parameter generation requires exponential time in the security parameter, thus CSI-FiSh is a HHS only in a practical sense for a specific security level, but not in the asymptotic sense.

In the next sections we are going to introduce threshold schemes based on HHS; then we will give more details on CSI-FiSh, and look at how the threshold schemes can be instantiated with it.

We now present threshold schemes based on Hard Homogeneous Spaces.

Let a group \(\mathcal {G}\) and a set \(\mathcal {E}\) be given, such that \(\mathcal {G}\) acts faithfully and transitively on \(\mathcal {E}\) and the HHS axioms are satisfied. We are going to require an additional property: that an element \(\mathfrak {g}\in \mathcal {G}\) of order q is known, and we shall write \(q_1\) for the smallest prime divisor of q. In particular, these hypotheses imply that there is an efficiently computable embedding \(\mathbb {Z}/q\mathbb {Z}\hookrightarrow \mathcal {G}\) defined by \(a\mapsto \mathfrak {g}^a\), which we are going to exploit to embed Shamir’s secret sharing in the HHS.

Notation. From now on we will use capital letters \(E,F,\dots \) to denote elements of the HHS \(\mathcal {E}\), and gothic letters \(\mathfrak {a},\mathfrak {b},\mathfrak {g},\dots \) to denote elements of the group \(\mathcal {G}\). Following [4], it will be convenient to see \(\mathbb {Z}/q\mathbb {Z}\) as acting directly on \(\mathcal {E}\): we will write \(\left[ a\right] \!\) for \(\mathfrak {g}^a\), and \(\left[ a\right] \!E\) for \(\mathfrak {g}^a*E\), where \(\mathfrak {g}\) is the distinguished element of order q in \(\mathcal {G}\).^{Footnote 2} Be wary that under this notation \([a][b]E = [a+b]E\).

Remark 1

The additional hypothesis excludes, in particular, HHS of unknown order, such as CSIDH (outside of the parameter set shared with CSI-FiSh).

Note that, assuming the factorization of q is known, given any element of \(\mathcal {G}\) it is easy to test whether it is of order q. Nevertheless, in some instances it may be difficult to decide whether an element \(\mathfrak {g}'\in \mathcal {G}\) belongs to \(\langle \mathfrak {g}\rangle \); this may happen, for example, if \(\mathcal {G}\simeq (\mathbb {Z}/q\mathbb {Z})^2\). This will not impact the protocols we define here, but is an important property to consider when designing threshold protocols in the general HHS setting. At any rate, for instantiations based on CSI-FiSh it is always easy to test membership of \(\langle \mathfrak {g}\rangle \).

On the other hand, unless \(\mathcal {G}=\langle \mathfrak {g}\rangle \), it is a well known hard problem (exponential in \(\log q\)) to decide whether given \(E,E'\in \mathcal {E}\) there exists \(a\in \mathbb {Z}/q\mathbb {Z}\) such that \(E'=\left[ a\right] \!E\). Indeed, a generic solution to this problem would imply an efficient generic algorithm for solving many instances of discrete logarithms [11].

We now describe a distributed algorithm to compute the group action of \(\langle \mathfrak {g}\rangle \) on \(\mathcal {E}\) in a threshold manner, and explain how it impacts the communication structure of threshold protocols. Then we present two simple threshold protocols, a KEM and a signature, directly adapted from their non-threshold counterparts.

3.1 Threshold Group Action

Like in Sect. 2, we assume that the participants \(\mathcal {P}_1,\mathcal {P}_2,\dots \) possess shares \(s_i=f(i)\) of a secret \(s\in \mathbb {Z}/q\mathbb {Z}\); their goal is to evaluate the group action \(\left[ s\right] \!E_0\) for any given \(E_0\in \mathcal {E}\), without communicating their shares \(s_i\).

Let \(S\subset \{1,\dots ,n\}\) be a set of cardinality at least k, and recall the definition of the Lagrange coefficients in Eq. (1):

Then the participants \(\mathcal {P}_i\) for \(i\in S\) determine the shared secret by \(s=\sum _{i\in S}s_i\cdot L_{0,i}^S\). For the sake of simplicity, we will assume that \(S=\{1,...,k\}\).

The participants coordinate as follows. First, \(E_0\) is sent to \(\mathcal {P}_1\), who starts by computing

The resulting \(E_1\) is passed on to \(\mathcal {P}_2\), who continues by computing

This procedure repeats analogously for the parties \(\mathcal {P}_3,...,\mathcal {P}_{k-1}\), and at last \(\mathcal {P}_k\) can compute

Communication Structure. Comparing the algorithm to classical threshold Diffie-Hellman protocols as in Sect. 2.1, it is obvious that there are differences in their structures. There, each party \(\mathcal {P}_i\) computes \(g_i=g^{s_i}\) from its secret share \(s_i\) and a common generator g. Anyone can then compute \(g_i^{L_{0,i}^S}\) for each \(i\in S\), and multiply the results to obtain \(g^{s}\).

In our HHS setting, the situation is different. First, \(\left[ s_i\cdot L_{0,i}^S\right] \!E\) cannot be computed from the knowledge of \(\left[ s_i\right] \!E\) and \(L_{0,i}^S\), thus only \(\mathcal {P}_i\) can compute it. Consequently, each participant has to know in advance the set S of parties taking part to the computation, in order to apply \(L_{0,i}^S\).

