On Tue, Nov 22, 2022 at 11:22:37AM +0100, Jiri Kosina wrote:
> On Sat, 19 Nov 2022, Jason A. Donenfeld wrote:
>
> > The U2F zero apparently has a real TRNG in it with maximum quality, not
> > one with quality of "1", which was likely a misinterpretation of the
> > field as a boolean. So remove the assignment entirely, so that we get
> > the default quality setting.
> >
> > In the u2f-zero firmware, the 0x21 RNG command used by this driver is
> > handled as such [1]:
> >
> > case U2F_CUSTOM_GET_RNG:
> > if (atecc_send_recv(ATECC_CMD_RNG,ATECC_RNG_P1,ATECC_RNG_P2,
> > NULL, 0,
> > appdata.tmp,
> > sizeof(appdata.tmp), &res) == 0 )
> > {
> > memmove(msg->pkt.init.payload, res.buf, 32);
> > U2FHID_SET_LEN(msg, 32);
> > usb_write((uint8_t*)msg, 64);
> > }
> > else
> > {
> > U2FHID_SET_LEN(msg, 0);
> > usb_write((uint8_t*)msg, 64);
> > }
> >
> > This same call to `atecc_send_recv(ATECC_CMD_RNG,ATECC_RNG_P1,
> > ATECC_RNG_P2,...)` is then also used in the token's cryptographically
> > critical "u2f_new_keypair" function, as its rather straightforward
> > source of random bytes [2]:
> >
> > int8_t u2f_new_keypair(uint8_t * handle, uint8_t * appid, uint8_t * pubkey)
> > {
> > struct atecc_response res;
> > uint8_t private_key[36];
> > int i;
> >
> > watchdog();
> >
> > if (atecc_send_recv(ATECC_CMD_RNG,ATECC_RNG_P1,ATECC_RNG_P2,
> > NULL, 0,
> > appdata.tmp,
> > sizeof(appdata.tmp), &res) != 0 )
> > {
> > return -1;
> > }
> >
> > So it seems rather plain that the ATECC RNG is considered to provide
> > good random numbers.
> >
> > [1] https://github.com/conorpp/u2f-zero/blob/master/firmware/src/custom.c
> > [2] https://github.com/conorpp/u2f-zero/blob/master/firmware/src/u2f_atecc.c
>
> Good catch, thanks Jason. Applied.
This should probably go via Herbert's tree, because it depends on some
changed handling for the zero quality field.
Jason
