# Elliptic curves over finite fields¶

AUTHORS:

• William Stein (2005): Initial version
• John Cremona (2008-02): Point counting and group structure for non-prime fields, Frobenius endomorphism and order, elliptic logs
• Mariah Lenox (2011-03): Added set_order method
class sage.schemes.elliptic_curves.ell_finite_field.EllipticCurve_finite_field(K, ainvs)

Elliptic curve over a finite field.

EXAMPLES:

sage: EllipticCurve(GF(101),[2,3])
Elliptic Curve defined by y^2  = x^3 + 2*x + 3 over Finite Field of size 101

sage: F=GF(101^2, 'a')
sage: EllipticCurve([F(2),F(3)])
Elliptic Curve defined by y^2  = x^3 + 2*x + 3 over Finite Field in a of size 101^2


Elliptic curves over $$\ZZ/N\ZZ$$ with $$N$$ prime are of type “elliptic curve over a finite field”:

sage: F = Zmod(101)
sage: EllipticCurve(F, [2, 3])
Elliptic Curve defined by y^2 = x^3 + 2*x + 3 over Ring of integers modulo 101
sage: E = EllipticCurve([F(2), F(3)])
sage: type(E)
<class 'sage.schemes.elliptic_curves.ell_finite_field.EllipticCurve_finite_field_with_category'>
sage: E.category()
Category of schemes over Ring of integers modulo 101


Elliptic curves over $$\ZZ/N\ZZ$$ with $$N$$ composite are of type “generic elliptic curve”:

sage: F = Zmod(95)
sage: EllipticCurve(F, [2, 3])
Elliptic Curve defined by y^2 = x^3 + 2*x + 3 over Ring of integers modulo 95
sage: E = EllipticCurve([F(2), F(3)])
sage: type(E)
<class 'sage.schemes.elliptic_curves.ell_generic.EllipticCurve_generic_with_category'>
sage: E.category()
Category of schemes over Ring of integers modulo 95
sage: TestSuite(E).run(skip=["_test_elements"])

abelian_group(debug=False)

Returns the abelian group structure of the group of points on this elliptic curve.

Warning

The algorithm is definitely not intended for use with large finite fields! The factorization of the orders of elements must be feasible. Also, baby-step-giant-step methods are used which have space and time requirements which are $$O(\sqrt{q})$$.

Also, the algorithm uses random points on the curve and hence the generators are likely to differ from one run to another; but the group is cached so the generators will not change in any one run of Sage.

INPUT:

• debug - (default: False): if True, print debugging messages

OUTPUT:

• an abelian group
• tuple of images of each of the generators of the abelian group as points on this curve

AUTHORS:

• John Cremona

EXAMPLES:

sage: E=EllipticCurve(GF(11),[2,5])
sage: E.abelian_group()
Additive abelian group isomorphic to Z/10 embedded in Abelian group of points on Elliptic Curve defined by y^2 = x^3 + 2*x + 5 over Finite Field of size 11

sage: E=EllipticCurve(GF(41),[2,5])
sage: E.abelian_group()
Additive abelian group isomorphic to Z/22 + Z/2 ...

sage: F.<a>=GF(3^6,'a')
sage: E=EllipticCurve([a^4 + a^3 + 2*a^2 + 2*a, 2*a^5 + 2*a^3 + 2*a^2 + 1])
sage: E.abelian_group()
Additive abelian group isomorphic to Z/26 + Z/26 ...

sage: F.<a>=GF(101^3,'a')
sage: E=EllipticCurve([2*a^2 + 48*a + 27, 89*a^2 + 76*a + 24])
sage: E.abelian_group()
Additive abelian group isomorphic to Z/1031352 ...


The group can be trivial:

sage: E=EllipticCurve(GF(2),[0,0,1,1,1])
sage: E.abelian_group()
Trivial group embedded in Abelian group of points on Elliptic Curve defined by y^2 + y = x^3 + x + 1 over Finite Field of size 2


Of course, there are plenty of points if we extend the field:

sage: E.cardinality(extension_degree=100)
1267650600228231653296516890625


This tests the patch for trac #3111, using 10 primes randomly selected:

sage: E = EllipticCurve('389a')
sage: for p in [5927, 2297, 1571, 1709, 3851, 127, 3253, 5783, 3499, 4817]:
...       G = E.change_ring(GF(p)).abelian_group()
sage: for p in prime_range(10000):  # long time (19s on sage.math, 2011)
...       if p != 389:
...           G = E.change_ring(GF(p)).abelian_group()


This tests that the bug reported in trac #3926 has been fixed:

sage: K.<i> = QuadraticField(-1)
sage: OK = K.ring_of_integers()
sage: P=K.factor(10007)[0][0]
sage: OKmodP = OK.residue_field(P)
sage: E = EllipticCurve([0,0,0,i,i+3])
sage: Emod = E.change_ring(OKmodP); Emod
Elliptic Curve defined by y^2  = x^3 + ibar*x + (ibar+3) over Residue field in ibar of Fractional ideal (10007)
sage: Emod.abelian_group() #random generators
(Multiplicative Abelian group isomorphic to C50067594 x C2,
((3152*ibar + 7679 : 7330*ibar + 7913 : 1), (8466*ibar + 1770 : 0 : 1)))

cardinality(algorithm='pari', extension_degree=1)

Return the number of points on this elliptic curve.