Further, it is not possible to introduce a combiner, who could proceed as in the classical case by receiving the different \(\left[ s_i\cdot L_{0,i}^S\right] \!E_0\) and combining them to obtain \(\left[ s\right] \!E_0\), since in general the set \(\mathcal {E}\) is not equipped with a compatible group operation \(\mathcal {E}\times \mathcal {E}\rightarrow \mathcal {E}\). Therefore, it is necessary to adopt a sequential round-robin communication structure:

Note that the order of the \(\mathcal {P}_i\) can be changed without affecting the final result.

However, this means that \(\mathcal {P}_k\) is the only party who ends up knowing the result of the group action. If a cryptographic protocol needs to handle this element secretly, our algorithm is only suitable for situations where only one participant is required to know the secret result. Algorithm 1 summarizes the described approach in the general case.

In a different setting where all participants are required to secretly know the final result, several modifications are possible. For example, when encrypted channels between the participants exist, the last participant can simply distribute through them the resulting \(\left[ s\right] \!E_0\).

Alternatively, k parallel executions of Algorithm 1, each arranging the participants in a different order, let all participants know the final result. The cost of this modification is rather high: \(O(k^2)\) elements of \(\mathcal {E}\) need to be transmitted, and \(O(k^2)\) group actions evaluated. This can be improved to \(O(k\log k)\) transmitted elements of \(\mathcal {E}\) (but still \(O(k^2)\) group actions) using a binary splitting strategy.

Remark 2

Algorithm 1 does nothing to prevent corrupted participants from leading to an incorrect output. While threshold schemes based on discrete logarithms can often detect and correct malicious behavior (using, e.g., error correcting codes [32]), this is more difficult for HHS. Indeed, there seems to be no way for a participant to verify the previous participant’s output in Algorithm 1, outside of generic zero-knowledge techniques.

3.2 Threshold HHS ElGamal Decryption

The first application we present for our threshold group action is threshold decryption, a direct adaptation of [21].

Inspired by the classical ElGamal encryption scheme [26], a PKE protocol in the HHS settings was first introduced by Stolbunov [49, 55, 56]. We briefly recall it here, using the terminology of KEMs.

• Public parameters: A HHS \((\mathcal {E},\mathcal {G})\), a starting element \(E_0\in \mathcal {E}\), and a hash function H from \(\mathcal {E}\) to \(\{0,1\}^\lambda \).

• Keygen: Sample a secret key \(\mathfrak {a}\in \mathcal {G}\), output \(\mathfrak {a}\) and the public key \(E_a=\mathfrak {a}*E_0\).

• Encaps: Sample \(\mathfrak {b}\in \mathcal {G}\), output \(K=H(\mathfrak {b}*E_a)\) and \(E_b=\mathfrak {b}*E_0\).

• Decaps: Given \(E_b\), if \(E_b\in \mathcal {E}\) output \(K = H(\mathfrak {a}*E_b)\), otherwise output \(\bot \).

The Decaps routine is easily adapted into a threshold algorithm requiring k participants to collaborate in order to recover the decryption key K. This also requires modifying Keygen, which must now be executed by a trusted dealer and integrate Shamir’s secret sharing.

• Public parameters: A HHS \((\mathcal {E},\mathcal {G})\) with a distinguished element \(\mathfrak {g}\in \mathcal {G}\) of order q, a starting element \(E_0\in \mathcal {E}\), and a hash function H from \(\mathcal {E}\) to \(\{0,1\}^\lambda \).

• Keygen:

  • Sample a secret \(s\in \mathbb {Z}/q\mathbb {Z}\) and generate shares \(s_i\in \mathbb {Z}/q\mathbb {Z}\) using Shamir’s secret sharing;

  • Distribute privately \(s_i\) to participant \(\mathcal {P}_i\);

  • Output public key \(E_a=\left[ s\right] \!E_0\).

• Encaps: Sample \(\mathfrak {b}\in \mathcal {G}\), output \(K=H(\mathfrak {b}*E_a)\) and \(E_b=\mathfrak {b}*E_0\).

• Decaps: Given \(E_b\) and a set S of participants, \(\#S\ge k\), run Algorithm 1 to compute \(E=\left[ s\right] \!E_b\); output \(\bot \) if the algorithm returns \(\bot \), otherwise output \(K = H(E)\).

The asymmetry of the scheme will not be lost on the reader: while the shared secret for the threshold group is restricted to be in \(\langle \mathfrak {g}\rangle \), there are no restrictions for Encaps. Although it would be completely possible (maybe even desirable for practical reasons) to restrict secrets to \(\langle \mathfrak {g}\rangle \) also in the encapsulation, we do not do so because there is no known way for decapsulation to test whether \(E_b\) has been generated this way.

It is clear that this scheme achieves the stated goal of threshold decryption: upon receiving a ciphertext, at least k participants must agree to decrypt in order to recover the key K; only the last participant in the chain learns K. If less than k participants agree to decrypt, the key K cannot be recovered; however this security property is only guaranteed when all participants behave honestly.

When allowing for corruptions, the scheme immediately becomes broken. Indeed in Algorithm 1, when a participant beyond the first receives an input, they are unable to link it to the ciphertext \(E_b\). This makes it possible to trick an unwilling participant \(\mathcal {P}\) into helping decrypt a message: let c be such a message, a group of \(k-1\) participants only has to wait for a message \(c'\) that \(\mathcal {P}\) is willing to decrypt; when \(\mathcal {P}\) agrees, they submit to it an intermediate value of a computation for c, which \(\mathcal {P}\) is unable to distinguish from one for \(c'\). Contrast this to the original El Gamal threshold decryption of Desmedt and Frankel [21], where each participant performs its computation directly on the input.

