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Date:   Tue, 22 Nov 2022 11:22:37 +0100 (CET)
From:   Jiri Kosina <jikos@kernel.org>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
cc:     herbert@gondor.apana.org.au, linux-crypto@vger.kernel.org,
        Andrej Shadura <andrew.shadura@collabora.co.uk>
Subject: Re: [PATCH] hwrng: u2fzero - account for high quality RNG
In-Reply-To: <20221119134259.2969204-1-Jason@zx2c4.com>
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References: <20221119134259.2969204-1-Jason@zx2c4.com>
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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