INPUT:

• algorithm – string (default: 'pari'), used only for point counting over prime fields:
• 'pari' – use the baby-step giant-step or Schoof-Elkies-Atkin methods as implemented in the PARI C-library function ellap
• 'bsgs' – use the baby-step giant-step method as implemented in Sage, with the Cremona-Sutherland version of Mestre’s trick
• 'all' – compute cardinality with both 'pari' and 'bsgs'; return result if they agree or raise a RuntimeError if they do not
• extension_degree – an integer $$d$$ (default: 1): if the base field is $$\GF{q}$$, return the cardinality of self over the extension $$\GF{q^d}$$ of degree $$d$$.

OUTPUT:

The order of the group of rational points of self over its base field, or over an extension field of degree $$d$$ as above. The result is cached.

Over prime fields, one of the above algorithms is used. Over non-prime fields, the serious point counting is done on a standard curve with the same $$j$$-invariant over the field $$\GF{p}(j)$$, then lifted to the base field, and finally account is taken of twists.

For $$j = 0$$ and $$j = 1728$$ special formulas are used instead.

EXAMPLES:

sage: EllipticCurve(GF(4, 'a'), [1,2,3,4,5]).cardinality()
8
sage: k.<a> = GF(3^3)
sage: l = [a^2 + 1, 2*a^2 + 2*a + 1, a^2 + a + 1, 2, 2*a]
sage: EllipticCurve(k,l).cardinality()
29

sage: l = [1, 1, 0, 2, 0]
sage: EllipticCurve(k, l).cardinality()
38


An even bigger extension (which we check against Magma):

sage: EllipticCurve(GF(3^100, 'a'), [1,2,3,4,5]).cardinality()
515377520732011331036459693969645888996929981504
sage: magma.eval("Order(EllipticCurve([GF(3^100)|1,2,3,4,5]))")    # optional - magma
'515377520732011331036459693969645888996929981504'

sage: EllipticCurve(GF(10007), [1,2,3,4,5]).cardinality()
10076
sage: EllipticCurve(GF(10007), [1,2,3,4,5]).cardinality(algorithm='pari')
10076
sage: EllipticCurve(GF(next_prime(10**20)), [1,2,3,4,5]).cardinality()
100000000011093199520


The cardinality is cached:

sage: E = EllipticCurve(GF(3^100, 'a'), [1,2,3,4,5])
sage: E.cardinality() is E.cardinality()
True
sage: E = EllipticCurve(GF(11^2, 'a'), [3,3])
sage: E.cardinality()
128
sage: EllipticCurve(GF(11^100, 'a'), [3,3]).cardinality()
137806123398222701841183371720896367762643312000384671846835266941791510341065565176497846502742959856128


TESTS:

sage: EllipticCurve(GF(10009), [1,2,3,4,5]).cardinality(algorithm='foobar')
Traceback (most recent call last):
...
ValueError: Algorithm is not known


If the cardinality has already been computed, then the algorithm keyword is ignored:

sage: E = EllipticCurve(GF(10007), [1,2,3,4,5])
sage: E.cardinality(algorithm='pari')
10076
sage: E.cardinality(algorithm='foobar')
10076

cardinality_bsgs(verbose=False)

Return the cardinality of self over the base field. Will be called by user function cardinality only when necessary, i.e. when the j_invariant is not in the prime field.

ALGORITHM: A variant of “Mestre’s trick” extended to all finite fields by Cremona and Sutherland, 2008.

Note

1. The Mestre-Schoof-Cremona-Sutherland algorithm may fail for a small finite number of curves over $$F_q$$ for $$q$$ at most 49, so for $$q<50$$ we use an exhaustive count.
2. Quadratic twists are not implemented in characteristic 2 when $$j=0 (=1728)$$; but this case is treated separately.

EXAMPLES:

sage: p=next_prime(10^3)
sage: E=EllipticCurve(GF(p),[3,4])
sage: E.cardinality_bsgs()
1020
sage: E=EllipticCurve(GF(3^4,'a'),[1,1])
sage: E.cardinality_bsgs()
64
sage: F.<a>=GF(101^3,'a')
sage: E=EllipticCurve([2*a^2 + 48*a + 27, 89*a^2 + 76*a + 24])
sage: E.cardinality_bsgs()
1031352

cardinality_exhaustive()

Return the cardinality of self over the base field. Simply adds up the number of points with each x-coordinate: only used for small field sizes!

EXAMPLES:

sage: p=next_prime(10^3)
sage: E=EllipticCurve(GF(p),[3,4])
sage: E.cardinality_exhaustive()
1020
sage: E=EllipticCurve(GF(3^4,'a'),[1,1])
sage: E.cardinality_exhaustive()
64

cardinality_pari()

Return the cardinality of self over the (prime) base field using PARI.

The result is not cached.

