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.
--
Jiri Kosina
SUSE Labs
