예제 #1
0
        /**
         * Computes the norm of an element <code>&#955;</code> of
         * <code><b>R</b>[&#964;]</code>, where <code>&#955; = u + v&#964;</code>
         * and <code>u</code> and <code>u</code> are real numbers (elements of
         * <code><b>R</b></code>).
         * @param mu The parameter <code>&#956;</code> of the elliptic curve.
         * @param u The real part of the element <code>&#955;</code> of
         * <code><b>R</b>[&#964;]</code>.
         * @param v The <code>&#964;</code>-adic part of the element
         * <code>&#955;</code> of <code><b>R</b>[&#964;]</code>.
         * @return The norm of <code>&#955;</code>.
         */
        public static SimpleBigDecimal Norm(sbyte mu, SimpleBigDecimal u, SimpleBigDecimal v)
        {
            SimpleBigDecimal norm;

            // s1 = u^2
            SimpleBigDecimal s1 = u.Multiply(u);

            // s2 = u * v
            SimpleBigDecimal s2 = u.Multiply(v);

            // s3 = 2 * v^2
            SimpleBigDecimal s3 = v.Multiply(v).ShiftLeft(1);

            if (mu == 1)
            {
                norm = s1.Add(s2).Add(s3);
            }
            else if (mu == -1)
            {
                norm = s1.Subtract(s2).Add(s3);
            }
            else
            {
                throw new ArgumentException("mu must be 1 or -1");
            }

            return(norm);
        }
예제 #2
0
        /**
         * Rounds an element <code>&#955;</code> of <code><b>R</b>[&#964;]</code>
         * to an element of <code><b>Z</b>[&#964;]</code>, such that their difference
         * has minimal norm. <code>&#955;</code> is given as
         * <code>&#955; = &#955;<sub>0</sub> + &#955;<sub>1</sub>&#964;</code>.
         * @param lambda0 The component <code>&#955;<sub>0</sub></code>.
         * @param lambda1 The component <code>&#955;<sub>1</sub></code>.
         * @param mu The parameter <code>&#956;</code> of the elliptic curve. Must
         * equal 1 or -1.
         * @return The rounded element of <code><b>Z</b>[&#964;]</code>.
         * @throws ArgumentException if <code>lambda0</code> and
         * <code>lambda1</code> do not have same scale.
         */
        public static ZTauElement Round(SimpleBigDecimal lambda0,
                                        SimpleBigDecimal lambda1, sbyte mu)
        {
            int scale = lambda0.Scale;

            if (lambda1.Scale != scale)
            {
                throw new ArgumentException("lambda0 and lambda1 do not have same scale");
            }

            if (!((mu == 1) || (mu == -1)))
            {
                throw new ArgumentException("mu must be 1 or -1");
            }

            BigInteger f0 = lambda0.Round();
            BigInteger f1 = lambda1.Round();

            SimpleBigDecimal eta0 = lambda0.Subtract(f0);
            SimpleBigDecimal eta1 = lambda1.Subtract(f1);

            // eta = 2*eta0 + mu*eta1
            SimpleBigDecimal eta = eta0.Add(eta0);

            if (mu == 1)
            {
                eta = eta.Add(eta1);
            }
            else
            {
                // mu == -1
                eta = eta.Subtract(eta1);
            }

            // check1 = eta0 - 3*mu*eta1
            // check2 = eta0 + 4*mu*eta1
            SimpleBigDecimal threeEta1 = eta1.Add(eta1).Add(eta1);
            SimpleBigDecimal fourEta1  = threeEta1.Add(eta1);
            SimpleBigDecimal check1;
            SimpleBigDecimal check2;

            if (mu == 1)
            {
                check1 = eta0.Subtract(threeEta1);
                check2 = eta0.Add(fourEta1);
            }
            else
            {
                // mu == -1
                check1 = eta0.Add(threeEta1);
                check2 = eta0.Subtract(fourEta1);
            }

            sbyte h0 = 0;
            sbyte h1 = 0;

            // if eta >= 1
            if (eta.CompareTo(BigInteger.One) >= 0)
            {
                if (check1.CompareTo(MinusOne) < 0)
                {
                    h1 = mu;
                }
                else
                {
                    h0 = 1;
                }
            }
            else
            {
                // eta < 1
                if (check2.CompareTo(BigInteger.Two) >= 0)
                {
                    h1 = mu;
                }
            }

            // if eta < -1
            if (eta.CompareTo(MinusOne) < 0)
            {
                if (check1.CompareTo(BigInteger.One) >= 0)
                {
                    h1 = (sbyte)-mu;
                }
                else
                {
                    h0 = -1;
                }
            }
            else
            {
                // eta >= -1
                if (check2.CompareTo(MinusTwo) < 0)
                {
                    h1 = (sbyte)-mu;
                }
            }

            BigInteger q0 = f0.Add(BigInteger.ValueOf(h0));
            BigInteger q1 = f1.Add(BigInteger.ValueOf(h1));

            return(new ZTauElement(q0, q1));
        }