Because of this, the security of the protocol can only be proven in a honest-but-curious model. We skip the easy security proof, and leave the search for more refined threshold decryption protocols for future work.

3.3 Threshold Signatures

An identification scheme in the HHS framework was first sketched by Couveignes [14]; in his PhD thesis [56] Stolbunov also suggested applying the Fiat-Shamir transform [29] to it to obtain a signature scheme. Nevertheless these schemes stood out of reach until recently, when the class group computation for CSIDH-512 was completed [4]; CSI-FiSh is effectively Stolbunov’s scheme, combined with optimizations introduced in SeaSign [17].

CSI-FiSh and its ancestors can be easily adapted into threshold protocols. We start by recalling the basic interactive zero-knowledge identification scheme: a prover Peggy wants to convince a verifier Vic that she knows a secret element \(\mathfrak {a}\in \mathcal {G}\) such that \(E_a=\mathfrak {a}*E_0\). They proceed as follows:

• Peggy samples a random \(\mathfrak {b}\in \mathcal {G}\) and commits to \(E_b=\mathfrak {b}*E_0\).

• Vic challenges with a random bit \(c\in \{0,1\}\).

• If \(c=0\), Peggy replies with \(\mathfrak {z}=\mathfrak {b}\); otherwise she replies with \(\mathfrak {z}=\mathfrak {b}\mathfrak {a}^{-1}\).

• If \(c=0\), Vic verifies that \(\mathfrak {z}*E_0=E_b\); otherwise, he verifies that \(\mathfrak {z}*E_a=E_b\).

It is immediately seen that the scheme is correct, thanks to the properties of homogeneous spaces, and that it has soundness 1/2. For the zero-knowledge property, it is crucial that elements in \(\mathcal {G}\) can be sampled uniformly, and that they have unique representation. See [4, 17, 56] for detailed proofs.

We now adapt this scheme into a threshold signature by applying the Fiat-Shamir transform and Shamir’s secret sharing as before.

We let again \((\mathcal {E},\mathcal {G})\) be a HHS with a distinguished element \(\mathfrak {g}\) of order q, we fix a starting element \(E_0\in \mathcal {E}\), and a hash function \(H:\{0,1\}^*\rightarrow \{0,1\}^\lambda \). We assume that a trusted dealer has sampled a random secret \(s\in \mathbb {Z}/q\mathbb {Z}\), securely distributed shares \(s_i\) to the participants \(\mathcal {P}_i\), and published the public key \(E_s=\left[ s\right] \!E_0\).

Here is a sketch of how participants \(\mathcal {P}_1,\dots ,\mathcal {P}_k\) can cooperate to sign a message m:

• In the commitment phase, the participants collaborate to produce a random element \(\left[ b\right] \!E_0\) in a way similar to Algorithm 1, by producing each a random value \(b_i\in \mathbb {Z}/q\mathbb {Z}\) and evaluating \(E_i=\left[ b_i\right] \!E_{i-1}\).

• Once \(E_k=\left[ b\right] \!E_0\) is computed, the challenge bit c is obtained from the hash \(H(E_k,m)\).

• If \(c=0\), each \(\mathcal {P}_i\) outputs \(z_i=b_i\), else each \(\mathcal {P}_i\) outputs \(z_i=b_i-s_i\cdot L_{0,i}^S\).

• The signature is \((c, z = \sum z_i)\).

To verify the signature it suffices to check that \(H(\left[ z\right] \!E_0,m)=0\dots \), if \(c=0\), or that \(H(\left[ z\right] \!E_s,m)=1\dots \), if \(c=1\). Of course, this sketch must be repeated \(\lambda \) times, in order to ensure the appropriate level of security.

The complete signing algorithm is summarized in Algorithm 2. As presented there, it is rather inefficient in terms of signature size and signing/verification time. All the key/signature size compromises presented in CSI-FiSh [4] are compatible with our threshold adaptation, and would produce a more efficient signature scheme. The details are left to the reader.

Security Analysis. We conclude with a study of the security of the threshold signature scheme. Like the other schemes presented here, it is only secure against (static) honest-but-curious adversaries; however the security proof is more technical, and we give it in more detail. Since our threshold signature has the same public key and produces the same signatures as the Stolbunov/CSI-FiSh non-threshold scheme, we are able to use Gennaro et al.’s security model [32], with the appropriate modifications to handle a trusted dealer. In a nutshell, security in this model is proven by showing that the transcript of the threshold protocol can be simulated given only the signature, even in presence of up to \(k-1\) corrupted participants; then, security follows from the unforgeability of the non-threshold signature scheme. We start with a brief description of the model.

Communication model. We assume the n participants \(\mathcal {P}_1,\dots ,\mathcal {P}_n\) have access to a broadcast channel they use to exchange messages when executing the signature protocol. On top of that, each participant has access to a private channel with the trusted dealer \(\mathcal {T}\), that they use to receive the secret shares.

The adversary. We consider a static honest-but-curious adversary, i.e., one that chooses up to \(k-1\) players to corrupt at the beginning of the unforgeability game, and then observes all their communications, including the secret shares received from the dealer; other than that, all parties strictly follow the protocol. In the literature, this type of adversary is often also called semi-honest or passive.

The view of an adversary is the probability distribution on the transcript of all the information seen by it during the protocol execution: this includes secret shares, the message m to sign, the messages received from other parties, and the resulting signature.

