// Shard produces a pseudorandom sharding of the network entity list
// based on a seed and a number of requested shards.
func (rh *RandHound) Shard(seed []byte, shards int) ([][]*onet.TreeNode, [][]abstract.Point, error) {
if shards == 0 || rh.nodes < shards {
return nil, nil, fmt.Errorf("Number of requested shards not supported")
}
// Compute a random permutation of [1,n-1]
prng := rh.Suite().Cipher(seed)
m := make([]int, rh.nodes-1)
for i := range m {
j := int(random.Uint64(prng) % uint64(i+1))
m[i] = m[j]
m[j] = i + 1
}
// Create sharding of the current roster according to the above permutation
el := rh.List()
sharding := make([][]*onet.TreeNode, shards)
keys := make([][]abstract.Point, shards)
for i, j := range m {
sharding[i%shards] = append(sharding[i%shards], el[j])
keys[i%shards] = append(keys[i%shards], el[j].ServerIdentity.Public)
}
return sharding, keys, nil
}
// Shard produces a pseudorandom sharding of the network entity list
// based on a seed and a number of requested shards.
func (rh *RandHound) Shard(seed []byte, shards uint32) ([][]*network.ServerIdentity, error) {
if shards == 0 || rh.Group.N < shards {
return nil, fmt.Errorf("Number of requested shards not supported")
}
// Compute a permutation of [0,n-1]
prng := rh.Suite().Cipher(seed)
m := make([]uint32, rh.Group.N)
for i := range m {
j := int(random.Uint64(prng) % uint64(i+1))
m[i] = m[j]
m[j] = uint32(i)
}
// Create sharding of the current Roster according to the above permutation
el := rh.Roster().List
n := int(rh.Group.N / shards)
sharding := [][]*network.ServerIdentity{}
shard := []*network.ServerIdentity{}
for i, j := range m {
shard = append(shard, el[j])
if (i%n == n-1) || (i == len(m)-1) {
sharding = append(sharding, shard)
shard = make([]*network.ServerIdentity, 0)
}
}
return sharding, nil
}
// permute the given y's and Y's
func permute(y []abstract.Secret, Y []abstract.Point) ([]abstract.Secret, []abstract.Point) {
amount := len(Y)
rand.Seed(int64(random.Uint64(random.Stream)))
permutation := rand.Perm(amount)
newy := make([]abstract.Secret, amount)
newY := make([]abstract.Point, amount)
for i := range Y {
newy[i] = y[permutation[i]]
newY[i] = Y[permutation[i]]
}
return newy, newY
}
// Randomly shuffle and re-randomize a set of ElGamal pairs,
// producing a correctness proof in the process.
// Returns (Xbar,Ybar), the shuffled and randomized pairs.
// If g or h is nil, the standard base point is used.
func Shuffle(group abstract.Group, g, h abstract.Point, X, Y []abstract.Point,
rand cipher.Stream) (XX, YY []abstract.Point, P proof.Prover) {
k := len(X)
if k != len(Y) {
panic("X,Y vectors have inconsistent length")
}
ps := PairShuffle{}
ps.Init(group, k)
// Pick a random permutation
pi := make([]int, k)
for i := 0; i < k; i++ { // Initialize a trivial permutation
pi[i] = i
}
for i := k - 1; i > 0; i-- { // Shuffle by random swaps
j := int(random.Uint64(rand) % uint64(i+1))
if j != i {
t := pi[j]
pi[j] = pi[i]
pi[i] = t
}
}
// Pick a fresh ElGamal blinding factor for each pair
beta := make([]abstract.Scalar, k)
for i := 0; i < k; i++ {
beta[i] = ps.grp.Scalar().Pick(rand)
}
// Create the output pair vectors
Xbar := make([]abstract.Point, k)
Ybar := make([]abstract.Point, k)
for i := 0; i < k; i++ {
Xbar[i] = ps.grp.Point().Mul(g, beta[pi[i]])
Xbar[i].Add(Xbar[i], X[pi[i]])
Ybar[i] = ps.grp.Point().Mul(h, beta[pi[i]])
Ybar[i].Add(Ybar[i], Y[pi[i]])
}
prover := func(ctx proof.ProverContext) error {
return ps.Prove(pi, g, h, beta, X, Y, rand, ctx)
}
return Xbar, Ybar, prover
}
func (rh *RandHound) chooseTrustees(Rc, Rs []byte) (map[uint32]uint32, []abstract.Point) {
// Seed PRNG for selection of trustees
var seed []byte
seed = append(seed, Rc...)
seed = append(seed, Rs...)
prng := rh.Suite().Cipher(seed)
// Choose trustees uniquely
shareIdx := make(map[uint32]uint32)
trustees := make([]abstract.Point, rh.Group.K)
tns := rh.List()
j := uint32(0)
for uint32(len(shareIdx)) < rh.Group.K {
i := uint32(random.Uint64(prng) % uint64(len(tns)))
// Add trustee only if not done so before; choosing yourself as an trustee is fine; ignore leader at index 0
if _, ok := shareIdx[i]; !ok && !tns[i].IsRoot() {
shareIdx[i] = j // j is the share index
trustees[j] = tns[i].ServerIdentity.Public
j++
}
}
return shareIdx, trustees
}
