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The Bouncy Castle Java APIs for TLS and DTLS.
There is a newer version: 1.78.1
package org.bouncycastle.tls.crypto.impl.jcajce;

import java.io.IOException;
import java.security.AlgorithmParameters;
import java.security.GeneralSecurityException;
import java.security.spec.AlgorithmParameterSpec;

import org.bouncycastle.asn1.ASN1Integer;
import org.bouncycastle.asn1.DERNull;
import org.bouncycastle.asn1.nist.NISTObjectIdentifiers;
import org.bouncycastle.asn1.pkcs.PKCSObjectIdentifiers;
import org.bouncycastle.asn1.pkcs.RSASSAPSSparams;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.jcajce.util.JcaJceHelper;
import org.bouncycastle.tls.crypto.CryptoHashAlgorithm;
import org.bouncycastle.tls.crypto.TlsCryptoUtils;

class RSAUtil
{
    static String getDigestSigAlgName(
        String name)
    {
        int dIndex = name.indexOf('-');
        if (dIndex > 0 && !name.startsWith("SHA3"))
        {
            return name.substring(0, dIndex) + name.substring(dIndex + 1);
        }

        return name;
    }

    static AlgorithmParameterSpec getPSSParameterSpec(int cryptoHashAlgorithm, String digestName, JcaJceHelper helper)
    {
        int saltLength = TlsCryptoUtils.getHashOutputSize(cryptoHashAlgorithm);

        // Used where providers can't handle PSSParameterSpec properly.
        AlgorithmIdentifier hashAlg = getHashAlgorithmID(cryptoHashAlgorithm);

        RSASSAPSSparams pssParams = new RSASSAPSSparams(
            hashAlg,
            new AlgorithmIdentifier(PKCSObjectIdentifiers.id_mgf1, hashAlg),
            new ASN1Integer(saltLength),
            RSASSAPSSparams.DEFAULT_TRAILER_FIELD);

        try
        {
            AlgorithmParameters params = helper.createAlgorithmParameters("PSS");

            params.init(pssParams.getEncoded(), "ASN.1");

            return params.getParameterSpec(AlgorithmParameterSpec.class);
        }
        catch (IOException e)
        {   // this should never happen!
            throw new IllegalStateException("cannot encode RSASSAPSSparams: " + e.getMessage());
        }
        catch (GeneralSecurityException e)
        {
            throw new IllegalStateException("cannot recover PSS paramSpec: " + e.getMessage());
        }

//        MGF1ParameterSpec mgf1Spec = new MGF1ParameterSpec(digestName);
//        return new PSSParameterSpec(digestName, "MGF1", mgf1Spec, saltLength, 1);
    }

    private static AlgorithmIdentifier getHashAlgorithmID(int cryptoHashAlgorithm)
    {
        switch (cryptoHashAlgorithm)
        {
        case CryptoHashAlgorithm.sha256:
            return new AlgorithmIdentifier(NISTObjectIdentifiers.id_sha256, DERNull.INSTANCE);
        case CryptoHashAlgorithm.sha384:
            return new AlgorithmIdentifier(NISTObjectIdentifiers.id_sha384, DERNull.INSTANCE);
        case CryptoHashAlgorithm.sha512:
            return new AlgorithmIdentifier(NISTObjectIdentifiers.id_sha512, DERNull.INSTANCE);
        default:
            return null;
        }
    }
}