Unforgeability. A threshold signature scheme is unforgeable if no polynomial-time adversary \(\mathcal {A}\) can produce a signature for a previously unsigned message m, given the view of \(\mathcal {A}\) for adaptively chosen messages \(m_1,...,m_Q\). This definition is analogous to the usual notion of UF-CMA. In other words, this means that \(\mathcal {A}\) does not learn enough information from transcripts of protocol executions to forge a valid signature.

Simulatability. Gennaro et al. proved that a threshold signature scheme is unforgeable if the underlying signature scheme is, and the threshold scheme is simulatable. This is defined as there being a polynomial time simulator \(\mathcal {S}\) that takes as input a message m, the public key \(E_s\), a valid signature on m, and the shares of the corrupted participants, and outputs transcripts that are computationally indistinguishable from the view of the adversary. Intuitively, this means that the adversary gains no more information from seeing the transcript, than from the signature alone.

The trusted dealer. Unlike the threshold scheme of Gennaro et al., our signature does not feature a distributed key generation. We thus adopt a hybrid model, where the generation of the trusted shares is modeled by an ideal functionality \(\mathcal {F}_\mathcal {T}\), that executes Shamir’s secret sharing, publishes the public key, and distributes the secret shares to each participant through the private channel.

In particular, the adversary is not able to tamper with \(\mathcal {F}_\mathcal {T}\), and the distinguisher has no knowledge of the master secret generated by it.

We will prove simulatability under a new assumption, that we call Power-DDHA. This decision version of the Scalar-HHS problem of Felderhoff [28] is a generalization of the Decision Diffie–Hellman Group Action (DDHA) introduced by Stolbunov [55], and is related to the P-DDH assumption introduced by Kiltz for discrete logarithm groups [38].

Problem 1 (Power-DDHA problem)

Let \((\mathcal {E},\mathcal {G})\) be a HHS. Let \(E\in \mathcal {E}\) and \(1<a<\#\mathcal {G}\) an integer; let \(\mathfrak {s}\) be a uniformly random element in \(\mathcal {G}\). The a-Power-DDHA problem is: given \((a,E,\mathfrak {s}*E,F)\), where \(F\in \mathcal {E}\) is an element, either sampled from the uniform distribution on \(\mathcal {E}\), or \(F=\mathfrak {s}^a*E\), decide from which distribution F is drawn.

Remark 3

The special case of \((-1)\)-Power-DDHA where the HHS is instantiated with a graph of \(\mathbb {F}_p\)-isomorphism classes of supersingular curves, and E is the special curve \(E:y^2=x^3+ x\), is known to be solvable efficiently. Other “special” curves in the graph also enjoy this property, see [12].

This obstacle is easy, but tedious, to circumvent in the proof of the next theorem. We leave the details to the reader.

Felderhoff proved that the search version of Power-DDHA (Scalar-HHS) is equivalent to Parallelization whenever the order of \(\mathcal {G}\) is known and odd [28]. We also recall the formal definition of the Vectorization problem, also known as Group Action Inverse Problem [55].

Problem 2 (GAIP)

Let \((\mathcal {E},\mathcal {G})\) be a HHS, let E, F be uniformly random elements of \(\mathcal {E}\). The Group Action Inverse Problem asks to compute \(\mathfrak {a}\in \mathcal {G}\) such that \(E=\mathfrak {a}*F\).

It is clear that GAIP is harder than Power-DDHA: given a GAIP solver one can simply apply it to \((E,\mathfrak {s}*E)\), and then use the answer to solve Power-DDHA.

Theorem 1

Under the Power-DDHA assumption, the signature scheme of Algorithm 2 is simulatable.

Stolbunov’s signature scheme is proven secure in the ROM under GAIP (see [4, 17, 56]); since GAIP is harder than Power-DDHA, we immediately get the following theorem.

Corollary 1

Under the Power-DDHA assumption, the signature scheme of Algorithm 2 is unforgeable, when the hash function H is modeled as a random oracle.

Proof of Theorem 1. Observe that the public key \(E_s= \left[ s\right] \!E_0\) uniquely determines s; but that, together with the \(k-1\) corrupted shares, uniquely determines the polynomial f in Shamir’s secret sharing, and thus all other shares. We shall denote by \(s_1,\dots ,s_n\) these uniquely determined shares, note however that the simulator only knows the corrupted ones.

Let \((c_1\cdots c_\lambda , z_1, \dots , z_\lambda )\) be a signature, and let S be the set of k signers (who signs a given message is decided by the adversary). To simulate a transcript, the simulator draws integers \(z_{i_1,j},\dots ,z_{i_{k-1},j}\in \mathbb {Z}/q\mathbb {Z}\) at random, for any \(1\le j\le \lambda \), and sets \(z_{i_k,j}=z_j-z_{i_1,j}-\cdots -z_{i_{k-1},j}\). Since \(z_j\) is uniformly distributed, it is clear all \(z_{i,j}\) also are. These values make the second part of the transcript (lines 12–15 in Algorithm 2).

To complete the transcript, the simulator now needs to output commitments \(E_j^{k_i}\) (line 8), where for each \(i\in S\) we denote by \(1\le k_i< k\) the position of i in S. We start with the case where S contains only one uncorrupted participant, which can be simulated perfectly.

