Creating a Signature

/**
* Takes a Message, an apiKey and apiSecret and uses the HMAC-SHA256
* algorithm to sign a FIX message by appending
* the sending-time sequenceNumber, SenderCompID and TargetCompID with
* \u0001 separator and signing using the
* secret, putting it into the RawBytes in Base64 encoding.
*/
private static void signLogon(final Message message,
                              final SessionID sessionId,
                              final String apiKey,
                              final String apiSecret) throws FieldNotFound {
    final var senderCompId = sessionId.getSenderCompID();
    final var targetCompId = sessionId.getTargetCompID();
    final var sendingTime = message.getString(SendingTime.FIELD);
    final var seqNum = message.getInt(MsgSeqNum.FIELD);
    final var sep = "\u0001";
    final var hmac = sign(apiSecret, sendingTime + sep + seqNum + sep +
            senderCompId + sep + targetCompId);
    message.setString(Password.FIELD, apiKey);
    message.setInt(RawDataLength.FIELD, hmac.length());
    message.setString(RawData.FIELD, hmac);
}

private static String sign(final String apiSecret, final String data) {
    final var mac = HmacUtils.getInitializedMac(HmacAlgorithms.HMAC_SHA_256,
            apiSecret.getBytes());
    final var encodedBytes = mac.doFinal(data.getBytes());
    final var encoder = Base64.getUrlEncoder();//URL Safe Base64
    return encoder.encodeToString(encodedBytes);
}