def compute_denominators(k, q):
"""Computes denominators for Lagrange basis polynomials.
Uses distinct points 1, ...,k
Arguments:
k -- number of basis polynomials
q -- the order of the group
"""
denominators = []
temp = Bn(1)
for i in range(1, k + 1):
if i == 1:
for j in range(2, k + 1):
elem = i - j
temp = temp.mod_mul(elem, q)
elif i == k:
elem = 1 - k
temp = temp.mod_mul(elem, q)
else:
inverse = Bn(i - 1 - k)
inverse = inverse.mod_inverse(q)
elem = i - 1
temp = temp.mod_mul(elem, q)
temp = temp.mod_mul(inverse, q)
denominators.append(temp)
return denominators
def generate_pis(chi, n, q):
"""Computes vector of elements P_i(chi) for i = 0, ..., n.
P_0 is special, defined as ln_plus1(chi) - 1, where ln_plus1 is the
(n+1)th Lagrange basis polynomial.
The rest P_i are defined as 2*ln_i(ch) + ln_plus1(chi), where ln_i is the
ith Lagrange basis polynomial.
It returns a list with the values of the n+1 polynomials P_i(chi).
Uses Lagrange basis polynomials with distinct points 1, ..., n + 1.
Arguments:
chi -- point of evaluation, must be a Bn
n -- the number of elements, a Python integer
q -- the order of the group, must be a Bn
"""
if chi <= n + 1:
raise ValueError("chi should be greater than n + 1, chi=%s n+1=%s" %
(chi, n + 1))
pis = []
prod = Bn(1)
# prod = (x - w_1) (x - w_2) ... (x - w_{n+1})
for j in range(1, n + 2):
prod = prod.mod_mul(chi - j, q)
# denoms[0] = 1 / (w_1 - w_2) (w_1 - w_3) ... (w_1 - w_{n + 1})
# denoms[1] = 1 / (w_2 - w_1) (w_2 - w_3) ... (w_2 - w_{n + 1})
# denoms[n] = 1 / (w_{n+1}- w_1) (w_{n+1} - w_2) ... (w_{n+1} - w_n)
denoms = compute_denominators(n + 1, q)
missing_factor = chi - (n + 1)
ln_plus1 = prod.mod_mul(missing_factor.mod_inverse(q), q)
ln_plus1 = ln_plus1.mod_mul(denoms[n].mod_inverse(q), q)
# P_0 is special
pis.append(ln_plus1.mod_sub(Bn(1), q))
two = Bn(2)
for i in range(1, n + 1):
missing_factor = chi - i
l_i = prod.mod_mul(missing_factor.mod_inverse(q), q)
l_i = l_i.mod_mul(denoms[i - 1].mod_inverse(q), q)
pis.append(two.mod_mul(l_i, q).mod_add(ln_plus1, q))
return pis
def __lagrange(i, index_list, p):
'''
Calculate lagrange coefficient
'''
top = Bn(1)
bottom = Bn(1)
for j in index_list:
if j != i:
top = (top * j)
bottom = (bottom * (j-i))
return top.mod_mul(bottom.mod_inverse(p), p)
def lagrangian(i, n, chi, q):
"""Evaluates Lagrange basis polynomial l_i at point x.
Distinct points are 1, 2, ..., n + 1.
Returns nonzero field element.
Arguments:
i -- polynomial index in range 1, 2, ..., n + 1
chi -- input value of the polynomial
q -- the order of the group
"""
numerator = Bn(1)
denominator = Bn(1)
for j in range(1, n + 2):
if i == j:
continue
numerator = numerator.mod_mul(chi - j, q)
elem = i - j
denominator = denominator.mod_mul(elem, q)
return numerator.mod_mul(denominator.mod_inverse(q), q)
def compute_denominators_slow(k, q):
"""Computes denominators for Lagrange basis polynomials.
This function can be used to test compute_denominators(k, q).