EXAMPLES:

sage: p=next_prime(10^3)
sage: E=EllipticCurve(GF(p),[3,4])
sage: E.cardinality_pari()
1020
sage: K=GF(next_prime(10^6))
sage: E=EllipticCurve(K,[1,0,0,1,1])
sage: E.cardinality_pari()
999945


TESTS:

sage: K.<a>=GF(3^20)
sage: E=EllipticCurve(K,[1,0,0,1,a])
sage: E.cardinality_pari()
Traceback (most recent call last):
...
ValueError: cardinality_pari() only works over prime fields.
sage: E.cardinality()
3486794310

count_points(n=1)

Returns the cardinality of this elliptic curve over the base field or extensions.

INPUT:

• n (int) – a positive integer

OUTPUT:

If $$n=1$$, returns the cardinality of the curve over its base field.

If $$n>1$$, returns a list $$[c_1, c_2, ..., c_n]$$ where $$c_d$$ is the cardinality of the curve over the extension of degree $$d$$ of its base field.

EXAMPLES:

sage: p = 101
sage: F = GF(p)
sage: E = EllipticCurve(F, [2,3])
sage: E.count_points(1)
96
sage: E.count_points(5)
[96, 10368, 1031904, 104053248, 10509895776]

sage: F.<a> = GF(p^2)
sage: E = EllipticCurve(F, [a,a])
sage: E.cardinality()
10295
sage: E.count_points()
10295
sage: E.count_points(1)
10295
sage: E.count_points(5)
[10295, 104072155, 1061518108880, 10828567126268595, 110462212555439192375]

frobenius()

Return the frobenius of self as an element of a quadratic order

Note

This computes the curve cardinality, which may be time-consuming.

Frobenius is only determined up to conjugacy.

EXAMPLES:

sage: E=EllipticCurve(GF(11),[3,3])
sage: E.frobenius()
phi
sage: E.frobenius().minpoly()
x^2 - 4*x + 11


For some supersingular curves, Frobenius is in Z:

sage: E=EllipticCurve(GF(25,'a'),[0,0,0,0,1])
sage: E.frobenius()
-5

frobenius_order()

Return the quadratic order Z[phi] where phi is the Frobenius endomorphism of the elliptic curve

Note

This computes the curve cardinality, which may be time-consuming.

EXAMPLES:

sage: E=EllipticCurve(GF(11),[3,3])
sage: E.frobenius_order()
Order in Number Field in phi with defining polynomial x^2 - 4*x + 11


For some supersingular curves, Frobenius is in Z and the Frobenius order is Z:

sage: E=EllipticCurve(GF(25,'a'),[0,0,0,0,1])
sage: R=E.frobenius_order()
sage: R
Order in Number Field in phi with defining polynomial x + 5
sage: R.degree()
1

frobenius_polynomial()

Return the characteristic polynomial of Frobenius.

The Frobenius endomorphism of the elliptic curve has quadratic characteristic polynomial. In most cases this is irreducible and defines an imaginary quadratic order; for some supersingular curves, Frobenius is an integer a and the polynomial is $$(x-a)^2$$.

Note

This computes the curve cardinality, which may be time-consuming.

EXAMPLES:

sage: E=EllipticCurve(GF(11),[3,3])
sage: E.frobenius_polynomial()
x^2 - 4*x + 11


For some supersingular curves, Frobenius is in Z and the polynomial is a square:

sage: E=EllipticCurve(GF(25,'a'),[0,0,0,0,1])
sage: E.frobenius_polynomial().factor()
(x + 5)^2

gens()

Returns a tuple of length up to 2 of points which generate the abelian group of points on this elliptic curve. See abelian_group() for limitations.

The algorithm uses random points on the curve, and hence the generators are likely to differ from one run to another; but they are cached so will be consistent in any one run of Sage.

AUTHORS:

• John Cremona

EXAMPLES:

sage: E=EllipticCurve(GF(11),[2,5])
sage: E.gens()                           # random output
((0 : 7 : 1),)
sage: EllipticCurve(GF(41),[2,5]).gens() # random output
((21 : 1 : 1), (8 : 0 : 1))
sage: F.<a>=GF(3^6,'a')
sage: E=EllipticCurve([a,a+1])
sage: pts=E.gens()
sage: len(pts)
1
sage: pts[0].order()==E.cardinality()
True

is_isogenous(other, field=None, proof=True)

Returns whether or not self is isogenous to other

INPUT:

• other – another elliptic curve.
• field (default None) – a field containing the base fields of the two elliptic curves into which the two curves may be extended to test if they are isogenous over this field. By default is_isogenous will not try to find this field unless one of the curves can be extended into the base field of the other, in which case it will test over the larger base field.
• proof (default True) – this parameter is here only to be consistent with versions for other types of elliptic curves.

OUTPUT:

(bool) True if there is an isogeny from curve self to curve other defined over field.

