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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 FIPS provider. The APIs may also be used with other providers although if being used in a FIPS context it is the responsibility of the user to ensure that any other providers used are FIPS certified and used appropriately.

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package org.bouncycastle.openpgp.operator;

import java.io.IOException;
import java.math.BigInteger;

import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.DEROctetString;
import org.bouncycastle.asn1.cryptlib.CryptlibObjectIdentifiers;
import org.bouncycastle.asn1.edec.EdECObjectIdentifiers;
import org.bouncycastle.asn1.pkcs.PrivateKeyInfo;
import org.bouncycastle.asn1.sec.SECObjectIdentifiers;
import org.bouncycastle.asn1.teletrust.TeleTrusTObjectIdentifiers;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.bcpg.HashAlgorithmTags;
import org.bouncycastle.bcpg.SymmetricKeyAlgorithmTags;
import org.bouncycastle.openpgp.PGPAlgorithmParameters;
import org.bouncycastle.openpgp.PGPKdfParameters;
import org.bouncycastle.util.BigIntegers;

public abstract class PGPKeyConverter
{
    protected PGPKeyConverter()
    {
        
    }

    /**
     * Reference: RFC Draft-ietf-openpgp-crypto-refresh-13
     * 
     * This class provides information about the recommended algorithms to use
     * depending on the key version and curve type in OpenPGP keys.
     *
     * 

     * For OpenPGP keys using the specified curves, the following algorithms are recommended:
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     * Recommended Algorithms for OpenPGP KeysCurve Hash Algorithm Symmetric Algorithm
NIST P-256 SHA2-256 AES-128
NIST P-384 SHA2-384 AES-192
NIST P-521 SHA2-512 AES-256
brainpoolP256r1 SHA2-256 AES-128
brainpoolP384r1 SHA2-384 AES-192
brainpoolP512r1 SHA2-512 AES-256
Curve25519Legacy SHA2-256 AES-128
Curve448Legacy (not in RFC Draft) SHA2-512 AES-256
     */
    protected PGPKdfParameters implGetKdfParameters(ASN1ObjectIdentifier curveID, PGPAlgorithmParameters algorithmParameters)
    {
        if (null == algorithmParameters)
        {
            if (curveID.equals(SECObjectIdentifiers.secp256r1) || curveID.equals(TeleTrusTObjectIdentifiers.brainpoolP256r1)
                || curveID.equals(CryptlibObjectIdentifiers.curvey25519) || curveID.equals(EdECObjectIdentifiers.id_X25519))
            {
                return new PGPKdfParameters(HashAlgorithmTags.SHA256, SymmetricKeyAlgorithmTags.AES_128);
            }
            else if (curveID.equals(SECObjectIdentifiers.secp384r1) || curveID.equals(TeleTrusTObjectIdentifiers.brainpoolP384r1))
            {
                return new PGPKdfParameters(HashAlgorithmTags.SHA384, SymmetricKeyAlgorithmTags.AES_192);
            }
            else if (curveID.equals(SECObjectIdentifiers.secp521r1) || curveID.equals(TeleTrusTObjectIdentifiers.brainpoolP512r1)
                || curveID.equals(EdECObjectIdentifiers.id_X448))
            {
                return new PGPKdfParameters(HashAlgorithmTags.SHA512, SymmetricKeyAlgorithmTags.AES_256);
            }
            else
            {
                throw new IllegalArgumentException("unknown curve");
            }
        }
        return (PGPKdfParameters)algorithmParameters;
    }

    protected PrivateKeyInfo getPrivateKeyInfo(ASN1ObjectIdentifier algorithm, int keySize, byte[] key)
        throws IOException
    {
        return (new PrivateKeyInfo(new AlgorithmIdentifier(algorithm),
            new DEROctetString(BigIntegers.asUnsignedByteArray(keySize, new BigInteger(1, key)))));
    }

    protected PrivateKeyInfo getPrivateKeyInfo(ASN1ObjectIdentifier algorithm, byte[] key)
        throws IOException
    {
        return (new PrivateKeyInfo(new AlgorithmIdentifier(algorithm), new DEROctetString(key)));
    }
}

Curve	Hash Algorithm	Symmetric Algorithm
NIST P-256	SHA2-256	AES-128
NIST P-384	SHA2-384	AES-192
NIST P-521	SHA2-512	AES-256
brainpoolP256r1	SHA2-256	AES-128
brainpoolP384r1	SHA2-384	AES-192
brainpoolP512r1	SHA2-512	AES-256
Curve25519Legacy	SHA2-256	AES-128
Curve448Legacy (not in RFC Draft)	SHA2-512	AES-256