Java tutorial
package org.bouncycastle.math.ec.custom.sec; import java.math.BigInteger; import org.bouncycastle.math.ec.AbstractECLookupTable; import org.bouncycastle.math.ec.ECConstants; import org.bouncycastle.math.ec.ECCurve; import org.bouncycastle.math.ec.ECFieldElement; import org.bouncycastle.math.ec.ECLookupTable; import org.bouncycastle.math.ec.ECPoint; import org.bouncycastle.math.raw.Nat256; import org.bouncycastle.util.encoders.Hex; public class SecP256K1Curve extends ECCurve.AbstractFp { public static final BigInteger q = SecP256K1FieldElement.Q; private static final int SECP256K1_DEFAULT_COORDS = COORD_JACOBIAN; private static final ECFieldElement[] SECP256K1_AFFINE_ZS = new ECFieldElement[] { new SecP256K1FieldElement(ECConstants.ONE) }; protected SecP256K1Point infinity; public SecP256K1Curve() { super(q); this.infinity = new SecP256K1Point(this, null, null); this.a = fromBigInteger(ECConstants.ZERO); this.b = fromBigInteger(BigInteger.valueOf(7)); this.order = new BigInteger(1, Hex.decodeStrict("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141")); this.cofactor = BigInteger.valueOf(1); this.coord = SECP256K1_DEFAULT_COORDS; } protected ECCurve cloneCurve() { return new SecP256K1Curve(); } public boolean supportsCoordinateSystem(int coord) { switch (coord) { case COORD_JACOBIAN: return true; default: return false; } } public BigInteger getQ() { return q; } public int getFieldSize() { return q.bitLength(); } public ECFieldElement fromBigInteger(BigInteger x) { return new SecP256K1FieldElement(x); } protected ECPoint createRawPoint(ECFieldElement x, ECFieldElement y) { return new SecP256K1Point(this, x, y); } protected ECPoint createRawPoint(ECFieldElement x, ECFieldElement y, ECFieldElement[] zs) { return new SecP256K1Point(this, x, y, zs); } public ECPoint getInfinity() { return infinity; } public ECLookupTable createCacheSafeLookupTable(ECPoint[] points, int off, final int len) { final int FE_INTS = 8; final int[] table = new int[len * FE_INTS * 2]; { int pos = 0; for (int i = 0; i < len; ++i) { ECPoint p = points[off + i]; Nat256.copy(((SecP256K1FieldElement) p.getRawXCoord()).x, 0, table, pos); pos += FE_INTS; Nat256.copy(((SecP256K1FieldElement) p.getRawYCoord()).x, 0, table, pos); pos += FE_INTS; } } return new AbstractECLookupTable() { public int getSize() { return len; } public ECPoint lookup(int index) { int[] x = Nat256.create(), y = Nat256.create(); int pos = 0; for (int i = 0; i < len; ++i) { int MASK = ((i ^ index) - 1) >> 31; for (int j = 0; j < FE_INTS; ++j) { x[j] ^= table[pos + j] & MASK; y[j] ^= table[pos + FE_INTS + j] & MASK; } pos += (FE_INTS * 2); } return createPoint(x, y); } public ECPoint lookupVar(int index) { int[] x = Nat256.create(), y = Nat256.create(); int pos = index * FE_INTS * 2; for (int j = 0; j < FE_INTS; ++j) { x[j] = table[pos + j]; y[j] = table[pos + FE_INTS + j]; } return createPoint(x, y); } private ECPoint createPoint(int[] x, int[] y) { return createRawPoint(new SecP256K1FieldElement(x), new SecP256K1FieldElement(y), SECP256K1_AFFINE_ZS); } }; } }