Uses distinct points 1, ...,k
Arguments:
k -- number of basis polynomials
q -- the order of the group
"""
denominators = []
for i in range(1, k + 1):
denominator = Bn(1)
for j in range(1, k + 1):
if i == j:
continue
elem = i - j
denominator = denominator.mod_mul(elem, q)
denominators.append(denominator)
return denominators
def VerifyOneOfN(ck, cis, proof, message=""):
""" Verify the ring signature on message """
n = int(math.ceil(math.log(len(cis)) / math.log(2)))
(G, g, h, o) = ck
(Celi, Cai, Cbi, cdi, fi, zai, zbi, zd) = proof
## Check all parts of the proof are in the right groups
assert 0 <= zd < o
for k in range(n):
assert 0 <= fi[k] < o
assert 0 <= zai[k] < o
assert 0 <= zbi[k] < o
assert G.check_point(Celi[k])
assert G.check_point(Cai[k])
assert G.check_point(Cbi[k])
assert G.check_point(cdi[k])
# Recompute the challenge
x = challenge(list(ck) + cis + Celi + Cai + Cbi + cdi + [message])
ret = True
for i in range(n):
ret &= x * Celi[i] + Cai[i] == Com(ck, fi[i], zai[i])
ret &= (x - fi[i]) * Celi[i] + Cbi[i] == Com(ck, Bn(0), zbi[i])
# acc = G.infinite()
bases = []
expons = []
for idx, ci in enumerate(cis):
idx = Bn(idx)
idxi = [Bn(int(idx.is_bit_set(i))) for i in range(n)]
acc_exp = Bn(1)
for k, ij in enumerate(idxi):
if ij == 0:
acc_exp = acc_exp.mod_mul(x - fi[k], o)
else:
acc_exp = acc_exp.mod_mul(fi[k], o)
bases += [ci]
expons += [acc_exp]
# acc = acc + acc_exp * ci
for k in range(n):
expi = (-pow(x, k, o))
# acc = acc + expi * cdi[k]
bases += [cdi[k]]
expons += [expi]
# assert G.wsum(expons, bases) == acc
acc = G.wsum(expons, bases)
ret &= acc == Com(ck, 0, zd)
return ret
def VerifyOneOfN(ck, cis, proof, message = ""):
""" Verify the ring signature on message """
n = int(math.ceil(math.log(len(cis)) / math.log(2)))
(G, g, h, o) = ck
(Celi, Cai, Cbi, cdi, fi, zai, zbi, zd) = proof
## Check all parts of the proof are in the right groups
assert 0 <= zd < o
for k in range(n):
assert 0 <= fi[k] < o
assert 0 <= zai[k] < o
assert 0 <= zbi[k] < o
assert G.check_point(Celi[k])
assert G.check_point(Cai[k])
assert G.check_point(Cbi[k])
assert G.check_point(cdi[k])
# Recompute the challenge
x = challenge(list(ck) + cis + Celi + Cai + Cbi + cdi + [ message ])
ret = True
for i in range(n):
ret &= x * Celi[i] + Cai[i] == Com(ck, fi[i], zai[i])
ret &= (x - fi[i]) * Celi[i] + Cbi[i] == Com(ck, Bn(0), zbi[i])
# acc = G.infinite()
bases = []
expons = []
for idx, ci in enumerate(cis):
idx = Bn(idx)
idxi = [Bn(int(idx.is_bit_set(i))) for i in range(n)]
acc_exp = Bn(1)
for k, ij in enumerate(idxi):
if ij == 0:
acc_exp = acc_exp.mod_mul(x - fi[k], o)
else:
acc_exp = acc_exp.mod_mul(fi[k], o)
bases += [ ci ]
expons += [ acc_exp ]
# acc = acc + acc_exp * ci
for k in range(n):
expi = (- pow(x,k,o))
# acc = acc + expi * cdi[k]
bases += [ cdi[k] ]
expons += [ expi ]
# assert G.wsum(expons, bases) == acc
acc = G.wsum(expons, bases)
ret &= acc == Com(ck, 0, zd)
return ret