EXAMPLES:

sage: E1 = EllipticCurve(GF(11^2,'a'),[2,7]); E1
Elliptic Curve defined by y^2 = x^3 + 2*x + 7 over Finite Field in a of size 11^2
sage: E1.is_isogenous(5)
Traceback (most recent call last):
...
ValueError: Second argument is not an Elliptic Curve.
sage: E1.is_isogenous(E1)
True

sage: E2 = EllipticCurve(GF(7^3,'b'),[3,1]); E2
Elliptic Curve defined by y^2 = x^3 + 3*x + 1 over Finite Field in b of size 7^3
sage: E1.is_isogenous(E2)
Traceback (most recent call last):
...
ValueError: The base fields must have the same characteristic.

sage: E3 = EllipticCurve(GF(11^2,'c'),[4,3]); E3
Elliptic Curve defined by y^2 = x^3 + 4*x + 3 over Finite Field in c of size 11^2
sage: E1.is_isogenous(E3)
False

sage: E4 = EllipticCurve(GF(11^6,'d'),[6,5]); E4
Elliptic Curve defined by y^2 = x^3 + 6*x + 5 over Finite Field in d of size 11^6
sage: E1.is_isogenous(E4)
True

sage: E5 = EllipticCurve(GF(11^7,'e'),[4,2]); E5
Elliptic Curve defined by y^2 = x^3 + 4*x + 2 over Finite Field in e of size 11^7
sage: E1.is_isogenous(E5)
Traceback (most recent call last):
...
ValueError: Curves have different base fields: use the field parameter.


When the field is given:

sage: E1 = EllipticCurve(GF(13^2,’a’),[2,7]); E1 Elliptic Curve defined by y^2 = x^3 + 2*x + 7 over Finite Field in a of size 13^2 sage: E1.is_isogenous(5,GF(13^6,’f’)) Traceback (most recent call last): ... ValueError: Second argument is not an Elliptic Curve. sage: E6 = EllipticCurve(GF(11^3,’g’),[9,3]); E6 Elliptic Curve defined by y^2 = x^3 + 9*x + 3 over Finite Field in g of size 11^3 sage: E1.is_isogenous(E6,QQ) Traceback (most recent call last): ... ValueError: The base fields must have the same characteristic. sage: E7 = EllipticCurve(GF(13^5,’h’),[2,9]); E7 Elliptic Curve defined by y^2 = x^3 + 2*x + 9 over Finite Field in h of size 13^5 sage: E1.is_isogenous(E7,GF(13^4,’i’)) Traceback (most recent call last): ... ValueError: Field must be an extension of the base fields of both curves sage: E1.is_isogenous(E7,GF(13^10,’j’)) False sage: E1.is_isogenous(E7,GF(13^30,’j’)) False
is_ordinary(proof=True)

Return True if this elliptic curve is ordinary, else False.

INPUT:

• proof (boolean, default True) – If True, returns a proved result. If False, then a return value of True is certain but a return value of False may be based on a probabilistic test. See the documentaion of the function is_j_supersingular() for more details.

EXAMPLES:

sage: F = GF(101)
sage: EllipticCurve(j=F(0)).is_ordinary()
False
sage: EllipticCurve(j=F(1728)).is_ordinary()
True
sage: EllipticCurve(j=F(66)).is_ordinary()
False
sage: EllipticCurve(j=F(99)).is_ordinary()
True

is_supersingular(proof=True)

Return True if this elliptic curve is supersingular, else False.

INPUT:

• proof (boolean, default True) – If True, returns a proved result. If False, then a return value of False is certain but a return value of True may be based on a probabilistic test. See the documentaion of the function is_j_supersingular() for more details.

EXAMPLES:

sage: F = GF(101)
sage: EllipticCurve(j=F(0)).is_supersingular()
True
sage: EllipticCurve(j=F(1728)).is_supersingular()
False
sage: EllipticCurve(j=F(66)).is_supersingular()
True
sage: EllipticCurve(j=F(99)).is_supersingular()
False


TESTS:

sage: from sage.schemes.elliptic_curves.ell_finite_field import supersingular_j_polynomial, is_j_supersingular
sage: F = GF(103)
sage: ssjlist = [F(1728)] + supersingular_j_polynomial(103).roots(multiplicities=False)
sage: Set([j for j in F if is_j_supersingular(j)]) == Set(ssjlist)
True

order(algorithm='pari', extension_degree=1)

Return the number of points on this elliptic curve.

INPUT:

• algorithm – string (default: 'pari'), used only for point counting over prime fields:
• 'pari' – use the baby-step giant-step or Schoof-Elkies-Atkin methods as implemented in the PARI C-library function ellap
• 'bsgs' – use the baby-step giant-step method as implemented in Sage, with the Cremona-Sutherland version of Mestre’s trick
• 'all' – compute cardinality with both 'pari' and 'bsgs'; return result if they agree or raise a RuntimeError if they do not
• extension_degree – an integer $$d$$ (default: 1): if the base field is $$\GF{q}$$, return the cardinality of self over the extension $$\GF{q^d}$$ of degree $$d$$.

OUTPUT:

The order of the group of rational points of self over its base field, or over an extension field of degree $$d$$ as above. The result is cached.

