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Efficient distributed key generation for threshold signatures
Mahnush Movahedi
# Threshold signatures
Threshold signature schemes are schemes where n parties and at least t are necessary and sufficient for signing. They want to make sure that t parties are necessary. Any threshold signature scheme typically has 3 protocols: key generation, signing, and verifying.
In key generation, one party creates a group public key as an output. Also, for each node that participates in the protocol, the protocol should output a secret key. In a signing phase, each party should sign a message with its own secret key. In a verification phase, the verifier should be able to gather all the signatures from all the parties. If they don't send the signature, it can be an empty signature. Using the group public key, he should be able to verify if the signature on the message is correct.
# Non-interactive protocols
I want to talk about non-interactiveness is important. This talk is about distributed key generation which is non-interactive. Non-interactive means we are only going to have one round in the protocol. It's important because generally you only create keys once, and then everyone goes home and everyone might sign multiple times. Also, not everyone might be online at the same time. A non-interactive scheme is important in this scenario so that not everyone needs to be online and synchronized at the same time.
Another interesting point for a non-interactive scheme is that most of the non-interactive protocols can be publicly verifiable which is another good property. Later, if the nodes are not available anymore to do the verification then anybody in the network can do the verification instead.
So that's the motivation for why we are looking into non-interactive key generation.
# Why threshold signature schemes?
Threshold signature schemes have many applications.
The first application was even before cryptographers thinking about cryptography. The first application was for democracy. They decided that instead of having only one person sign a bill, they would choose a group and if a threshold of this group or a fraction of them signed a bill then we would assume the bill is passed. Later, the threshold signature scheme was used for voting-based consensus protocols. Instead of everybody trusting only one person to help them reach agreement, they work together and so they sign the block or the value they want together and if a fraction of them or a threshold of them agree on a block then they assume it's accepted and decided.
Another nice application of threshold signature scheme is creating random numbers. If this threshold signature scheme have this property, then it can be used as a random beacon. It requires a signature scheme with a uniqueness property. The signature is unique and uniformly chosen from the group. It's a good random value that is unpredictable. You can sign this random value again and it's unpredictable again. Random beacons are really useful tools in distributed computing in general.
# Trivial threshold signature scheme
Let's try to create a trivial threshold signature scheme. The first step is to do distributed key generation. Before doing that, let's see how the keys are really used. Very trivial threshold signature scheme is that each party creates a public and secret key, then use a well-defined signature scheme to sign independently. For the verification, if some of them have a valid signature on the message then you consider it a valid signature as a group on the signature. This is correct, but it's not an efficient scheme.
It's inefficient because group public key size is O(n), and signature size is O(t) and verifier should verify O(t) individual signatures. Can we combine the signatures to make them shorter and to make the verifier verify less data and to reduce messages over the network?
# Secret sharing
One way to do the distributed key sharing is to use secret sharing. Keys generally are secrets. Assume that in a secret sharing scheme, there's one secret we want to share among n parties such that t of them are necessary and sufficient to reconstruct a secret. Here, it's very important that less than t people can gather together but they can't learn any information about the secret. So it has a complete secrecy property.
The first secret sharing scheme was introduced by Shamir and it's called Shamir's secret sharing scheme. It's based on polynomials. If you have a polynomial of degree t - 1 then you can assume f(0) of the polynomial is your secret. You can choose the coefficients of the polynomial randomly, and create a polynomial of the .. over .... t points are necessary and sufficient to reconstruct the polynomial and the secret. Up to t points reveals no information about the secret so it has perfect secrecy.
(t,n)-secret sharing scheme is based on Shamir's secret sharing scheme. Say we choose points on a random polynomial. We send one point to each party. Each party has one point on the polynomial. Later in the reconstruction phase, some of them come together and reconstruct the polynomial and the secret. A signature in this scheme has secrecy because less than t shares carry no information.
# From secret sharing to key generation
Threshold signatures requires key generation. Use secret sharing to create a master public key and a set of partial signing keys. Assume there's a public key and an associated secret key for the group. Then we can share the secret key for the group. Say we have an oracle and this oracle is trusted, then he can create a public key and an associated secret key for the group and he can share the group secret key to the group. This sharing happens once. And then later people can come together and sign at different times.