If \(c_j=0\) the simulator simply sets

as in Algorithm 2. If \(c_j=1\), define the sequence

so that \(E_s = E_s^{k}\). The simulator can compute all curves \(E_s^{k_i}\) as follows: assume the uncorrupted participant \(\mathcal {P}_i\) is in position \(k_i\) in S, for any \(k'<k_i\) it computes \(E_s^{k'}\) directly:

whereas for all \(k'\ge k_i\) it computes it backwards from \(E_s\):

Then, the commitments are computed as

which is immediately seen as being the same as in Algorithm 2, thanks to \(b_{i,j}=z_{i,j}+s_i\cdot L_{0,i}^S\). An example of this computation where participants \(\mathcal {P}_1\) and \(\mathcal {P}_3\) are corrupted and participant \(\mathcal {P}_2\) is not is pictured in Fig. 1.

Recomputation of \(E_j^{k_i}\) given \(z_{i,j}\).

Full size image

Because all the choices are uniquely determined once the values \(z_{i,j}\) have been chosen, it is clear that this transcript is perfectly indistinguishable from a real one, even for a computationally unbounded distinguisher.

We are left with the case where the set S contains more than one uncorrupted participant; in this case, we will resort to random sampling. For simplicity, we will assume that sets S are always sorted in increasing order, so that the relative order of the participants’ actions does not change from one signature to another.

Like above, we start one direct chain from \(E_0\), and one backwards from \(E_s\); both chains stop when they encounter an uncorrupted participant \(\mathcal {P}_i\). Now, let \(E_s^{k_i-1}\) be the last curve in the direct chain, we set the next curve \(E_s^{k_i}=\left[ r_i\right] \!E_0\), where \(r_i\) is sampled uniformly from \(\mathbb {Z}/q\mathbb {Z}\). We also store \(r_i\) in association with S, and keep it for reuse the next time the adversary queries for the set S.

We continue the direct chain from \(E_s^{k_i}\), either using the knowledge of \(s_i\cdot L_{0,i}^S\) for corrupted participants, or sampling a random \(r_i\) for uncorrupted ones; we stop when we meet the backwards chain. An example of this process is pictured below:

we write in bold data that is obtained through random sampling; the value \(r_4\) is implicitly determined by the other four values. After we have determined this data, we compute the \(E_j^{k_i}\)’s and complete the transcript as before.

Now, this transcript is no longer indistinguishable from the real view of the adversary, however we argue that it still is computationally indistinguishable assuming Power-DDHA. Indeed, when \(c_j=1\), the distinguisher is able to recover \(E_s^{k_i}\) from \(E_j^{k_i}\) as \(E_s^{k_i}=\left[ -z_{k_1,j}-z_{k_2,j}-\cdots -z_{k_i,j}\right] \!E_j^{k_i}\). This means that the distinguisher will collect many pairs of the form \(\left( E,\left[ s_i\cdot L_{0,i}^S\right] \!E\right) \) (in queries where \(\mathcal {P}_i\) is the only uncorrupted participant in S), and many others of the form \(\left( E',\left[ r_i\right] \!E'\right) \) (where the expected relation would be \(\left( E',\left[ s_i\cdot L_{0,i}^{S'}\right] \!E'\right) \) instead). In general, it will be the case that \(E'=\left[ b\right] \!E\) for some \(b\in \mathbb {Z}/q\mathbb {Z}\) not necessarily known to the distinguisher; however, by subtracting known factors coming from corrupted players, the distinguisher can reduce to a distinguishing problem between \(\left( \left[ \sum s_i'\right] \!E_0, \left[ \sum s_i'a_i\right] \!E_0\right) \) and \(\left( \left[ \sum s_i'\right] \!E_0, \left[ r\right] \!E_0\right) \), where the \(s_i'\) are unknowns related to uncorrupted shares \(s_i\), the \(a_i\) are known (and possibly 0), and r is random. This is an instance of a problem more general than Power-DDHA, and is thus at least as hard as Power-DDHA.

Hence, assuming Power-DDHA is hard, no polynomial time algorithm can distinguish between the simulated transcript and the real interaction, thus proving that the threshold scheme is simulatable.    \(\square \)

Remark 4

It is evident from the proof that the security of the (n, n)-threshold signature scheme can be proven without assuming Power-DDHA. The appearance of this surprising assumption seems an artifact related to the limitations of the HHS framework; indeed, the analogous scheme based on discrete logarithms can be proven as hard as standard Schnorr signatures without additional assumptions [54]. We hope that further research will improve the state of security proofs for HHS threshold schemes.

Remark 5

Although our scheme is unforgeable in a (static) honest-but-curious model, it is obviously non-robust: any participant can lead to an invalid signature without being detected. Robustness can be added using generic zero-knowledge techniques, however it would be interesting to achieve it in a more efficient bespoke fashion.

Another desirable improvement would be to prove security in a stronger adaptive corruptions model, where the adversary can query the signing oracle before choosing which participants to corrupt.

We now describe an instantiation of the previous schemes based on a principal homogeneous space of supersingular elliptic curves defined over a finite field \(\mathbb {F}_p\).

It was first observed by Delfs and Galbraith [20] that the set of all supersingular curves defined over a prime field \(\mathbb {F}_p\) partitions into one or two levels, each level being a principal homogeneous space for the class group of an order of the quadratic imaginary field \(\mathbb {Q}(\sqrt{-p})\), in a way analogous to the well known theory of complex multiplication.

These principal homogeneous spaces were first used for a cryptographic purpose in the key-exchange scheme CSIDH [11], however only the precomputation performed recently by Beullens et al. for the signature scheme CSI-FiSh [4] permits to turn one of these into a true HHS.

We now briefly recall some key facts on CSIDH and CSI-FiSh, before turning to the instantiation of our threshold schemes. More details on the mathematical background of isogeny-based cryptography can be found in [16].

