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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.ByteArrayOutputStream;
import java.io.IOException;

import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.edec.EdECObjectIdentifiers;
import org.bouncycastle.asn1.nist.NISTObjectIdentifiers;
import org.bouncycastle.asn1.ntt.NTTObjectIdentifiers;
import org.bouncycastle.bcpg.ECDHPublicBCPGKey;
import org.bouncycastle.bcpg.HashAlgorithmTags;
import org.bouncycastle.bcpg.PublicKeyPacket;
import org.bouncycastle.bcpg.SymmetricKeyAlgorithmTags;
import org.bouncycastle.bcpg.X25519PublicBCPGKey;
import org.bouncycastle.bcpg.X448PublicBCPGKey;
import org.bouncycastle.openpgp.PGPException;
import org.bouncycastle.util.encoders.Hex;


public class RFC6637Utils
{
    private RFC6637Utils()
    {

    }

    // "Anonymous Sender    ", which is the octet sequence
    private static final byte[] ANONYMOUS_SENDER = Hex.decode("416E6F6E796D6F75732053656E64657220202020");

    public static String getXDHAlgorithm(PublicKeyPacket pubKeyData)
    {
        if (pubKeyData.getKey() instanceof X25519PublicBCPGKey)
        {
            return "X25519withSHA256CKDF";
        }
        else if (pubKeyData.getKey() instanceof X448PublicBCPGKey)
        {
            return "X448withSHA512CKDF";
        }
        ECDHPublicBCPGKey ecKey = (ECDHPublicBCPGKey)pubKeyData.getKey();
        String curve = ecKey.getCurveOID().equals(EdECObjectIdentifiers.id_X448) ? "X448" : "X25519";
        switch (ecKey.getHashAlgorithm())
        {
        case HashAlgorithmTags.SHA256:
            return curve + "withSHA256CKDF";
        case HashAlgorithmTags.SHA384:
            return curve + "withSHA384CKDF";
        case HashAlgorithmTags.SHA512:
            return curve + "withSHA512CKDF";
        default:
            throw new IllegalArgumentException("Unknown hash algorithm specified: " + ecKey.getHashAlgorithm());
        }
    }

    public static String getAgreementAlgorithm(PublicKeyPacket pubKeyData)
    {
        ECDHPublicBCPGKey ecKey = (ECDHPublicBCPGKey)pubKeyData.getKey();

        switch (ecKey.getHashAlgorithm())
        {
        case HashAlgorithmTags.SHA256:
            return "ECCDHwithSHA256CKDF";
        case HashAlgorithmTags.SHA384:
            return "ECCDHwithSHA384CKDF";
        case HashAlgorithmTags.SHA512:
            return "ECCDHwithSHA512CKDF";
        default:
            throw new IllegalArgumentException("Unknown hash algorithm specified: " + ecKey.getHashAlgorithm());
        }
    }


    public static ASN1ObjectIdentifier getKeyEncryptionOID(int algID)
        throws PGPException
    {
        switch (algID)
        {
        case SymmetricKeyAlgorithmTags.AES_128:
            return NISTObjectIdentifiers.id_aes128_wrap;
        case SymmetricKeyAlgorithmTags.AES_192:
            return NISTObjectIdentifiers.id_aes192_wrap;
        case SymmetricKeyAlgorithmTags.AES_256:
            return NISTObjectIdentifiers.id_aes256_wrap;
        //RFC3657
        case SymmetricKeyAlgorithmTags.CAMELLIA_128:
            return NTTObjectIdentifiers.id_camellia128_wrap;
        case SymmetricKeyAlgorithmTags.CAMELLIA_192:
            return NTTObjectIdentifiers.id_camellia192_wrap;
        case SymmetricKeyAlgorithmTags.CAMELLIA_256:
            return NTTObjectIdentifiers.id_camellia256_wrap;
        default:
            throw new PGPException("unknown symmetric algorithm ID: " + algID);
        }
    }

    // RFC 6637 - Section 8
    // curve_OID_len = (byte)len(curve_OID);
    // Param = curve_OID_len || curve_OID || public_key_alg_ID || 03
    // || 01 || KDF_hash_ID || KEK_alg_ID for AESKeyWrap || "Anonymous
    // Sender    " || recipient_fingerprint;
    // Z_len = the key size for the KEK_alg_ID used with AESKeyWrap
    // Compute Z = KDF( S, Z_len, Param );
    public static byte[] createUserKeyingMaterial(PublicKeyPacket pubKeyData, KeyFingerPrintCalculator fingerPrintCalculator)
        throws IOException, PGPException
    {
        ByteArrayOutputStream pOut = new ByteArrayOutputStream();

        ECDHPublicBCPGKey ecKey = (ECDHPublicBCPGKey)pubKeyData.getKey();
        byte[] encOid = ecKey.getCurveOID().getEncoded();

        pOut.write(encOid, 1, encOid.length - 1);
        pOut.write(pubKeyData.getAlgorithm());
        pOut.write(0x03);
        pOut.write(0x01);
        pOut.write(ecKey.getHashAlgorithm());
        pOut.write(ecKey.getSymmetricKeyAlgorithm());
        pOut.write(ANONYMOUS_SENDER);
        byte[] fp = fingerPrintCalculator.calculateFingerprint(pubKeyData);
        if (pubKeyData.getVersion() == PublicKeyPacket.LIBREPGP_5)
        {
            pOut.write(fp, 0, 20);
        }
        else
        {
            pOut.write(fp);
        }
        return pOut.toByteArray();
    }

}