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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;

import org.bouncycastle.bcpg.AEADUtils;
import org.bouncycastle.bcpg.SymmetricKeyUtils;
import org.bouncycastle.crypto.KDFCalculator;
import org.bouncycastle.crypto.fips.FipsKDF;
import org.bouncycastle.crypto.fips.FipsKDFOperatorFactory;

class AEADUtil
{
    /**
     * Derive a message key and IV from the given session key.
     * The result is a byte array containing the key bytes followed by the IV.
     * To split them, use {@link org.bouncycastle.bcpg.AEADUtils#splitMessageKeyAndIv(byte[], int, int)}.
     *
     * @param aeadAlgo   AEAD algorithm
     * @param cipherAlgo symmetric cipher algorithm
     * @param sessionKey session key
     * @param salt       salt
     * @param hkdfInfo   HKDF info
     * @return message key and appended IV
     */
    static byte[] deriveMessageKeyAndIv(int aeadAlgo, int cipherAlgo, byte[] sessionKey, byte[] salt, byte[] hkdfInfo)
    {
        // Is it okay to have this common logic be implemented using BCs lightweight API?
        // Should we move it to BcAEADUtil instead and also provide a JCE implementation?
        FipsKDFOperatorFactory kdfOpt = new FipsKDF.AgreementOperatorFactory();
        FipsKDF.HKDFKey key = FipsKDF.HKDF_KEY_BUILDER
            .withPrf(FipsKDF.AgreementKDFPRF.SHA256_HMAC)
            .withSalt(salt)
            .build(sessionKey);

        KDFCalculator kdfCalculator = kdfOpt.createKDFCalculator(
            FipsKDF.HKDF.withPRF(key.getPRF()).using(key.getKey()).withIV(hkdfInfo));

        int keyLen = SymmetricKeyUtils.getKeyLengthInOctets(cipherAlgo);
        int ivLen = AEADUtils.getIVLength(aeadAlgo);
        byte[] messageKeyAndIv = new byte[keyLen + ivLen - 8];
        kdfCalculator.generateBytes(messageKeyAndIv, 0, messageKeyAndIv.length);
        return messageKeyAndIv;
    }
}