But oracles are not real. We cannot trust a single party to do this sharing phase for us. We wanted to avoid that. We wanted a decentralized system. We want to go from distributed key generation step by step.
# Securing distributed key generation step by step
Since one dealer is not trusted, we need multiple dealers. We need a verifiable secret sharing scheme. Then we can assume that the DKG is multiple instances of verifiable secret sharing. We need verifiable secret sharing to prevent adversarial dealers.
All the nodes agree on the commitment on the polynomial. We can't put shares on the chain. We have to encrypt the shares and put them on the chain. So now we have a polynomial, we have points on the polynomial, we encrypt the points with the secret keys of the parties, we commit to the polynomial, and we put everything on the blockchain. Still this isn't enough because the encryption and the commitment may be completely on different polynomials. A dishonest dealer might commit to something but encrypt something else. So we have to have a proof that if I have an encrypted share, then this share opens to a correct point on the committed polynomial. So we use a zero-knowledge proof for that.
The dealer creates a polynomial. He commits to the coefficients of the polynomial. He has to encrypt the share. The way he decides to encrypt the shares, because later we want to use a zero-knowledge proof on the shares, we chunk the shares and use exponential ElGamal encryption of each chunk. A zero-knowledge proof can prove that if you can decypher the ciphertext then the encryption is going to be a point on the committed polynomial.
# Distributed key generation again
Up to this point, I only talked about verifiable secret sharing. But how can we go from verifiable secret sharing to distributed key generation? The next step is how to have multiple dealers. To have multiple dealers, we can repeat the same procedure for each dealer. Now they receive multiple shares, and then we can add the shares to each other. Since Shamir secret sharing has homomorphic property, then the addition works here for us.
We have polynomials. If you have a polynomial and add it to another polynomial the result is a polynomial of the same degree. It's very simple. But now if I have a random polynomial and add it to a non-random polynomial, the result is a random polynomial. Why is this true? Nobody can guess my first random polynomial so nobody can guess what is the result of the addition.
Assume I have two dealers. The first dealer chooses a secret and he is honest. A second dealer has another secret and he is dishonest. So now I have public key and s1 for the honest people. And I have public key 2 and secret 2 for the dishonest player. The secret key of each party is a corresponding point-- the addition of the corresponding points of the polynomials. This way, I can have multiple dealers. If I have enough dealers, I can make sure that at least one of them is honest and created the correct random polynomial and this way I can make sure the public key and secret key is going to be random. The small secret keys are also random.
# Rushing adversary attack
So far, we don't know. There's an attack on this protocol usually called a rushing adversary attack where the adversary waits until it sees all the public keys before choosing his own. The resulting group public key is then not random.
# DKG under the presence of rushing adversary
These kinds of attacks happen on other DKGs before. Gennaro et al proved that a similar scheme which is a discrete log based DKG protocol cannot produce a uniform public key. They show that this attack holds. They also have a solution for that problem. They solved that problem and created a new DKG protocol that is producing correct random output. The problem is that the new scheme is not non-interactive, it has an additional round. The original protocol had 3 rounds and they were only requiring one additional round. But for us we're looking for a non-interactive protocol.
There's another interesting observation as well. They also observed that some of the discrete log crypto systems remain secure despite a non-uniform public keys. So similarly, we investigated BLS threshold siganture scheme and it seems to remain secure when using our non-interactive distributed key generation protocol which creates a non-random public key in the presence of a random oracle model and CDH oracles.
# Rushing attack, not a problem
Here's the intuition behind the proof. It's similar to Gennaro et al.'s proof. Threshold signature reduces to d signature scheme,s where d is the number of dealers. To break the scheme, adversary has to break d individual signature schemes. But, at least one of the dealers is honest.
Our BLS threshold signature scheme can be reduced to multiple instances of a single signature scheme (not a threshold signature scheme).
# Performance
The cost per dealer was analyzed for bulletproofs, bulletproofs with optimization, and SNARKs. We looked at bandwidth, prover computation, and verifier computation.
The bandwidth for bulletproofs is 174 kilobytes. Bulletproofs with optimization is 27 kilobytes, but the prover and verifier time is going to be much higher. Verifier computation is 13 minutes in that case, compared to 11.5 seconds with 174 kilobyte bulletproofs. With a SNARK, it takes 32 kilobytes and prover computation takes up to an hour while verification takes 6 milliseconds.
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