4.1 Supersingular Complex Multiplication

From now on we let p be a prime, \(\mathbb {F}_p\) the field with p elements, and \(\bar{\mathbb {F}}_p\) an algebraic closure. An elliptic curve E defined over \(\mathbb {F}_p\) is said to be supersingular if and only if \(\#E(\mathbb {F}_p)=p+1\). It is well known that there are approximately p/12 isomorphism classes of supersingular curves, all defined over \(\mathbb {F}_{p^2}\); of these, \(O(\sqrt{p})\) are defined over \(\mathbb {F}_p\).

Let E be a supersingular curve defined over \(\mathbb {F}_p\), an endomorphism is an isogeny from E to itself, and it is said to be defined over \(\mathbb {F}_p\) (or \(\mathbb {F}_p\)-rational) if it commutes with the Frobenius endomorphism \(\pi \). The \(\mathbb {F}_p\)-rational endomorphisms of E form a ring, denoted by \(\text {End}_{\mathbb {F}_p}(E)\), isomorphic to an order^{Footnote 3} of \(\mathbb {Q}(\sqrt{-p})\); more precisely, it is isomorphic to either \(\mathbb {Z}[\sqrt{-p}]\) or \(\mathbb {Z}[(\sqrt{-p}+1)/2]\). Let \(\mathcal {O}\) be such an order, the class group \(\mathrm {cl}(\mathcal {O})\) is the quotient of the group of invertible ideals of \(\mathcal {O}\) by the group of its principal ideals; it is a finite abelian group.

The set of all supersingular curves with \(\text {End}_{\mathbb {F}_p}(E)\) isomorphic to a given order \(\mathcal {O}\subset \mathbb {Q}(\sqrt{-p})\) is called the horizontal isogeny class associated to \(\mathcal {O}\). A straightforward extension to the theory of complex multiplication states that the horizontal isogeny class of \(\mathcal {O}\), up to \(\mathbb {F}_p\)-isomorphism, is a principal homogeneous space for \(\mathrm {cl}(\mathcal {O})\). To make this into a HHS, an efficient (e.g., polynomial in \(\log (p)\)) algorithm to evaluate the action of \(\mathrm {cl}(\mathcal {O})\) is needed. This is where isogenies play an important role. Fix an isomorphism \(\text {End}_{\mathbb {F}_p}(E)\simeq \mathcal {O}\), for any invertible ideal \(\mathfrak {a}\), the action \(\mathfrak {a}*E\) can be computed as follows: first define the \(\mathfrak {a}\)-torsion subgroup of E as

this is a finite subgroup of E, and it is stabilized by the Frobenius endomorphism \(\pi \); then the unique isogeny \(\phi :E\rightarrow E/\langle E[\mathfrak {a}]\rangle \) with kernel \(E[\mathfrak {a}]\) is such that \(\mathfrak {a}*E=E/\langle E[\mathfrak {a}]\rangle \). It follows that, if \(\mathfrak {a}\) and \(\mathfrak {b}\) are two ideals in the same class, i.e., such that \(\mathfrak {a}=(\alpha )\cdot \mathfrak {b}\) for some element \(\alpha \in \mathcal {O}\), then \(E/\langle E[\mathfrak {a}]\rangle \simeq E/\langle E[\mathfrak {b}]\rangle \).

The curve \(E/\langle E[\mathfrak {a}]\rangle \) can be efficiently computed using an isogeny evaluation algorithm [27, 57], however the complexity of this operation is polynomial in the degree of the isogeny, or, equivalently, in the norm \(N(\mathfrak {a})=\#(\mathcal {O}/\mathfrak {a})\). This implies that the action of an element \(\mathfrak {a}\in \mathrm {cl}(\mathcal {O})\) can only be efficiently computed when a representative of small norm of \(\mathfrak {a}\) is known, or, more generally, when a decomposition

with all \(\mathfrak {l}_i\) of small norm is known.

Now, for any prime \(\ell \), the ideal \((\ell )\subset \mathcal {O}\) is either prime, or it splits into a product of two (possibly equal) conjugate prime ideals \(\mathfrak {l}\bar{\mathfrak {l}}=(\ell )\) of norm \(\ell \). In the former case, there are no invertible ideals of norm \(\ell \) in \(\mathcal {O}\); in the latter, \(\mathfrak {l}\) and \(\bar{\mathfrak {l}}\) are the only ideals of norm \(\ell \), and they are the inverse of one another in \(\mathrm {cl}(\mathcal {O})\). Asymptotically, about \(50\%\) of the primes \(\ell \) split, thus we may hope to form a basis of generators of \(\mathrm {cl}(\mathcal {O})\) of norms bounded by \(\text {polylog}(p)\), such that any element of \(\mathrm {cl}(\mathcal {O})\) can be represented as a product of \(\text {polylog}(p)\) elements of the basis.^{Footnote 4}

This representation for the elements of \(\mathrm {cl}(\mathcal {O})\) using a smooth basis is at the heart of the Couveignes–Rostovtsev–Stolbunov key exchange scheme, and of CSIDH. However, having a smooth basis may not be enough: to have a HHS, one still needs to be able to rewrite any element of \(\mathrm {cl}(\mathcal {O})\) as a compact product of smooth elements. This is the key difference between CSIDH and CSI-FiSh, as we shall see next.