Over prime fields, one of the above algorithms is used. Over non-prime fields, the serious point counting is done on a standard curve with the same $$j$$-invariant over the field $$\GF{p}(j)$$, then lifted to the base field, and finally account is taken of twists.

For $$j = 0$$ and $$j = 1728$$ special formulas are used instead.

EXAMPLES:

sage: EllipticCurve(GF(4, 'a'), [1,2,3,4,5]).cardinality()
8
sage: k.<a> = GF(3^3)
sage: l = [a^2 + 1, 2*a^2 + 2*a + 1, a^2 + a + 1, 2, 2*a]
sage: EllipticCurve(k,l).cardinality()
29

sage: l = [1, 1, 0, 2, 0]
sage: EllipticCurve(k, l).cardinality()
38


An even bigger extension (which we check against Magma):

sage: EllipticCurve(GF(3^100, 'a'), [1,2,3,4,5]).cardinality()
515377520732011331036459693969645888996929981504
sage: magma.eval("Order(EllipticCurve([GF(3^100)|1,2,3,4,5]))")    # optional - magma
'515377520732011331036459693969645888996929981504'

sage: EllipticCurve(GF(10007), [1,2,3,4,5]).cardinality()
10076
sage: EllipticCurve(GF(10007), [1,2,3,4,5]).cardinality(algorithm='pari')
10076
sage: EllipticCurve(GF(next_prime(10**20)), [1,2,3,4,5]).cardinality()
100000000011093199520


The cardinality is cached:

sage: E = EllipticCurve(GF(3^100, 'a'), [1,2,3,4,5])
sage: E.cardinality() is E.cardinality()
True
sage: E = EllipticCurve(GF(11^2, 'a'), [3,3])
sage: E.cardinality()
128
sage: EllipticCurve(GF(11^100, 'a'), [3,3]).cardinality()
137806123398222701841183371720896367762643312000384671846835266941791510341065565176497846502742959856128


TESTS:

sage: EllipticCurve(GF(10009), [1,2,3,4,5]).cardinality(algorithm='foobar')
Traceback (most recent call last):
...
ValueError: Algorithm is not known


If the cardinality has already been computed, then the algorithm keyword is ignored:

sage: E = EllipticCurve(GF(10007), [1,2,3,4,5])
sage: E.cardinality(algorithm='pari')
10076
sage: E.cardinality(algorithm='foobar')
10076

plot(*args, **kwds)

Draw a graph of this elliptic curve over a prime finite field.

INPUT:

• *args, **kwds - all other options are passed to the circle graphing primitive.

EXAMPLES:

sage: E = EllipticCurve(FiniteField(17), [0,1])
sage: P = plot(E, rgbcolor=(0,0,1))

points()

All the points on this elliptic curve. The list of points is cached so subsequent calls are free.

EXAMPLES:

sage: p = 5
sage: F = GF(p)
sage: E = EllipticCurve(F, [1, 3])
sage: a_sub_p = E.change_ring(QQ).ap(p); a_sub_p
2

sage: len(E.points())
4
sage: p + 1 - a_sub_p
4
sage: E.points()
[(0 : 1 : 0), (1 : 0 : 1), (4 : 1 : 1), (4 : 4 : 1)]

sage: K = GF(p**2,'a')
sage: E = E.change_ring(K)
sage: len(E.points())
32
sage: (p + 1)**2 - a_sub_p**2
32
sage: w = E.points(); w
[(0 : 1 : 0), (0 : 2*a + 4 : 1), (0 : 3*a + 1 : 1), (1 : 0 : 1), (2 : 2*a + 4 : 1), (2 : 3*a + 1 : 1), (3 : 2*a + 4 : 1), (3 : 3*a + 1 : 1), (4 : 1 : 1), (4 : 4 : 1), (a : 1 : 1), (a : 4 : 1), (a + 2 : a + 1 : 1), (a + 2 : 4*a + 4 : 1), (a + 3 : a : 1), (a + 3 : 4*a : 1), (a + 4 : 0 : 1), (2*a : 2*a : 1), (2*a : 3*a : 1), (2*a + 4 : a + 1 : 1), (2*a + 4 : 4*a + 4 : 1), (3*a + 1 : a + 3 : 1), (3*a + 1 : 4*a + 2 : 1), (3*a + 2 : 2*a + 3 : 1), (3*a + 2 : 3*a + 2 : 1), (4*a : 0 : 1), (4*a + 1 : 1 : 1), (4*a + 1 : 4 : 1), (4*a + 3 : a + 3 : 1), (4*a + 3 : 4*a + 2 : 1), (4*a + 4 : a + 4 : 1), (4*a + 4 : 4*a + 1 : 1)]


Note that the returned list is an immutable sorted Sequence:

sage: w[0] = 9
Traceback (most recent call last):
...

random_element()

Return a random point on this elliptic curve, uniformly chosen among all rational points.

ALGORITHM:

Choose the point at infinity with probability $$1/(2q + 1)$$. Otherwise, take a random element from the field as x-coordinate and compute the possible y-coordinates. Return the i’th possible y-coordinate, where i is randomly chosen to be 0 or 1. If the i’th y-coordinate does not exist (either there is no point with the given x-coordinate or we hit a 2-torsion point with i == 1), try again.

