[messaging] An Insecure Messaging Algorithm

[Trevor Perrin]

On Thu, Oct 30, 2014 at 2:30 PM, Jason A. Donenfeld <Jason at zx2c4.com> wrote:
> Hi folks,
>
> Not sure the best place to ask about this,


Seems like a general crypto question, I'd recommend:

http://lists.randombit.net/mailman/listinfo/cryptography
http://www.metzdowd.com/mailman/listinfo/cryptography


Trevor



> 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; // Fixed!
>
> 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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