4.2 CSIDH and CSI-FiSh

CSIDH was designed to make evaluating the group action of \(\mathrm {cl}(\mathcal {O})\) as efficient as possible. To this end, a prime p of the form

is selected, where \(\ell _1,\dots ,\ell _{n-1}\) are the first \(n-1\) odd primes, and \(\ell _n\) is chosen so to make p prime. This choice guarantees several desirable properties:

• The curve \(E\;:\;y^2=x^3+x\) has \(\mathbb {F}_p\)-rational endomorphism ring isomorphic to \(\mathbb {Z}[\pi ]\), where \(\pi =\sqrt{-p}\) is the image of the Frobenius endomorphism of E;

• All curves in the horizontal isogeny class of \(\mathbb {Z}[\pi ]\) can be written in the form \(y^2=x^3+Ax^2+x\), and the coefficient A uniquely characterizes the \(\mathbb {F}_p\)-isomorphism class;

• All \(\ell _i\) split in \(\mathbb {Z}[\pi ]\) as \((\ell _i)=\mathfrak {l}_i\bar{\mathfrak {l}}_i=\langle \ell _i,\pi -1\rangle \cdot \langle \ell _i,\pi +1\rangle \);

• For any curve E, the \(\mathfrak {l}_i\)-torsion subgroup is easily found as \(E[\mathfrak {l}_i]=E[\ell _i]\cap E(\mathbb {F}_p)\).

The first two properties ensure that supersingular isomorphism classes are easy to construct and represent uniquely, the third guarantees^{Footnote 5} that a number exponential in n of ideal classes of \(\mathbb {Z}[\pi ]\) can be efficiently represented and its action evaluated, the fourth enables some important optimizations for computing isogenies of degree \(\ell _i\).

In CSIDH and optimized variants [13, 41, 42, 44], all ideal classes are implicitly represented as products

with the exponents \(e_i\) in some box \([-B_i,B_i]\) (negative exponents are interpreted as powers of \(\bar{\mathfrak {l}}_i\)). Explicitly, the representation of an ideal class \(\mathfrak {a}\) is simply the vector of exponents \((e_1,\dots ,e_n)\). The action of such ideals can be evaluated in time \(\text {poly}(B_i,e_i,n)\) using isogeny formulas.

In practice, a single parameter set has been fully specified for CSIDH, corresponding to the NIST post-quantum level 1.^{Footnote 6} The set has \(n=74\), \(\ell _{73}=373\), and \(\ell _{74}=587\), yielding a prime p of approximately 512 bits; we shall refer to it as CSIDH-512. Protocols based on CSIDH-512 usually sample exponents in a box \([-5,5]\), which heuristically covers almost all the class group, and which permits to evaluate one class group action in under 30 ms [42].

However, based on this data only, CSIDH is not a HHS. Indeed, all axioms of an HHS are satisfied but two: it is not possible to efficiently evaluate the action of any element of \(\mathrm {cl}(\mathbb {Z}[\pi ])\), and it is not always possible to test equality of two elements of \(\mathrm {cl}(\mathbb {Z}[\pi ])\). Take for example the exponent vector \((2^{128},0,\dots ,0)\), corresponding to the ideal \(\mathfrak {a}=\langle 3,\pi -1\rangle ^{2^{128}}\); this is a valid element of \(\mathrm {cl}(\mathbb {Z}[\pi ])\), however without further knowledge its action can only be evaluated through \(2^{128}\) isogeny evaluations. Hopefully, \(\mathfrak {a}\) has an equivalent representation on the basis \(\mathfrak {l}_1,\dots ,\mathfrak {l}_n\) with much smaller exponents, however we have no way to compute it and, even if we were given it, we could not test their equality.

These problems go away once we have computed the group structure of \(\mathrm {cl}(\mathbb {Z}[\pi ])\). More precisely, we need to know the relation lattice of \(\mathfrak {l}_1,\dots ,\mathfrak {l}_n\), i.e., the lattice

which yields a representation of the class group as \(\mathrm {cl}(\mathbb {Z}[\pi ])\simeq \mathbb {Z}^n/\varLambda \). Now, equality of two exponent vectors \(\mathbf {e},\mathbf {f}\) can be tested by checking that \(\mathbf {e}-\mathbf {f}\in \varLambda \), and any exponent vector \(\mathbf {e}\) can be evaluated efficiently by finding an (approximate) closest vector \(\mathbf {f}\in \varLambda \) and evaluating \(\mathbf {e}-\mathbf {f}\) instead.

Neither computing the relation lattice, nor computing a good reduced basis for it are easy tasks: the former requires subexponential time in \(\log (p)\), and the latter exponential time in n.^{Footnote 7} Nevertheless, the computation for the CSIDH-512 parameter set happens to be just within reach of contemporary computers, as proven by Beullens et al. [4]: they managed to compute the structure of the class group, which happens to be cyclic of order

and a BKZ-reduced basis for the relation lattice. In particular, they found out that the ideal \(\mathfrak {l}_1=\langle 3,\pi -1\rangle \) generates \(\mathrm {cl}(\mathbb {Z}[\pi ])\).

Thanks to CSI-FiSh, we thus dispose of a HHS with quantum security estimated at the NIST-1 security level, although scaling to higher security levels currently looks problematic.

4.3 Instantiation of the Threshold Schemes

Given the CSI-FiSh data, we can now instantiate our threshold schemes. However, it is evident by Eq. (2) that the full group \(\langle \mathfrak {l}_1\rangle =\mathrm {cl}(\mathbb {Z}[\pi ])\) is not suitable for them, because the smallest prime factor of its order is 3, thus limiting the schemes to just 2 participants. We may instead choose as generator \(\mathfrak {l}_1^3\), which limits the schemes to 36 participants, or \(\mathfrak {l}_1^{111}\), allowing more than a million participants.^{Footnote 8}

Efficiency. The performance of our schemes can be readily inferred from that of the CSI-FiSh signature scheme.

