[messaging] An Insecure Messaging Algorithm

[Jason A. Donenfeld]

Hi folks,

Not sure the best place to ask about this, so I figure I'll give it a stab
here. I've recently come across what appears to be a rather insecure
ciphering system between a computer and a device. I've managed to figure
out how the algorithm works, and my work with it has been successful.
However, I have no idea what this algorithm is, or if it has a name, or
where I can read more about it.

I'm inclined to think its a LFSR, but I'm really not sure.

I've translated it into C-psuedocode. Could anybody here identify it?

Thanks,
Jason


==========
Important parts involve cipher, which updates stored_key1, and the the
updating of stored_key2 when receiving messages.
==========

uint16_t stored_key1;
uint16_t stored_key2;

void cipher(uint8_t *buffer, size_t len)
{
    uint16_t key1 = stored_key1;
    uint16_t key2 = stored_key2;
    uint8_t xor_byte, multiplier;

    while (len--) {
        xor_byte = 0;
        for (int i = 0; i < 4; ++i) {
            multiplier = 2 * xor_byte;
            if (key1 & 1) {
                multiplier |= 1;
                key1 = ((key1 ^ key2) >> 1) | 0x8000;
            } else
                key1 >>= 1;
            xor_byte = 2 * multiplier;
            if (key1 & 0x80)
                xor_byte |= 1;
        }
        *(buffer++) ^= xor_byte;
    }
    stored_key1 = key1;
}

void make_initial_connection_to_device(void)
{
    stored_key1 = (uint16_t)rand();
    stored_key2 = 0xA0CB;

    uint8_t buffer[] = { stored_key1 };
    somehow_send_it_to_the_device(buffer, 1);
    receive_message_from_device();
}


void send_message_to_device(uint8_t *buffer_to_send, size_t len)
{
    cipher(buffer_to_send, len);
    somehow_send_it_to_the_device(buffer_to_send, len);
}

void receive_message_from_device(void)
{
    if (len < 2)
        return;

    uint8_t *buffer_received;
    size_t len;
    somehow_receive_data_from_device(&buffer_received, &len);

    cipher(buffer_received, len);

    uint8_t mutation = *buffer_received;
    ++buffer_received;
    --len;

    stored_key2 = (stored_key2 & 0xFF) | (mutation << 8);

    somehow_do_something_else_with_received_data(buffer_received, len);
}
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