func Sign(msg []byte, key dh.Element) []byte {
h := hmac.New(sha1.New, []byte(key.String()))
return h.Sum(msg)
}
func main() {
p, _ := new(big.Int).SetString("233970423115425145524320034830162017933", 10)
groupOrder, _ := new(big.Int).SetString("233970423115425145498902418297807005944", 10)
a := big.NewInt(-95051)
b := big.NewInt(11279326)
EC := dh.NewEllipticCurve(a, b, p)
gx := big.NewInt(182)
gy, _ := new(big.Int).SetString("85518893674295321206118380980485522083", 10)
g := dh.NewEllipticCurveElement(EC, gx, gy)
MC := dh.NewMontgomeryCurve(big.NewInt(534), big.NewInt(1), p)
mg := dh.NewMontgomeryCurveElement(MC, big.NewInt(4))
q, _ := new(big.Int).SetString("29246302889428143187362802287225875743", 10)
GG := dh.NewGeneratedGroup(MC, mg, *q)
twistOrder := new(big.Int).Mul(p, big.NewInt(2))
twistOrder.Add(twistOrder, big.NewInt(2))
twistOrder.Sub(twistOrder, groupOrder)
factors := dh.FindFactors(twistOrder, twistOrder, q, MC)
total := big.NewInt(1)
for factor, _ := range factors {
total.Mul(total, big.NewInt(factor))
}
// Find an element that is order=total. We will use this to guess which
// of the one or two remainders for each small prime is correct.
log.Printf("Finding an element of order %d...", total)
pow := new(big.Int)
pow.Div(twistOrder, total)
var h dh.Element = nil
for {
h = MC.Pow(MC.Random(), pow)
if h.Cmp(MC.Identity()) != 0 {
bad := false
for factor, _ := range factors {
if MC.Pow(h, big.NewInt(factor)).Cmp(MC.Identity()) == 0 {
bad = true
break
}
}
if !bad {
log.Printf("%s^%d == %s", h, total, MC.Pow(h, total))
break
}
}
}
d := dh.NewDiffieHellman(GG)
mac := Alice(d, h)
// Determine the possible remainders for each small prime. There are usually
// two because (u, v) and (u, -v) are conflated by the ladder.
type factorModuli struct {
factor int64
possibleModuli []int64
}
smallFactorModuli := make([]*factorModuli, 0)
for factor, h := range factors {
fm := &factorModuli{factor, make([]int64, 0)}
mac := Alice(d, h)
log.Printf("Guessing the shared secret in the subgroup of order %d", factor)
found := false
for i := int64(0); i < factor; i++ {
k := MC.Pow(h, big.NewInt(i))
if hmac.Equal(mac, Sign(secretMessage, k)) {
found = true
fm.possibleModuli = append(fm.possibleModuli, i)
if i != 0 {
// If h^i = (u, v) then (u, -v) = (h^i)^-1 = h^-i
fm.possibleModuli = append(fm.possibleModuli, factor-i)
}
break
}
}
if !found {
log.Fatal("Could not guess the shared secret")
}
smallFactorModuli = append(smallFactorModuli, fm)
}
// Figure out which one of the various choices for each small remainder is correct.
// For each permutation:
// - use CRT to compute remainder x mod "total"
// - check if the saved MAC corresponds to h^x.
indices := make([]int, len(smallFactorModuli))
bigModuli := make([]*big.Int, 0)
for {
carry := true
for i := 0; carry && i < len(indices); i++ {
indices[i] = (indices[i] + 1) % len(smallFactorModuli[i].possibleModuli)
carry = indices[i] == 0
}
moduli := make(map[int64]int64)
for i, fm := range smallFactorModuli {
moduli[fm.factor] = fm.possibleModuli[indices[i]]
}
// From Wikipedia CRT page
x := new(big.Int)
for n, a := range moduli {
N := new(big.Int).Set(total)
N.Div(N, big.NewInt(n))
N.ModInverse(N, big.NewInt(n))
N.Mul(N, total)
N.Div(N, big.NewInt(n))
N.Mul(N, big.NewInt(a))
x.Add(x, N)
x.Mod(x, total)
}
k := MC.Pow(h, x)
if hmac.Equal(mac, Sign(secretMessage, k)) {
log.Printf("x mod r may be %s [r = %s]", x, total)
bigModuli = append(bigModuli, x)
if x.Cmp(new(big.Int)) != 0 {
// Again there are probably two answers, because
// h^x = h^-x using the montgomery ladder.
y := new(big.Int).Sub(total, x)
log.Printf("x mod r may be %s [r = %s]", y, total)
bigModuli = append(bigModuli, y)
}
break
}
}
// We now have two possible remainders mod "total". For each of those,
// use the kangaroo algorithm to try to guess the secret. Note that
// the public key that Alice gives us will *also* correspond to two
// different points (u, v) and (u, -v) so we need to try plugging both
// of them in and see which one works.
ch := make(chan bool)
for _, x := range bigModuli {
for _, negate := range []bool{true, false} {
go func(x *big.Int, negate bool) {
n := new(big.Int)
n.Sub(q, x)
// Convert to Weierstrass form so Op is implemented and so
// we can specify which of the two collapsed points (u, v)
// and (u, -v) to use for this trial.
w1 := dh.Q60MontgomeryToWeierstrass(d.PublicKey(), EC)
if negate {
dh.NegateWeierstrass(w1, EC)
}
w2 := EC.Pow(g, n)
hp := EC.Op(w1, w2)
gp := EC.Pow(g, total)
B := new(big.Int)
B.Div(q, total)
log.Printf("Searching for index of %s in 0..%d", hp, B.Int64())
m := dh.Pollard(EC, gp, hp, 0, int(B.Int64()))
if m == nil {
log.Printf("The wild kangaroo escaped!")
return
} else {
m.Mod(m, q)
log.Printf("index = %s", m)
check := EC.Pow(gp, m)
log.Printf("%s^%d mod %s = %s", gp, m, p, check)
if check.Cmp(hp) != 0 {
log.Printf("ERROR! Expected %s but got %s", hp, check)
}
}
m.Mul(m, total)
x.Add(x, m)
log.Printf("Predicted key: %d", x)
log.Printf("%s", d)
ch <- true
}(x, negate)
}
}
<-ch
}