DTLS notifyHandshakeComplete

    SinglePacketTransformer initializeSRTPTransformer(int srtpProtectionProfile, TlsContext tlsContext) {
        logger.debug("initializeSRTPTransformer");
        int cipher, cipher_key_length, cipher_salt_length;
        int auth_function, auth_key_length;
        int RTCP_auth_tag_length, RTP_auth_tag_length;
        // https://groups.google.com/forum/#!searchin/discuss-webrtc/dtls|sort:date/discuss-webrtc/DQnIWm5x5e0/CcRUadIyCAAJ
        switch (srtpProtectionProfile) {
            case SRTPProtectionProfile.SRTP_AES128_CM_HMAC_SHA1_32:
                cipher_key_length = 128 / 8;
                cipher_salt_length = 112 / 8;
                cipher = SRTPPolicy.AESCM_ENCRYPTION;
                auth_function = SRTPPolicy.HMACSHA1_AUTHENTICATION;
                auth_key_length = 160 / 8;
                RTCP_auth_tag_length = 80 / 8;
                RTP_auth_tag_length = 32 / 8;
                break;
            case SRTPProtectionProfile.SRTP_AES128_CM_HMAC_SHA1_80:
                cipher_key_length = 128 / 8;
                cipher_salt_length = 112 / 8;
                cipher = SRTPPolicy.AESCM_ENCRYPTION;
                auth_function = SRTPPolicy.HMACSHA1_AUTHENTICATION;
                auth_key_length = 160 / 8;
                RTCP_auth_tag_length = RTP_auth_tag_length = 80 / 8;
                break;
            case SRTPProtectionProfile.SRTP_NULL_HMAC_SHA1_32:
                cipher_key_length = 0;
                cipher_salt_length = 0;
                cipher = SRTPPolicy.NULL_ENCRYPTION;
                auth_function = SRTPPolicy.HMACSHA1_AUTHENTICATION;
                auth_key_length = 160 / 8;
                RTCP_auth_tag_length = 80 / 8;
                RTP_auth_tag_length = 32 / 8;
                break;
            case SRTPProtectionProfile.SRTP_NULL_HMAC_SHA1_80:
                cipher_key_length = 0;
                cipher_salt_length = 0;
                cipher = SRTPPolicy.NULL_ENCRYPTION;
                auth_function = SRTPPolicy.HMACSHA1_AUTHENTICATION;
                auth_key_length = 160 / 8;
                RTCP_auth_tag_length = RTP_auth_tag_length = 80 / 8;
                break;
            default:
                throw new IllegalArgumentException("srtpProtectionProfile");
        }
        // https://www.bouncycastle.org/docs/tlsdocs1.5on/org/bouncycastle/tls/package-summary.html
        // this may only be called inside notifyHandshakeComplete with BC > 1.55
        byte[] keyingMaterial = tlsContext.exportKeyingMaterial(ExporterLabel.dtls_srtp, null, 2 * (cipher_key_length + cipher_salt_length));
        byte[] client_write_SRTP_master_key = new byte[cipher_key_length];
        byte[] server_write_SRTP_master_key = new byte[cipher_key_length];
        byte[] client_write_SRTP_master_salt = new byte[cipher_salt_length];
        byte[] server_write_SRTP_master_salt = new byte[cipher_salt_length];
        byte[][] keyingMaterialValues = { client_write_SRTP_master_key, server_write_SRTP_master_key, client_write_SRTP_master_salt, server_write_SRTP_master_salt };
        for (int i = 0, keyingMaterialOffset = 0; i < keyingMaterialValues.length; i++) {
            byte[] keyingMaterialValue = keyingMaterialValues[i];
            System.arraycopy(keyingMaterial, keyingMaterialOffset, keyingMaterialValue, 0, keyingMaterialValue.length);
            keyingMaterialOffset += keyingMaterialValue.length;
        }
        SRTPPolicy srtpPolicy = new SRTPPolicy(cipher, cipher_key_length, auth_function, auth_key_length, RTP_auth_tag_length, cipher_salt_length);
        SRTPPolicy srtcpPolicy = new SRTPPolicy(cipher, cipher_key_length, auth_function, auth_key_length, RTCP_auth_tag_length, cipher_salt_length);
        SRTPContextFactory clientSRTPContextFactory = new SRTPContextFactory(tlsContext instanceof TlsClientContext, client_write_SRTP_master_key, client_write_SRTP_master_salt, srtpPolicy, srtcpPolicy);
        SRTPContextFactory serverSRTPContextFactory = new SRTPContextFactory(tlsContext instanceof TlsServerContext, server_write_SRTP_master_key, server_write_SRTP_master_salt, srtpPolicy, srtcpPolicy);
        SRTPContextFactory forwardSRTPContextFactory;
        SRTPContextFactory reverseSRTPContextFactory;
        if (tlsContext instanceof TlsClientContext) {
            forwardSRTPContextFactory = clientSRTPContextFactory;
            reverseSRTPContextFactory = serverSRTPContextFactory;
        } else if (tlsContext instanceof TlsServerContext) {
            forwardSRTPContextFactory = serverSRTPContextFactory;
            reverseSRTPContextFactory = clientSRTPContextFactory;
        } else {
            throw new IllegalArgumentException("tlsContext");
        }
        // handles rtp and rtcp
        return new SRTPTransformer(forwardSRTPContextFactory, reverseSRTPContextFactory);
    }