This gives a uniform distribution because you can imagine $$2q + 1$$ buckets, one for the point at infinity and 2 for each element of the field (representing the x-coordinates). This gives a 1-to-1 map of elliptic curve points into buckets. At every iteration, we simply choose a random bucket until we find a bucket containing a point.

AUTHOR:

EXAMPLES:

sage: k = GF(next_prime(7^5))
sage: E = EllipticCurve(k,[2,4])
sage: P = E.random_element(); P
(16740 : 12486 : 1)
sage: type(P)
<class 'sage.schemes.elliptic_curves.ell_point.EllipticCurvePoint_finite_field'>
sage: P in E
True

sage: k.<a> = GF(7^5)
sage: E = EllipticCurve(k,[2,4])
sage: P = E.random_element(); P
(5*a^4 + 3*a^3 + 2*a^2 + a + 4 : 2*a^4 + 3*a^3 + 4*a^2 + a + 5 : 1)
sage: type(P)
<class 'sage.schemes.elliptic_curves.ell_point.EllipticCurvePoint_finite_field'>
sage: P in E
True

sage: k.<a> = GF(2^5)
sage: E = EllipticCurve(k,[a^2,a,1,a+1,1])
sage: P = E.random_element(); P
(a^4 + a : a^4 + a^3 + a^2 : 1)
sage: type(P)
<class 'sage.schemes.elliptic_curves.ell_point.EllipticCurvePoint_finite_field'>
sage: P in E
True


Ensure that the entire point set is reachable:

sage: E = EllipticCurve(GF(11), [2,1])
sage: len(set(E.random_element() for _ in range(100)))
16
sage: E.cardinality()
16


TESTS:

See trac #8311:

sage: E = EllipticCurve(GF(3), [0,0,0,2,2])
sage: E.random_element()
(0 : 1 : 0)
sage: E.cardinality()
1

sage: E = EllipticCurve(GF(2), [0,0,1,1,1])
sage: E.random_point()
(0 : 1 : 0)
sage: E.cardinality()
1

sage: F.<a> = GF(4)
sage: E = EllipticCurve(F, [0, 0, 1, 0, a])
sage: E.random_point()
(0 : 1 : 0)
sage: E.cardinality()
1

random_point()

Return a random point on this elliptic curve, uniformly chosen among all rational points.

ALGORITHM:

Choose the point at infinity with probability $$1/(2q + 1)$$. Otherwise, take a random element from the field as x-coordinate and compute the possible y-coordinates. Return the i’th possible y-coordinate, where i is randomly chosen to be 0 or 1. If the i’th y-coordinate does not exist (either there is no point with the given x-coordinate or we hit a 2-torsion point with i == 1), try again.

This gives a uniform distribution because you can imagine $$2q + 1$$ buckets, one for the point at infinity and 2 for each element of the field (representing the x-coordinates). This gives a 1-to-1 map of elliptic curve points into buckets. At every iteration, we simply choose a random bucket until we find a bucket containing a point.

AUTHOR:

EXAMPLES:

sage: k = GF(next_prime(7^5))
sage: E = EllipticCurve(k,[2,4])
sage: P = E.random_element(); P
(16740 : 12486 : 1)
sage: type(P)
<class 'sage.schemes.elliptic_curves.ell_point.EllipticCurvePoint_finite_field'>
sage: P in E
True

sage: k.<a> = GF(7^5)
sage: E = EllipticCurve(k,[2,4])
sage: P = E.random_element(); P
(5*a^4 + 3*a^3 + 2*a^2 + a + 4 : 2*a^4 + 3*a^3 + 4*a^2 + a + 5 : 1)
sage: type(P)
<class 'sage.schemes.elliptic_curves.ell_point.EllipticCurvePoint_finite_field'>
sage: P in E
True

sage: k.<a> = GF(2^5)
sage: E = EllipticCurve(k,[a^2,a,1,a+1,1])
sage: P = E.random_element(); P
(a^4 + a : a^4 + a^3 + a^2 : 1)
sage: type(P)
<class 'sage.schemes.elliptic_curves.ell_point.EllipticCurvePoint_finite_field'>
sage: P in E
True


Ensure that the entire point set is reachable:

sage: E = EllipticCurve(GF(11), [2,1])
sage: len(set(E.random_element() for _ in range(100)))
16
sage: E.cardinality()
16


TESTS:

See trac #8311:

sage: E = EllipticCurve(GF(3), [0,0,0,2,2])
sage: E.random_element()
(0 : 1 : 0)
sage: E.cardinality()
1

sage: E = EllipticCurve(GF(2), [0,0,1,1,1])
sage: E.random_point()
(0 : 1 : 0)
sage: E.cardinality()
1

sage: F.<a> = GF(4)
sage: E = EllipticCurve(F, [0, 0, 1, 0, a])
sage: E.random_point()
(0 : 1 : 0)
sage: E.cardinality()
1

rational_points()

All the points on this elliptic curve. The list of points is cached so subsequent calls are free.

