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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.ByteArrayOutputStream;
import java.io.IOException;

import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.nist.NISTObjectIdentifiers;
import org.bouncycastle.bcpg.ECDHPublicBCPGKey;
import org.bouncycastle.bcpg.HashAlgorithmTags;
import org.bouncycastle.bcpg.PublicKeyPacket;
import org.bouncycastle.bcpg.SymmetricKeyAlgorithmTags;
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)
    {
        ECDHPublicBCPGKey ecKey = (ECDHPublicBCPGKey)pubKeyData.getKey();

        switch (ecKey.getHashAlgorithm())
        {
        case HashAlgorithmTags.SHA256:
            return "X25519withSHA256CKDF";
        case HashAlgorithmTags.SHA384:
            return "X25519withSHA384CKDF";
        case HashAlgorithmTags.SHA512:
            return "X25519withSHA512CKDF";
        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;
        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);
        pOut.write(fingerPrintCalculator.calculateFingerprint(pubKeyData));

        return pOut.toByteArray();
    }
}