From 83478938f78fd640c72af83d739a90c840e1b876 Mon Sep 17 00:00:00 2001 From: "Jason A. Donenfeld" Date: Sat, 19 Nov 2022 14:42:59 +0100 Subject: [PATCH] hwrng: u2fzero - account for high quality RNG 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 Cc: Andrej Shadura Cc: Jiri Kosina Cc: Herbert Xu Signed-off-by: Jason A. Donenfeld Acked-by: Andrej Shadura Signed-off-by: Herbert Xu --- drivers/hid/hid-u2fzero.c | 1 - 1 file changed, 1 deletion(-) diff --git a/drivers/hid/hid-u2fzero.c b/drivers/hid/hid-u2fzero.c index ad489ca..744a91e 100644 --- a/drivers/hid/hid-u2fzero.c +++ b/drivers/hid/hid-u2fzero.c @@ -261,7 +261,6 @@ static int u2fzero_init_hwrng(struct u2fzero_device *dev, dev->hwrng.name = dev->rng_name; dev->hwrng.read = u2fzero_rng_read; - dev->hwrng.quality = 1; return devm_hwrng_register(&dev->hdev->dev, &dev->hwrng); } -- 2.7.4