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Spoof TLS/JA3/JA4 and HTTP/2 fingerprints in Java
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package org.bouncycastle.tls.crypto.impl.jcajce;

import java.math.BigInteger;
import java.security.AlgorithmParameters;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.interfaces.ECKey;
import java.security.interfaces.ECPrivateKey;
import java.security.spec.AlgorithmParameterSpec;
import java.security.spec.ECField;
import java.security.spec.ECFieldF2m;
import java.security.spec.ECFieldFp;
import java.security.spec.ECGenParameterSpec;
import java.security.spec.ECParameterSpec;
import java.security.spec.EllipticCurve;

import org.bouncycastle.math.ec.ECCurve;

class ECUtil
{
    static ECCurve convertCurve(EllipticCurve ec, BigInteger order, int cofactor)
    {
        ECField field = ec.getField();
        BigInteger a = ec.getA();
        BigInteger b = ec.getB();

        if (field instanceof ECFieldFp)
        {
            return new ECCurve.Fp(((ECFieldFp)field).getP(), a, b, order, BigInteger.valueOf(cofactor));
        }
        else
        {
            ECFieldF2m fieldF2m = (ECFieldF2m)field;
            int m = fieldF2m.getM();
            int ks[] = convertMidTerms(fieldF2m.getMidTermsOfReductionPolynomial());
            return new ECCurve.F2m(m, ks[0], ks[1], ks[2], a, b, order, BigInteger.valueOf(cofactor));
        }
    }

    static int[] convertMidTerms(int[] k)
    {
        int[] res = new int[3];

        if (k.length == 1)
        {
            res[0] = k[0];
        }
        else
        {
            if (k.length != 3)
            {
                throw new IllegalArgumentException("Only Trinomials and pentanomials supported");
            }

            if (k[0] < k[1] && k[0] < k[2])
            {
                res[0] = k[0];
                if (k[1] < k[2])
                {
                    res[1] = k[1];
                    res[2] = k[2];
                }
                else
                {
                    res[1] = k[2];
                    res[2] = k[1];
                }
            }
            else if (k[1] < k[2])
            {
                res[0] = k[1];
                if (k[0] < k[2])
                {
                    res[1] = k[0];
                    res[2] = k[2];
                }
                else
                {
                    res[1] = k[2];
                    res[2] = k[0];
                }
            }
            else
            {
                res[0] = k[2];
                if (k[0] < k[1])
                {
                    res[1] = k[0];
                    res[2] = k[1];
                }
                else
                {
                    res[1] = k[1];
                    res[2] = k[0];
                }
            }
        }

        return res;
    }

    static AlgorithmParameterSpec createInitSpec(String curveName)
    {
        return new ECGenParameterSpec(curveName);
    }

    static AlgorithmParameters getAlgorithmParameters(JcaTlsCrypto crypto, String curveName)
    {
        return getAlgorithmParameters(crypto, new ECGenParameterSpec(curveName));
    }

    static AlgorithmParameters getAlgorithmParameters(JcaTlsCrypto crypto, AlgorithmParameterSpec initSpec)
    {
        try
        {
            AlgorithmParameters ecAlgParams = crypto.getHelper().createAlgorithmParameters("EC");
            ecAlgParams.init(initSpec);

            ECParameterSpec ecSpec = ecAlgParams.getParameterSpec(ECParameterSpec.class);
            if (null != ecSpec)
            {
                return ecAlgParams;
            }
        }
        catch (AssertionError e)
        {
        }
        catch (Exception e)
        {
        }

        return null;
    }

    static ECParameterSpec getECParameterSpec(JcaTlsCrypto crypto, String curveName)
    {
        return getECParameterSpec(crypto, createInitSpec(curveName));
    }

    static ECParameterSpec getECParameterSpec(JcaTlsCrypto crypto, AlgorithmParameterSpec initSpec)
    {
        KeyPairGenerator kpGen;
        try
        {
            kpGen = crypto.getHelper().createKeyPairGenerator("EC");
            kpGen.initialize(initSpec, crypto.getSecureRandom());
        }
        catch (AssertionError e)
        {
            return null;
        }
        catch (Exception e)
        {
            return null;
        }

        // Try the "modern" way
        try
        {
            AlgorithmParameters ecAlgParams = crypto.getHelper().createAlgorithmParameters("EC");
            ecAlgParams.init(initSpec);

            ECParameterSpec ecSpec = ecAlgParams.getParameterSpec(ECParameterSpec.class);
            if (null != ecSpec)
            {
                return ecSpec;
            }
        }
        catch (AssertionError e)
        {
        }
        catch (Exception e)
        {
        }

        /*
         * Try a more round about way (the IBM JCE is an example of this).
         * 
         * NOTE: For these providers, we will not be able to provide an AlgorithmParameters object
         * to BCJSSE for use with AlgorithmConstraints checks, so curve constraints will not work.
         */
        try
        {
            KeyPair kp = kpGen.generateKeyPair();
            return ((ECKey)kp.getPrivate()).getParams();
        }
        catch (AssertionError e)
        {
        }
        catch (Exception e)
        {
        }

        return null;
    }

    static boolean isECPrivateKey(PrivateKey key)
    {
        return key instanceof ECPrivateKey || "EC".equalsIgnoreCase(key.getAlgorithm());
    }

    static boolean isCurveSupported(JcaTlsCrypto crypto, String curveName)
    {
        return null != curveName && isCurveSupported(crypto, new ECGenParameterSpec(curveName));
    }

    static boolean isCurveSupported(JcaTlsCrypto crypto, ECGenParameterSpec initSpec)
    {
        return null != getECParameterSpec(crypto, initSpec);
    }
}