From 1cceec0ae2fd8324b97e0908c292b92a0e0d4aab Mon Sep 17 00:00:00 2001
From: Jochen Hoenicke <hoenicke@gmail.com>
Date: Tue, 10 Jan 2017 21:51:57 +0100
Subject: [PATCH] Check that U2F key only uses hardened derivation (#139)

We generate only U2F keys with hardened derivation.  However, we
didn't check incoming keys if they used hardened derivation.
This patch fixes this.
---
 firmware/u2f.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/firmware/u2f.c b/firmware/u2f.c
index ddf5602..6c24617 100644
--- a/firmware/u2f.c
+++ b/firmware/u2f.c
@@ -505,6 +505,12 @@ const HDNode *validateKeyHandle(const uint8_t app_id[], const uint8_t key_handle
 	uint32_t key_path[KEY_PATH_ENTRIES];
 	key_path[0] = U2F_KEY_PATH;
 	memcpy(&key_path[1], key_handle, KEY_PATH_LEN);
+	for (unsigned int i = 1; i < KEY_PATH_ENTRIES; i++) {
+		// check high bit for hardened keys
+		if (! (key_path[i] & 0x80000000)) {
+			return NULL;
+		}
+	}
 
 	const HDNode *node = getDerivedNode(key_path, KEY_PATH_ENTRIES);
 	if (!node)