org.bouncycastle.crypto.hash2curve.impl

Class MontgomeryCurveProcessor

java.lang.Object

org.bouncycastle.crypto.hash2curve.impl.MontgomeryCurveProcessor

All Implemented Interfaces:

CurveProcessor

public class MontgomeryCurveProcessor
extends java.lang.Object
implements CurveProcessor

Curve processor for Montgomery curves of the form B * y^2 = x^3 + A * x^2 + x Internally we treat this as a long Weierstrass curve y^2 = x^3 + a2 * x^2 + a4 * x + a6 with a2 = A / B, a4 = 1 / B, a6 = 0. All arithmetic is done explicitly in F_p using these formulas, not via the ECPoint group operations, because BouncyCastle's Montgomery implementation does not use this model directly.

Constructor Summary

Constructors

Constructor and Description
MontgomeryCurveProcessor(ECCurve curve, int J, int K, int hEff) Constructs a MontgomeryCurveProcessor object for processing elliptic curves represented in the Montgomery model.

Method Summary

Modifier and Type	Method and Description
ECPoint	add(ECPoint P, ECPoint Q) Adds two elliptic curve points on the Montgomery curve model and returns the resulting point.
ECPoint	clearCofactor(ECPoint P) Clears the cofactor of the given elliptic curve point using the efficient cofactor value.
AffineXY	mapToAffineXY(ECPoint p) Converts an elliptic-curve point into the affine (x, y) coordinate representation defined by the hash-to-curve suite.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

MontgomeryCurveProcessor

public MontgomeryCurveProcessor(ECCurve curve,
                                int J,
                                int K,
                                int hEff)

Constructs a MontgomeryCurveProcessor object for processing elliptic curves represented in the Montgomery model. Computes and initializes curve parameters for use in point operations and transformations.

Parameters:

curve - the elliptic curve to be processed, represented using the ECCurve class

J - parameter J of the Montgomery curve equation, used for internal calculations

K - parameter K of the Montgomery curve equation, used for internal calculations

hEff - the effective cofactor value for the curve, utilized in certain operations

Method Detail

add

public ECPoint add(ECPoint P,
                   ECPoint Q)

Adds two elliptic curve points on the Montgomery curve model and returns the resulting point. The method internally converts Montgomery coordinates to Weierstrass coordinates to perform the group law, then converts the result back to Montgomery coordinates.

Specified by:

add in interface CurveProcessor

Parameters:

P - the first elliptic curve point on the Montgomery curve model

Q - the second elliptic curve point on the Montgomery curve model

Returns:

the resulting elliptic curve point on the Montgomery curve model after addition

clearCofactor

public ECPoint clearCofactor(ECPoint P)

Clears the cofactor of the given elliptic curve point using the efficient cofactor value. If the input point is at infinity, the same point is returned. Otherwise, it transforms the point into the short-Weierstrass model, multiplies by the effective cofactor, and normalizes the resulting point.

Specified by:

clearCofactor in interface CurveProcessor

Parameters:

P - the elliptic curve point on the Montgomery curve model

Returns:

the resulting elliptic curve point in the short-Weierstrass model with the cofactor cleared

mapToAffineXY

public AffineXY mapToAffineXY(ECPoint p)

Description copied from interface: CurveProcessor

Converts an elliptic-curve point into the affine (x, y) coordinate representation defined by the hash-to-curve suite.

The returned coordinates are intended for serialization, testing, and interoperability with the reference outputs defined in RFC 9380. For most Weierstrass curves, this is simply the affine (x, y) coordinates of the given point. For curves that use a different coordinate model in the specification (e.g. Montgomery curves such as curve25519), this method applies the appropriate coordinate transformation.

This method does not change the underlying group element represented by the point. It only changes how that point is expressed as field elements. The input point is expected to be a valid point on the curve used by the implementation.

Specified by:

mapToAffineXY in interface CurveProcessor

Parameters:

p - a valid elliptic-curve point

Returns:

the affine (x, y) coordinates corresponding to the suite-specific representation of the given point