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The Bouncy Castle Java APIs for the OpenPGP Protocol. The APIs are designed primarily to be used in conjunction with the BC LTS provider but may also be used with other providers providing cryptographic services.
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package org.bouncycastle.openpgp.operator;

import java.io.IOException;
import java.io.OutputStream;

import org.bouncycastle.bcpg.HashAlgorithmTags;
import org.bouncycastle.bcpg.S2K;
import org.bouncycastle.bcpg.SymmetricKeyAlgorithmTags;
import org.bouncycastle.crypto.generators.Argon2BytesGenerator;
import org.bouncycastle.crypto.params.Argon2Parameters;
import org.bouncycastle.openpgp.PGPException;
import org.bouncycastle.util.Strings;

/**
 * Basic utility class
 */
class PGPUtil
    implements HashAlgorithmTags
{
    static byte[] makeKeyFromPassPhrase(
        PGPDigestCalculator digestCalculator,
        int algorithm,
        S2K s2k,
        char[] passPhrase)
        throws PGPException
    {
        int keySize;

        switch (algorithm)
        {
        case SymmetricKeyAlgorithmTags.TRIPLE_DES:
        case SymmetricKeyAlgorithmTags.AES_192:
        case SymmetricKeyAlgorithmTags.CAMELLIA_192:
            keySize = 192;
            break;
        case SymmetricKeyAlgorithmTags.IDEA:
        case SymmetricKeyAlgorithmTags.CAST5:
        case SymmetricKeyAlgorithmTags.BLOWFISH:
        case SymmetricKeyAlgorithmTags.SAFER:
        case SymmetricKeyAlgorithmTags.AES_128:
        case SymmetricKeyAlgorithmTags.CAMELLIA_128:
            keySize = 128;
            break;
        case SymmetricKeyAlgorithmTags.DES:
            keySize = 64;
            break;
        case SymmetricKeyAlgorithmTags.AES_256:
        case SymmetricKeyAlgorithmTags.TWOFISH:
        case SymmetricKeyAlgorithmTags.CAMELLIA_256:
            keySize = 256;
            break;
        default:
            throw new PGPException("unknown symmetric algorithm: " + algorithm);
        }

        byte[] pBytes = Strings.toUTF8ByteArray(passPhrase);
        byte[] keyBytes = new byte[(keySize + 7) / 8];

        int generatedBytes = 0;
        int loopCount = 0;

        if (s2k != null)
        {
            if (s2k.getType() == S2K.ARGON_2)
            {
                Argon2Parameters.Builder builder = new Argon2Parameters
                    .Builder(Argon2Parameters.ARGON2_id)
                    .withSalt(s2k.getIV())
                    .withIterations(s2k.getPasses())
                    .withParallelism(s2k.getParallelism())
                    .withMemoryPowOfTwo(s2k.getMemorySizeExponent())
                    .withVersion(Argon2Parameters.ARGON2_VERSION_13);

                Argon2BytesGenerator argon2 = new Argon2BytesGenerator();
                argon2.init(builder.build());
                argon2.generateBytes(passPhrase, keyBytes);

                return keyBytes;
            }
            else if (s2k.getHashAlgorithm() != digestCalculator.getAlgorithm())
            {
                throw new PGPException("s2k/digestCalculator mismatch");
            }
        }
        else
        {
            if (digestCalculator.getAlgorithm() != HashAlgorithmTags.MD5)
            {
                throw new PGPException("digestCalculator not for MD5");
            }
        }

        OutputStream dOut = digestCalculator.getOutputStream();

        try
        {
            byte[] iv = s2k != null? s2k.getIV() : null;
            while (generatedBytes < keyBytes.length)
            {
                if (s2k != null)
                {
                    for (int i = 0; i != loopCount; i++)
                    {
                        dOut.write(0);
                    }

                    switch (s2k.getType())
                    {
                    case S2K.SIMPLE:
                        dOut.write(pBytes);
                        break;
                    case S2K.SALTED:
                        dOut.write(iv);
                        dOut.write(pBytes);
                        break;
                    case S2K.SALTED_AND_ITERATED:
                        long count = s2k.getIterationCount();
                        dOut.write(iv);
                        dOut.write(pBytes);

                        count -= iv.length + pBytes.length;

                        while (count > 0)
                        {
                            if (count < iv.length)
                            {
                                dOut.write(iv, 0, (int)count);
                                break;
                            }
                            else
                            {
                                dOut.write(iv);
                                count -= iv.length;
                            }

                            if (count < pBytes.length)
                            {
                                dOut.write(pBytes, 0, (int)count);
                                count = 0;
                            }
                            else
                            {
                                dOut.write(pBytes);
                                count -= pBytes.length;
                            }
                        }
                        break;
                    default:
                        throw new PGPException("unknown S2K type: " + s2k.getType());
                    }
                }
                else
                {
                    for (int i = 0; i != loopCount; i++)
                    {
                        dOut.write((byte)0);
                    }

                    dOut.write(pBytes);
                }

                dOut.close();

                byte[] dig = digestCalculator.getDigest();

                if (dig.length > (keyBytes.length - generatedBytes))
                {
                    System.arraycopy(dig, 0, keyBytes, generatedBytes, keyBytes.length - generatedBytes);
                }
                else
                {
                    System.arraycopy(dig, 0, keyBytes, generatedBytes, dig.length);
                }

                generatedBytes += dig.length;

                loopCount++;
            }
        }
        catch (IOException e)
        {
            throw new PGPException("exception calculating digest: " + e.getMessage(), e);
        }

        for (int i = 0; i != pBytes.length; i++)
        {
            pBytes[i] = 0;
        }

        return keyBytes;
    }

    public static byte[] makeKeyFromPassPhrase(
        PGPDigestCalculatorProvider digCalcProvider,
        int algorithm,
        S2K s2k,
        char[] passPhrase)
        throws PGPException
    {
        PGPDigestCalculator digestCalculator;

        if (s2k != null && s2k.getType() != S2K.ARGON_2)
        {
            digestCalculator = digCalcProvider.get(s2k.getHashAlgorithm());
        }
        else
        {
            digestCalculator = digCalcProvider.get(HashAlgorithmTags.MD5);
        }

        return makeKeyFromPassPhrase(digestCalculator, algorithm, s2k, passPhrase);
    }
}