[PATCH] pre-shared passcode: secure pairing for "no keyboard, no display" devices

From: Pavel Machek
Date: Wed Feb 13 2019 - 17:49:04 EST


Currently, "no keyboard, no display" devices can be paired, but
pairing is not secure against active attacker.

Can we do better? Not for the first pairing; but for the next ones --
yes, I believe we can.

BLE device in this case has internal storage, and Linux running
there. From factory, random 6-digit number is stored in the
flash. Legitimate user knows the number, and system is manipulated so
that pairing passkey will be this pre-shared passkey. After pairing,
user is allowed to change it.

[Or maybe passkey is 000000 from the factory; this is still win for
the user, as long as he can change the key to something random in a
secure cave.]

Fortunately, kernel support for this is rather easy; patch is attached

Does someone see a security issue with proposal above?

What would be suitable interface for setting pre-shared passkey?
Module parameter is really easy.

Signed-off-by: Pavel Machek <pavel@xxxxxxx>

diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c
index 621146d..7a2b06595 100644
--- a/net/bluetooth/smp.c
+++ b/net/bluetooth/smp.c
@@ -2674,6 +2674,11 @@ static u8 sc_select_method(struct smp_chan *smp)
return method;

+static int preshared_passkey = -1;
+module_param(preshared_passkey, int, 0600);
+MODULE_PARM_DESC(preshared_passkey, "Preshared passkey for device w/o keyboard or display");
static int smp_cmd_public_key(struct l2cap_conn *conn, struct sk_buff *skb)
struct smp_cmd_public_key *key = (void *) skb->data;
@@ -2752,9 +2757,11 @@ static int smp_cmd_public_key(struct l2cap_conn *conn, struct sk_buff *skb)
set_bit(SMP_FLAG_DEBUG_KEY, &smp->flags);

if (smp->method == DSP_PASSKEY) {
+ if (preshared_passkey != -1)
+ hcon->passkey_notify = preshared_passkey;
hcon->passkey_notify %= 1000000;
hcon->passkey_entered = 0;
smp->passkey_round = 0;
if (mgmt_user_passkey_notify(hdev, &hcon->dst, hcon->type,

(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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