EXAMPLES:

sage: p = 5
sage: F = GF(p)
sage: E = EllipticCurve(F, [1, 3])
sage: a_sub_p = E.change_ring(QQ).ap(p); a_sub_p
2

sage: len(E.points())
4
sage: p + 1 - a_sub_p
4
sage: E.points()
[(0 : 1 : 0), (1 : 0 : 1), (4 : 1 : 1), (4 : 4 : 1)]

sage: K = GF(p**2,'a')
sage: E = E.change_ring(K)
sage: len(E.points())
32
sage: (p + 1)**2 - a_sub_p**2
32
sage: w = E.points(); w
[(0 : 1 : 0), (0 : 2*a + 4 : 1), (0 : 3*a + 1 : 1), (1 : 0 : 1), (2 : 2*a + 4 : 1), (2 : 3*a + 1 : 1), (3 : 2*a + 4 : 1), (3 : 3*a + 1 : 1), (4 : 1 : 1), (4 : 4 : 1), (a : 1 : 1), (a : 4 : 1), (a + 2 : a + 1 : 1), (a + 2 : 4*a + 4 : 1), (a + 3 : a : 1), (a + 3 : 4*a : 1), (a + 4 : 0 : 1), (2*a : 2*a : 1), (2*a : 3*a : 1), (2*a + 4 : a + 1 : 1), (2*a + 4 : 4*a + 4 : 1), (3*a + 1 : a + 3 : 1), (3*a + 1 : 4*a + 2 : 1), (3*a + 2 : 2*a + 3 : 1), (3*a + 2 : 3*a + 2 : 1), (4*a : 0 : 1), (4*a + 1 : 1 : 1), (4*a + 1 : 4 : 1), (4*a + 3 : a + 3 : 1), (4*a + 3 : 4*a + 2 : 1), (4*a + 4 : a + 4 : 1), (4*a + 4 : 4*a + 1 : 1)]


Note that the returned list is an immutable sorted Sequence:

sage: w[0] = 9
Traceback (most recent call last):
...

set_order(value, num_checks=8)

Set the value of self._order to value.

Use this when you know a priori the order of the curve to avoid a potentially expensive order calculation.

INPUT:

• value - Integer in the Hasse-Weil range for this curve.
• num_checks - Integer (default: 8) number of times to check whether value*(a random point on this curve) is equal to the identity.

OUTPUT:

None

EXAMPLES:

This example illustrates basic usage.

sage: E = EllipticCurve(GF(7), [0, 1]) # This curve has order 6
sage: E.set_order(6)
sage: E.order()
6
sage: E.order() * E.random_point()
(0 : 1 : 0)


We now give a more interesting case, the NIST-P521 curve. Its order is too big to calculate with Sage, and takes a long time using other packages, so it is very useful here.

sage: p = 2^521 - 1
sage: prev_proof_state = proof.arithmetic()
sage: proof.arithmetic(False) # turn off primality checking
sage: F = GF(p)
sage: A = p - 3
sage: B = 1093849038073734274511112390766805569936207598951683748994586394495953116150735016013708737573759623248592132296706313309438452531591012912142327488478985984
sage: q = 6864797660130609714981900799081393217269435300143305409394463459185543183397655394245057746333217197532963996371363321113864768612440380340372808892707005449
sage: E = EllipticCurve([F(A), F(B)])
sage: E.set_order(q)
sage: G = E.random_point()
sage: E.order() * G  # This takes practically no time.
(0 : 1 : 0)
sage: proof.arithmetic(prev_proof_state) # restore state


It is an error to pass a value which is not an integer in the Hasse-Weil range:

sage: E = EllipticCurve(GF(7), [0, 1]) # This curve has order 6
sage: E.set_order("hi")
Traceback (most recent call last):
...
ValueError: Value hi illegal (not an integer in the Hasse range)
sage: E.set_order(3.14159)
Traceback (most recent call last):
...
ValueError: Value 3.14159000000000 illegal (not an integer in the Hasse range)
sage: E.set_order(0)
Traceback (most recent call last):
...
ValueError: Value 0 illegal (not an integer in the Hasse range)
sage: E.set_order(1000)
Traceback (most recent call last):
...
ValueError: Value 1000 illegal (not an integer in the Hasse range)


It is also very likely an error to pass a value which is not the actual order of this curve. How unlikely is determined by num_checks, the factorization of the actual order, and the actual group structure:

sage: E = EllipticCurve(GF(7), [0, 1]) # This curve has order 6
sage: E.set_order(11)
Traceback (most recent call last):
...
ValueError: Value 11 illegal (multiple of random point not the identity)


However, set_order can be fooled, though it’s not likely in “real cases of interest”. For instance, the order can be set to a multiple of the actual order:

sage: E = EllipticCurve(GF(7), [0, 1]) # This curve has order 6
sage: E.set_order(12)  # 12 just fits in the Hasse range
sage: E.order()
12


Or, the order can be set incorrectly along with num_checks set too small:

sage: E = EllipticCurve(GF(7), [0, 1]) # This curve has order 6
sage: E.set_order(4, num_checks=0)
WARNING: No checking done in set_order
sage: E.order()
4


The value of num_checks must be an integer. Negative values are interpreted as zero, which means don’t do any checking:

sage: E = EllipticCurve(GF(7), [0, 1]) # This curve has order 6
sage: E.set_order(4, num_checks=-12)
WARNING: No checking done in set_order
sage: E.order()
4


NOTES:

The implementation is based on the fact that orders of elliptic curves are cached in the (pseudo-private) _order slot.

