org.bouncycastle.crypto.hash2curve.impl

Class GenericSqrtRatioCalculator

java.lang.Object

org.bouncycastle.crypto.hash2curve.impl.GenericSqrtRatioCalculator

All Implemented Interfaces:

SqrtRatioCalculator

public class GenericSqrtRatioCalculator
extends java.lang.Object
implements SqrtRatioCalculator

Generic implementation of the sqrt_ratio(u, v) operation defined in RFC 9380.

This computes a square root of u/v in the prime field Fp associated with an elliptic curve, when such a square root exists, and otherwise returns a valid square root of z·u/v for a fixed quadratic non-residue z. This function is a required component of all map-to-curve constructions in RFC 9380, including the Simplified SWU and Elligator 2 maps.

RFC 9380 defines optimized sqrt_ratio formulas for certain curves where the field prime p satisfies special congruences (e.g. p ≡ 3 mod 4 or p ≡ 5 mod 8). However, those optimizations are curve-specific and do not apply to all hash-to-curve suites. This implementation instead follows the fully generic algorithm from Section 5.6.3 of RFC 9380, which is valid for any elliptic curve defined over a prime field Fp.

This generic version supports all curves used in the RFC 9830 test vectors, including the NIST P-256 / P-384 / P-521 curves, Curve25519, Edwards25519 (Ristretto255), Curve448, and Edwards448 (Decaf448). It provides a single uniform implementation suitable for all supported hash-to-curve suites.

Constructor Summary

Constructors

Constructor and Description
GenericSqrtRatioCalculator(ECCurve curve, java.math.BigInteger z) Constructs a GenericSqrtRatioCalculator instance with the specified elliptic curve and a given value z.

Method Summary

Modifier and Type	Method and Description
SqrtRatio	sqrtRatio(java.math.BigInteger u, java.math.BigInteger v) he sqrtRatio subroutine of hash2Curve in the field F

Methods inherited from class java.lang.Object

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

Constructor Detail

GenericSqrtRatioCalculator

public GenericSqrtRatioCalculator(ECCurve curve,
                                  java.math.BigInteger z)

Constructs a GenericSqrtRatioCalculator instance with the specified elliptic curve and a given value z.

Parameters:

curve - the elliptic curve over a finite field to be used for square root and ratio calculations

z - a non-square element of F

Method Detail

sqrtRatio

public SqrtRatio sqrtRatio(java.math.BigInteger u,
                           java.math.BigInteger v)

he sqrtRatio subroutine of hash2Curve in the field F

Specified by:

sqrtRatio in interface SqrtRatioCalculator

Parameters:

u - u parameter, element of F

v - v parameter, element of F, such that v != 0

Returns:

SqrtRatio result