To evaluate the action of an ideal in \(\mathrm {cl}(\mathbb {Z}[\pi ])\), CSI-FiSh first solves an approximate closest vector problem using Babai’s nearest plane algorithm [1], and an algorithm by Doulgerakis, Laarhoven and de Weger [25]; then uses the isogeny evaluation algorithm of CSIDH. The average cost for one evaluation is reported to be \(135.3\cdot 10^6\) cycles (40–50 ms on a commercial CPU), which is only 15% slower than the original CSIDH evaluation.^{Footnote 9}

In the encryption scheme, each participant computes exactly one class group action. Since the participants must do their computations sequentially, the total time for decryption is multiplied by the number of participants; the time for encryption, on the other hand, is unaffected by the number of participants, indeed the threshold nature of the protocol is transparent to the user.

In the signature scheme, using the optimization described in [4], depending on the choice of parameters each participant computes between 6 and 56 group actions. Since the group action largely dominates the cost of the whole signing algorithm, we can expect to complete a (k, n)-threshold signature in approximately \(k\cdot t \cdot 135.3\cdot 10^6\) cycles, where \(6\le t\le 56\). However, the t group actions by a each participant are independent and can be computed in parallel; since the round-robin evaluation in the threshold scheme leaves plenty of idle cycles for participants while they wait for other participants’ results, by carefully staggering the threshold group evaluations the k participants can evaluate the t group actions with the same efficiency as the non-threshold scheme, as long as \(k\le t\). According to [4, Tables 3, 4], this would provide, for example, quantum-resistant threshold signatures for up to 16 participants in under 1 s, with public keys of 4 KB and signature size of only 560 B. Another example are 1880 B signatures with public key size of 128 B and k up to 56 in under 3 s; other interesting compromises are possible. These numbers compare favorably to other post-quantum threshold signatures that are expected to run in seconds [15], and may be especially interesting for side-channel protected implementations of CSI-FiSh.

Attacks. The security of the threshold schemes is essentially the same as that of the original single-participant signature and encryption schemes.

The fact that secrets are sampled in a subgroup of \(\mathrm {cl}(\mathbb {Z}[\pi ])\) of index 3 or 111 has a minor impact on security, as cryptanalyses can exploit this information to speed-up their searches.

In the classical setting, the best algorithm for both Vectorization and Parallelization is a random-walk approach [20] that finds a path between two supersingular curves in \(O(\sqrt{\#\mathrm {cl}(\mathbb {Z}[\pi ])})=O(\root 4 \of {p})\). If, like in our case, we restrict to a vertex set that is x times smaller, the random walk algorithm will find a collision approximately \(\sqrt{x}\) times faster. Hence, we expect a loss in classical security of less than 4 bits.^{Footnote 10}

Note that this gain is optimal: if an algorithm could solve the Vectorization problem in a subgroup of size N/x more than \(O(\sqrt{x})\) times faster, then by a divide and conquer approach the Vectorization problem in the full group of size N could be solved in less than \(O(\sqrt{N})\) operations.

A similar gain can also be obtained in the best quantum algorithm for solving the Vectorization problem [39, 40, 48]. However, since its complexity is sub-exponential, the final gain is even less than 4 bits. The exact quantum security of CSIDH and CSI-FiSh is currently debated [3, 6, 8, 11, 47], nevertheless whatever the final consensus turns out to be, the quantum security of our threshold schemes will be extremely close to it.

We introduced threshold variants of encryption and signature schemes based on Hard Homogeneous Spaces, and efficient quantum-safe instantiations thereof based on isogeny graphs of supersingular curves (CSIDH).

Our schemes are similar to well known Diffie–Hellman-style threshold schemes, however they are sharply different in the communication structure: whereas classical schemes have participants output messages in parallel with little coordination, our schemes impose a strictly sequential round-robin message passing style. Apparently, this limitation trickles down, negatively affecting many aspects: security properties, security proofs, efficiency.

In our ElGamal-style decryption algorithm, only one participant learns the cleartext, and we are only able to prove security in a honest-but-curious setting. While the communication structure is slightly less problematic for the signature scheme, its security too can only be proven in a honest-but-curious setting with static corruptions. Interesting questions for future research are efficient protocols where all participants learn the cleartext, or with stronger security properties, such as the ability to detect malicious participants.

Another topic we did not address in this work are verifiable distributed key generation algorithms, which would allow to run the threshold schemes without resorting to a trusted dealer. As observed by Benaloh [2], Shamir’s secret sharing is \((+,+)\)-homomorphic: given two secrets s and \(s'\) with respective shares \(s_i\) and \(s_i'\), the sums of shares \(s_i+s_i'\) form valid shares of \(s+s'\). Based on this observation, Pedersen [45] constructed a DKG scheme without a trusted dealer, by having each party set up its own (k, n)-Shamir secret sharing scheme, and then combining these schemes using the homomorphic property.

While the same homomorphic property also applies to HHS threshold schemes, it seems difficult to achieve verifiability of the DKG like in [30, 33, 46]. An interesting research question is the construction of a verifiable DKG in the general HHS framework, or for specific isogeny-based instantiations.

Finally, the instantiation of our schemes is limited by the feasibility of parameter generation: to the present date the only available parameter set is the CSIDH-512 HHS, as computed by Beullens et al., with security currently estimated at the NIST-1 level. Higher security levels would require extremely intensive computations that are currently out of reach.