AUTHORS:

• Mariah Lenox (2011-02-16)
trace_of_frobenius()

Return the trace of Frobenius acting on this elliptic curve.

Note

This computes the curve cardinality, which may be time-consuming.

EXAMPLES:

sage: E=EllipticCurve(GF(101),[2,3])
sage: E.trace_of_frobenius()
6
sage: E=EllipticCurve(GF(11^5,'a'),[2,5])
sage: E.trace_of_frobenius()
802


The following shows that the issue from trac #2849 is fixed:

sage: E=EllipticCurve(GF(3^5,'a'),[-1,-1])
sage: E.trace_of_frobenius()
-27

sage.schemes.elliptic_curves.ell_finite_field.is_j_supersingular(j, proof=True)

Return True if $$j$$ is a supersingular $$j$$-invariant.

INPUT:

• j (finite field element) – an element of a finite field
• proof (boolean, default True) – If True, returns a proved result. If False, then a return value of False is certain but a return value of True may be based on a probabilistic test. See the ALGORITHM section below for more details.

OUTPUT:

(boolean) True if $$j$$ is supersingular, else False.

ALGORITHM:

For small characteristics $$p$$ we check whether the $$j$$-invariant is in a precomputed list of supersingular values. Otherwise we next check the $$j$$-invariant. If $$j=0$$, the curve is supersingular if and only if $$p=2$$ or $$p\equiv3\pmod{4}$$; if $$j=1728$$, the curve is supersingular if and only if $$p=3$$ or $$p\equiv2\pmod{3}$$. Next, if the base field is the prime field $${\rm GF}(p)$$, we check that $$(p+1)P=0$$ for several random points $$P$$, returning False if any fail: supersingular curves over $${\rm GF}(p)$$ have cardinality $$p+1$$. If Proof is false we now return True. Otherwise we compute the cardinality and return True if and only if it is divisible by $$p$$.

EXAMPLES:

sage: from sage.schemes.elliptic_curves.ell_finite_field import is_j_supersingular, supersingular_j_polynomials
sage: [(p,[j for j in GF(p) if is_j_supersingular(j)]) for p in prime_range(30)]
[(2, [0]), (3, [0]), (5, [0]), (7, [6]), (11, [0, 1]), (13, [5]), (17, [0, 8]), (19, [7, 18]), (23, [0, 3, 19]), (29, [0, 2, 25])]

sage: [j for j in GF(109) if is_j_supersingular(j)]
[17, 41, 43]
sage: PolynomialRing(GF(109),'j')(supersingular_j_polynomials[109]).roots()
[(43, 1), (41, 1), (17, 1)]

sage: [p for p in prime_range(100) if is_j_supersingular(GF(p)(0))]
[2, 3, 5, 11, 17, 23, 29, 41, 47, 53, 59, 71, 83, 89]
sage: [p for p in prime_range(100) if is_j_supersingular(GF(p)(1728))]
[2, 3, 7, 11, 19, 23, 31, 43, 47, 59, 67, 71, 79, 83]
sage: [p for p in prime_range(100) if is_j_supersingular(GF(p)(123456))]
[2, 3, 59, 89]

sage.schemes.elliptic_curves.ell_finite_field.supersingular_j_polynomial(p)

Return a polynomial whose roots are the supersingular $$j$$-invariants in characteristic $$p$$, other than 0, 1728.

INPUT:

• $$p$$ (integer) – a prime number.

ALGORITHM:

First compute H(X) whose roots are the Legendre $$\lambda$$-invariants of supersingular curves (Silverman V.4.1(b)) in charactersitic $$p$$. Then, using a resultant computation with the polynomial relating $$\lambda$$ and $$j$$ (Silverman III.1.7(b)), we recover the polynomial (in variable j) whose roots are the $$j$$-invariants. Factors of $$j$$ and $$j-1728$$ are removed if present.

EXAMPLES:

sage: from sage.schemes.elliptic_curves.ell_finite_field import supersingular_j_polynomial
sage: f = supersingular_j_polynomial(67); f
j^5 + 53*j^4 + 4*j^3 + 47*j^2 + 36*j + 8
sage: f.factor()
(j + 1) * (j^2 + 8*j + 45) * (j^2 + 44*j + 24)

sage: [supersingular_j_polynomial(p) for p in prime_range(30)]
[1, 1, 1, 1, 1, j + 8, j + 9, j + 12, j + 4, j^2 + 2*j + 21]


TESTS:

sage: supersingular_j_polynomial(6)
Traceback (most recent call last):
...
ValueError: p (=6) should be a prime number


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