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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.bcpg;

public class AEADUtils
    implements AEADAlgorithmTags
{
    private AEADUtils()
    {
    }

    /**
     * Return the length of the IV used by the given AEAD algorithm in octets.
     *
     * @param aeadAlgorithmTag AEAD algorithm identifier
     * @return length of the IV
     */
    public static int getIVLength(int aeadAlgorithmTag)
    {
        switch (aeadAlgorithmTag)
        {
        case EAX:
            return 16;
        case OCB:
            return 15;
        case GCM:
            return 12;
        default:
            throw new IllegalArgumentException("Invalid AEAD algorithm tag: " + aeadAlgorithmTag);
        }
    }

    /**
     * Return the length of the authentication tag used by the given AEAD algorithm in octets.
     *
     * @param aeadAlgorithmTag AEAD algorithm identifier
     * @return length of the auth tag
     */
    public static int getAuthTagLength(int aeadAlgorithmTag)
    {
        switch (aeadAlgorithmTag)
        {
        case EAX:
        case OCB:
        case GCM:
            return 16;
        default:
            throw new IllegalArgumentException("Invalid AEAD algorithm tag: " + aeadAlgorithmTag);
        }
    }

    /**
     * Split a given byte array containing m
 bytes of key and n-8 bytes of IV into
     * two separate byte arrays.
     * m
 is the key length of the cipher algorithm, while n is the IV length of the AEAD algorithm.
     * Note, that the IV is filled with n-8 bytes only, the remainder is left as 0s.
     * Return an array of both arrays with the key and index 0 and the IV at index 1.
     *
     * @param messageKeyAndIv m+n-8 bytes of concatenated message key and IV
     * @param cipherAlgo      symmetric cipher algorithm
     * @param aeadAlgo        AEAD algorithm
     * @return array of arrays containing message key and IV
     */
    public static byte[][] splitMessageKeyAndIv(byte[] messageKeyAndIv, int cipherAlgo, int aeadAlgo)
    {
        int keyLen = SymmetricKeyUtils.getKeyLengthInOctets(cipherAlgo);
        int ivLen = AEADUtils.getIVLength(aeadAlgo);
        byte[] messageKey = new byte[keyLen];
        byte[] iv = new byte[ivLen];
        System.arraycopy(messageKeyAndIv, 0, messageKey, 0, messageKey.length);
        System.arraycopy(messageKeyAndIv, messageKey.length, iv, 0, ivLen - 8);

        return new byte[][]{messageKey, iv};
    }
}