Org.BouncyCastle.Math.EC.FpFieldElement.Square C# (CSharp) Method

		public override ECFieldElement Square()
		{
			return new FpFieldElement(q, x.Multiply(x).Mod(q));
		}

        /**
         * Decode a point on this curve from its ASN.1 encoding. The different
         * encodings are taken account of, including point compression for
         * <code>F<sub>p</sub><code> (X9.62 s 4.2.1 pg 17).
         * @return The decoded point.
         */
        public override ECPoint DecodePoint(
            byte[] encoded)
        {
            ECPoint p = null;

            switch (encoded[0])
            {
            // compressed
            case 0x02:
            case 0x03:
                int    ytilde = encoded[0] & 1;
                byte[] i      = new byte[encoded.Length - 1];

                Array.Copy(encoded, 1, i, 0, i.Length);

                ECFieldElement x     = new FpFieldElement(this.q, new BigInteger(1, i));
                ECFieldElement alpha = x.Multiply(x.Square()).Add(x.Multiply(a).Add(b));
                ECFieldElement beta  = alpha.Sqrt();

                //
                // if we can't find a sqrt we haven't got a point on the
                // curve - run!
                //
                if (beta == null)
                {
                    throw new ArithmeticException("Invalid point compression");
                }

                BigInteger betaValue = beta.ToBigInteger();
                int        bit0      = betaValue.TestBit(0) ? 1 : 0;

                if (bit0 != ytilde)
                {
                    // Use the other root
                    beta = new FpFieldElement(q, q.Subtract(betaValue));
                }

                p = new FpPoint(this, x, beta, true);
                break;

            case 0x04:
                byte[] xEnc = new byte[(encoded.Length - 1) / 2];
                byte[] yEnc = new byte[(encoded.Length - 1) / 2];

                Array.Copy(encoded, 1, xEnc, 0, xEnc.Length);
                Array.Copy(encoded, xEnc.Length + 1, yEnc, 0, yEnc.Length);

                p = new FpPoint(this,
                                new FpFieldElement(this.q, new BigInteger(1, xEnc)),
                                new FpFieldElement(this.q, new BigInteger(1, yEnc)));
                break;

            default:
                throw new FormatException("Invalid point encoding " + encoded[0]);
            }

            return(p);
        }