constantine/sage/frobenius_bn254_snarks.sage

# Constantine
# Copyright (c) 2018-2019    Status Research & Development GmbH
# Copyright (c) 2020-Present Mamy André-Ratsimbazafy
# Licensed and distributed under either of
#   * MIT license (license terms in the root directory or at http://opensource.org/licenses/MIT).
#   * Apache v2 license (license terms in the root directory or at http://www.apache.org/licenses/LICENSE-2.0).
# at your option. This file may not be copied, modified, or distributed except according to those terms.

# ############################################################
#
#                         BLS12-381
#                   Frobenius Endomorphism
#              Untwist-Frobenius-Twist isogeny
#
# ############################################################

# Parameters
x = Integer('0x44E992B44A6909F1')
p = 36*x^4 + 36*x^3 + 24*x^2 + 6*x + 1
r = 36*x^4 + 36*x^3 + 18*x^2 + 6*x + 1
t = 6*x^2 + 1
cofactor = 1
print('p  : ' + p.hex())
print('r  : ' + r.hex())
print('t  : ' + t.hex())
print('p (mod r) == t-1 (mod r) == 0x' + (p % r).hex())

# Finite fields
Fp       = GF(p)
K2.<u>  = PolynomialRing(Fp)
Fp2.<beta>  = Fp.extension(u^2+1)

# Curves
b = 3
SNR = Fp2([9, 1])
G1 = EllipticCurve(Fp, [0, b])
G2 = EllipticCurve(Fp2, [0, b/SNR])

# https://crypto.stackexchange.com/questions/64064/order-of-twisted-curve-in-pairings
# https://math.stackexchange.com/questions/144194/how-to-find-the-order-of-elliptic-curve-over-finite-field-extension
cofactorG1 = G1.order() // r
cofactorG2 = G2.order() // r

print('')
print('cofactor G1: ' + cofactorG1.hex())
print('cofactor G2: ' + cofactorG2.hex())
print('')

# Utilities
def fp2_to_hex(a):
    v = vector(a)
    return Integer(v[0]).hex() + ' + β * ' + Integer(v[1]).hex()

# Frobenius constants (D type: use SNR, M type use 1/SNR)
FrobConst_psi = SNR^((p-1)/6)
FrobConst_psi_2 = FrobConst_psi * FrobConst_psi
FrobConst_psi_3 = FrobConst_psi_2 * FrobConst_psi
print('FrobConst_psi   : ' + fp2_to_hex(FrobConst_psi))
print('FrobConst_psi_2  : ' + fp2_to_hex(FrobConst_psi_2))
print('FrobConst_psi_3  : ' + fp2_to_hex(FrobConst_psi_3))

print('')
FrobConst_psi2_2 = FrobConst_psi_2 * FrobConst_psi_2^p
FrobConst_psi2_3 = FrobConst_psi_3 * FrobConst_psi_3^p
print('FrobConst_psi2_2  : ' + fp2_to_hex(FrobConst_psi2_2))
print('FrobConst_psi2_3  : ' + fp2_to_hex(FrobConst_psi2_3))

print('')
FrobConst_psi3_2 = FrobConst_psi_2 * FrobConst_psi2_2^p
FrobConst_psi3_3 = FrobConst_psi_3 * FrobConst_psi2_3^p
print('FrobConst_psi3_2  : ' + fp2_to_hex(FrobConst_psi3_2))
print('FrobConst_psi3_3  : ' + fp2_to_hex(FrobConst_psi3_3))

