ed448goldilocks/aux/decaffeinate_curve25519.sage

# This is as sketch of how to decaffeinate Curve25519

F = GF(2^255-19)
d = -121665
M = EllipticCurve(F,[0,2-4*d,0,1,0])

def maybe(): return randint(0,1)

def qpositive(x):
    return int(x) <= (2^255-19-1)/2

def M_to_E(P):
    # P must be even
    (x,y) = P.xy()
    assert x.is_square()
    
    s = sqrt(x)
    if s == 0: t = 1
    else: t = y/s
    
    X,Y = 2*s / (1+s^2), (1-s^2) / t
    if maybe(): X,Y = -X,-Y
    if maybe(): X,Y = Y,-X
    # OK, have point in ed
    return X,Y

def decaf_encode_from_E(X,Y):
    assert X^2 + Y^2 == 1 + d*X^2*Y^2
    if not qpositive(X*Y): X,Y = Y,-X
    assert qpositive(X*Y)
    
    assert (1-X^2).is_square()
    sx = sqrt(1-X^2)
    tos = -2*sx/X/Y
    if not qpositive(tos): sx = -sx
    s = (1 + sx) / X
    if not qpositive(s): s = -s
    
    return s

def isqrt(x):
    assert(x.is_square())
    if x == 0: return 0
    else: return 1/sqrt(x)

def decaf_encode_from_E_c(X,Y):
    Z = F.random_element()
    T = X*Y*Z
    X = X*Z
    Y = Y*Z
    assert X^2 + Y^2 == Z^2 + d*T^2
    
    # Precompute
    sd = sqrt(F(1-d))
    
    zx = Z^2-X^2
    TZ = T*Z
    assert zx.is_square
    ooAll = isqrt(zx*TZ^2)
    osx = ooAll * TZ
    ooTZ = ooAll * zx * osx
    
    floop = qpositive(T^2 * ooTZ)
    if floop:
        frob = zx * ooTZ
    else:
        frob = sd
        Y = -X
        
    osx *= frob
    
    if qpositive(-2*osx*Z) != floop: osx = -osx
    s = Y*(ooTZ*Z + osx)
    if not qpositive(s): s = -s
    
    return s
    
def is_rotation((X,Y),(x,y)):
    return x*Y == X*y or x*X == -y*Y
    
def decaf_decode_to_E(s):
    assert qpositive(s)
    t = sqrt(s^4 + (2-4*d)*s^2 + 1)
    if not qpositive(t/s): t = -t
    X,Y = 2*s / (1+s^2), (1-s^2) / t
    assert qpositive(X*Y)
    return X,Y
    
def decaf_decode_to_E_c(s):
    assert qpositive(s)
    
    s2 = s^2
    s21 = 1+s2
    t2 = s21^2 - 4*d*s2
    
    alt  = s21*s
    the  = isqrt(t2*alt^2)
    oot  = the * alt
    the *= t2
    tos  = the * s21
    X = 2 * (tos-the) * oot
    Y = (1-s2) * oot
    
    if not qpositive(tos): Y = -Y
    assert qpositive(X*Y)
    
    return X,Y

def test():
    P = 2*M.random_point()
    X,Y = M_to_E(P)
    s = decaf_encode_from_E(X,Y)
    assert s == decaf_encode_from_E_c(X,Y)
    XX,YY = decaf_decode_to_E(s)
    XX2,YY2 = decaf_decode_to_E_c(s)
    assert is_rotation((X,Y),(XX,YY))
    assert is_rotation((X,Y),(XX2,YY2))