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Diffstat (limited to 'libraries/spongycastle/core/src/main/java/org/spongycastle/math/ec/WTauNafMultiplier.java')
| -rw-r--r-- | libraries/spongycastle/core/src/main/java/org/spongycastle/math/ec/WTauNafMultiplier.java | 118 |
1 files changed, 118 insertions, 0 deletions
diff --git a/libraries/spongycastle/core/src/main/java/org/spongycastle/math/ec/WTauNafMultiplier.java b/libraries/spongycastle/core/src/main/java/org/spongycastle/math/ec/WTauNafMultiplier.java new file mode 100644 index 000000000..351cfa19b --- /dev/null +++ b/libraries/spongycastle/core/src/main/java/org/spongycastle/math/ec/WTauNafMultiplier.java @@ -0,0 +1,118 @@ +package org.spongycastle.math.ec; + +import java.math.BigInteger; + +/** + * Class implementing the WTNAF (Window + * <code>τ</code>-adic Non-Adjacent Form) algorithm. + */ +public class WTauNafMultiplier extends AbstractECMultiplier +{ + /** + * Multiplies a {@link org.spongycastle.math.ec.ECPoint.F2m ECPoint.F2m} + * by <code>k</code> using the reduced <code>τ</code>-adic NAF (RTNAF) + * method. + * @param p The ECPoint.F2m to multiply. + * @param k The integer by which to multiply <code>k</code>. + * @return <code>p</code> multiplied by <code>k</code>. + */ + protected ECPoint multiplyPositive(ECPoint point, BigInteger k) + { + if (!(point instanceof ECPoint.F2m)) + { + throw new IllegalArgumentException("Only ECPoint.F2m can be " + + "used in WTauNafMultiplier"); + } + + ECPoint.F2m p = (ECPoint.F2m)point; + ECCurve.F2m curve = (ECCurve.F2m)p.getCurve(); + int m = curve.getM(); + byte a = curve.getA().toBigInteger().byteValue(); + byte mu = curve.getMu(); + BigInteger[] s = curve.getSi(); + + ZTauElement rho = Tnaf.partModReduction(k, m, a, s, mu, (byte)10); + + return multiplyWTnaf(p, rho, curve.getPreCompInfo(p), a, mu); + } + + /** + * Multiplies a {@link org.spongycastle.math.ec.ECPoint.F2m ECPoint.F2m} + * by an element <code>λ</code> of <code><b>Z</b>[τ]</code> using + * the <code>τ</code>-adic NAF (TNAF) method. + * @param p The ECPoint.F2m to multiply. + * @param lambda The element <code>λ</code> of + * <code><b>Z</b>[τ]</code> of which to compute the + * <code>[τ]</code>-adic NAF. + * @return <code>p</code> multiplied by <code>λ</code>. + */ + private ECPoint.F2m multiplyWTnaf(ECPoint.F2m p, ZTauElement lambda, + PreCompInfo preCompInfo, byte a, byte mu) + { + ZTauElement[] alpha; + if (a == 0) + { + alpha = Tnaf.alpha0; + } + else + { + // a == 1 + alpha = Tnaf.alpha1; + } + + BigInteger tw = Tnaf.getTw(mu, Tnaf.WIDTH); + + byte[]u = Tnaf.tauAdicWNaf(mu, lambda, Tnaf.WIDTH, + BigInteger.valueOf(Tnaf.POW_2_WIDTH), tw, alpha); + + return multiplyFromWTnaf(p, u, preCompInfo); + } + + /** + * Multiplies a {@link org.spongycastle.math.ec.ECPoint.F2m ECPoint.F2m} + * by an element <code>λ</code> of <code><b>Z</b>[τ]</code> + * using the window <code>τ</code>-adic NAF (TNAF) method, given the + * WTNAF of <code>λ</code>. + * @param p The ECPoint.F2m to multiply. + * @param u The the WTNAF of <code>λ</code>.. + * @return <code>λ * p</code> + */ + private static ECPoint.F2m multiplyFromWTnaf(ECPoint.F2m p, byte[] u, + PreCompInfo preCompInfo) + { + ECCurve.F2m curve = (ECCurve.F2m)p.getCurve(); + byte a = curve.getA().toBigInteger().byteValue(); + + ECPoint.F2m[] pu; + if ((preCompInfo == null) || !(preCompInfo instanceof WTauNafPreCompInfo)) + { + pu = Tnaf.getPreComp(p, a); + curve.setPreCompInfo(p, new WTauNafPreCompInfo(pu)); + } + else + { + pu = ((WTauNafPreCompInfo)preCompInfo).getPreComp(); + } + + // q = infinity + ECPoint.F2m q = (ECPoint.F2m) p.getCurve().getInfinity(); + for (int i = u.length - 1; i >= 0; i--) + { + q = Tnaf.tau(q); + if (u[i] != 0) + { + if (u[i] > 0) + { + q = q.addSimple(pu[u[i]]); + } + else + { + // u[i] < 0 + q = q.subtractSimple(pu[-u[i]]); + } + } + } + + return q; + } +} |