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Make map_to_curve_simple_swu take a single input again (since we no longer need batch inversion). 

Also make it clearer that we don't depend on Sage's elliptic curve impl except for debugging.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
This commit is contained in:
Daira Hopwood 2021-01-02 00:50:42 +00:00
parent c0f2b2d8b6
commit fd7283a979
1 changed files with 82 additions and 80 deletions
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32
hashtocurve.sage
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@ -93,7 +93,7 @@ def select_z_nz(s, ifz, ifnz):
# This should be constant-time in a real implementation.
return ifz if (s == 0) else ifnz
def map_to_curve_simple_swu(F, E, Z, us, c):
def map_to_curve_simple_swu(F, E, Z, u, c):
# would be precomputed
h = F.g
(0, 0, 0, A, B) = E.a_invariants()
@ -103,8 +103,6 @@ def map_to_curve_simple_swu(F, E, Z, us, c):
assert (Z/h).is_square()
theta = sqrt(Z/h)
Qs = []
for u in us:
# 1. tv1 = inv0(Z^2 * u^4 + Z * u^2)
# 2. x1 = (-B / A) * (1 + tv1)
# 3. If tv1 == 0, set x1 = B / (Z * A)
@ -177,9 +175,8 @@ def map_to_curve_simple_swu(F, E, Z, us, c):
# 9. If sgn0(u) != sgn0(y), set y = -y
y = select_z_nz((int(u) % 2) - (int(y) % 2), y, -y)
Qs.append(E((x, y)))
return Qs
return (x, y)
# iso_Ep = Isogeny of degree 3 from Elliptic Curve defined by y^2 = x^3 + 10949663248450308183708987909873589833737836120165333298109615750520499732811*x + 1265 over Fp
@ -303,38 +300,43 @@ def hash_to_curve_affine(msg, DST, uniform=True):
c = Cost()
us = hash_to_field(msg, DST, 2 if uniform else 1)
#print("u = ", u)
Qs = map_to_curve_simple_swu(F_p, E_isop, Z_isop, us, c)
(Q0x, Q0y) = map_to_curve_simple_swu(F_p, E_isop, Z_isop, us[0], c)
if uniform:
(Q1x, Q1y) = map_to_curve_simple_swu(F_p, E_isop, Z_isop, us[1], c)
# Complete addition using affine coordinates: I + 2M + 2S
# (S for x1^2; compute numerator and denominator of the division for the correct case;
# I + M to divide; S + M to compute x and y of the result.)
R = Qs[0] + Qs[1]
# Just use Sage's implementation, since this is mainly for comparison to the Jacobian impl.
R = E_isop((Q0x, Q0y)) + E_isop((Q1x, Q1y))
#print("R = ", R)
c.invs += 1
c.sqrs += 2
c.muls += 2
(Rx, Ry) = R.xy()
else:
R = Qs[0]
(Rx, Ry) = (Q0x, Q0y)
# no cofactor clearing needed since Pallas and Vesta are prime-order
(x, y) = R.xy()
P = E_p(isop_map_affine(x, y, c))
P = E_p(isop_map_affine(Rx, Ry, c))
return (P, c)
def hash_to_curve_jacobian(msg, DST):
c = Cost()
us = hash_to_field(msg, DST, 2)
#print("u = ", u)
Qs = map_to_curve_simple_swu(F_p, E_isop, Z_isop, us, c)
(Q0x, Q0y) = map_to_curve_simple_swu(F_p, E_isop, Z_isop, us[0], c)
(Q1x, Q1y) = map_to_curve_simple_swu(F_p, E_isop, Z_isop, us[1], c)
R = Qs[0] + Qs[1]
if DEBUG:
R = E_isop((Q0x, Q0y)) + E_isop((Q1x, Q1y))
#print("R = ", R)
(Q0x, Q0y) = Qs[0].xy()
(Q1x, Q1y) = Qs[1].xy()
(Rx, Ry, Rz) = unified_mmadd_jacobian(E_isop_A, Q0x, Q0y, Q1x, Q1y, c)
assert E_isop((Rx / Rz^2, Ry / Rz^3)) == R
if DEBUG: assert E_isop((Rx / Rz^2, Ry / Rz^3)) == R
# no cofactor clearing needed since Pallas and Vesta are prime-order
(Px, Py, Pz) = isop_map_jacobian(Rx, Ry, Rz, c)