# Recap, with ξ (xi) the sextic non-residue
# psi_2 = (ξ^((p-1)/6))^2 = ξ^((p-1)/3)
# psi_3 = psi_2 * ξ^((p-1)/6) = ξ^((p-1)/3) * ξ^((p-1)/6) = ξ^((p-1)/2)
#
# Reminder, in 𝔽p2, frobenius(a) = a^p = conj(a)
# psi2_2 = psi_2 * psi_2^p = ξ^((p-1)/3) * ξ^((p-1)/3)^p = ξ^((p-1)/3) * frobenius(ξ)^((p-1)/3)
#        = norm(ξ)^((p-1)/3)
# psi2_3 = psi_3 * psi_3^p = ξ^((p-1)/2) * ξ^((p-1)/2)^p = ξ^((p-1)/2) * frobenius(ξ)^((p-1)/2)
#        = norm(ξ)^((p-1)/2)
#
# In Fp²:
# - quadratic non-residues respect the equation a^((p²-1)/2) ≡ -1 (mod p²) by the Legendre symbol
# - sextic non-residues are also quadratic non-residues so ξ^((p²-1)/2) ≡ -1 (mod p²)
#
# We have norm(ξ)^((p-1)/2) = (ξ*frobenius(ξ))^((p-1)/2) = (ξ*(ξ^p))^((p-1)/2) = ξ^(p+1)^(p-1)/2
#                           = ξ^((p²-1)/2)
# And ξ^((p²-1)/2) ≡ -1 (mod p²)
# So psi2_3 ≡ -1 (mod p²)

# Frobenius Fp2
A = Fp2([5, 7])
Aconj = Fp2([5, -7])
AF = A.frobenius(1) # or pth_power(1)
AF2 = A.frobenius(2)
AF3 = A.frobenius(3)
print('')
print('A          : ' + fp2_to_hex(A))
print('A conjugate: ' + fp2_to_hex(Aconj))
print('')
print('AF1        : ' + fp2_to_hex(AF))
print('AF2        : ' + fp2_to_hex(AF2))
print('AF3        : ' + fp2_to_hex(AF3))

def psi(P):
    (Px, Py, Pz) = P
    return G2([
        FrobConst_psi_2 * Px.frobenius(1),
        FrobConst_psi_3 * Py.frobenius(1)
        # Pz.frobenius() - Always 1 after extract
    ])

def psi2(P):
    (Px, Py, Pz) = P
    return G2([
        FrobConst_psi2_2 * Px.frobenius(2),
        FrobConst_psi2_3 * Py.frobenius(2)
        # Pz - Always 1 after extract
    ])

def clearCofactorG2(P):
    return cofactorG2 * P

# Test generator
set_random_seed(1337)

# Vectors
print('\nTest vectors:')
for i in range(4):
    P = G2.random_point()
    P = clearCofactorG2(P)

    (Px, Py, Pz) = P
    vPx = vector(Px)
    vPy = vector(Py)
    # vPz = vector(Pz)
    print(f'\nTest {i}')
    print('  Px: ' + Integer(vPx[0]).hex() + ' + β * ' + Integer(vPx[1]).hex())
    print('  Py: ' + Integer(vPy[0]).hex() + ' + β * ' + Integer(vPy[1]).hex())
    # print('  Pz: ' + Integer(vPz[0]).hex() + ' + β * ' + Integer(vPz[1]).hex())

    # Galbraith-Lin-Scott, 2008, Theorem 1
    # Fuentes-Castaneda et al, 2011, Equation (2)
    assert psi(psi(P)) - t*psi(P) + p*P == G2([0, 1, 0])

    # Galbraith-Scott, 2008, Lemma 1
    # k-th cyclotomic polynomial with k = 12
    assert psi2(psi2(P)) - psi2(P) + P == G2([0, 1, 0])

    assert psi(psi(P)) == psi2(P)

    (Qx, Qy, Qz) = psi(P)
    vQx = vector(Qx)
    vQy = vector(Qy)
    print('  Qx: ' + Integer(vQx[0]).hex() + ' + β * ' + Integer(vQx[1]).hex())
    print('  Qy: ' + Integer(vQy[0]).hex() + ' + β * ' + Integer(vQy[1]).hex())

    assert psi(P) == (p % r) * P, "Can be false if the cofactor was not cleared"