fpsensor: Encrypt the FP templates with HW key

We use AES-GCM crypto to encrypt the FP templates between the AP and the
MCU.
For every single template, we generate a nonce/salt that is updated on
every template update (i.e. when the AP requests an encrypted template).
We then derive the symmetric key using HMAC-SHA256 and encrypt/decrypt
the template.

Design doc at go/cros-fp-dd

BUG=b:73337313
BRANCH=nocturne
TEST=enroll a finger, log out, log back in, unlock the device.
TEST=retrieve MQT frame on unlocked system

Change-Id: I29c66c6dc660242a423af02066f5aba671613300
Signed-off-by: Nicolas Norvez <norvez@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1194999
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/1364318
Reviewed-by: YH Lin <yueherngl@chromium.org>
Commit-Queue: YH Lin <yueherngl@chromium.org>
Tested-by: YH Lin <yueherngl@chromium.org>

1 files changed, 20 insertions, 10 deletions

diff --git a/include/ec_commands.h b/include/ec_commands.h
index fe72340fb8..2000afd52a 100644
--- a/include/ec_commands.h
+++ b/include/ec_commands.h
@@ -4987,10 +4987,29 @@ struct __ec_align4 ec_response_fp_info {
 
 /* constants defining the 'offset' field which also contains the frame index */
 #define FP_FRAME_INDEX_SHIFT       28
+/* Frame buffer where the captured image is stored */
 #define FP_FRAME_INDEX_RAW_IMAGE    0
-#define FP_FRAME_TEMPLATE_INDEX(offset) ((offset) >> FP_FRAME_INDEX_SHIFT)
+/* First frame buffer holding a template */
+#define FP_FRAME_INDEX_TEMPLATE     1
+#define FP_FRAME_GET_BUFFER_INDEX(offset) ((offset) >> FP_FRAME_INDEX_SHIFT)
 #define FP_FRAME_OFFSET_MASK       0x0FFFFFFF
 
+/* Constants for encryption parameters */
+#define FP_CONTEXT_NONCE_BYTES 12
+#define FP_CONTEXT_USERID_WORDS (32 / sizeof(uint32_t))
+#define FP_CONTEXT_TAG_BYTES 16
+#define FP_CONTEXT_SALT_BYTES 16
+
+struct ec_fp_template_encryption_metadata {
+	/*
+	 * The salt is *only* ever used for key derivation. The nonce is unique,
+	 * a different one is used for every message.
+	 */
+	uint8_t nonce[FP_CONTEXT_NONCE_BYTES];
+	uint8_t salt[FP_CONTEXT_SALT_BYTES];
+	uint8_t tag[FP_CONTEXT_TAG_BYTES];
+};
+
 struct __ec_align4 ec_params_fp_frame {
 	/*
 	 * The offset contains the template index or FP_FRAME_INDEX_RAW_IMAGE
@@ -5016,17 +5035,8 @@ struct __ec_align4 ec_params_fp_template {
 /* Clear the current fingerprint user context and set a new one */
 #define EC_CMD_FP_CONTEXT 0x0406
 
-#define FP_CONTEXT_USERID_WORDS (32 / sizeof(uint32_t))
-#define FP_CONTEXT_NONCE_WORDS (32 / sizeof(uint32_t))
-
 struct __ec_align4 ec_params_fp_context {
 	uint32_t userid[FP_CONTEXT_USERID_WORDS];
-	/* TODO(b/73337313) mostly a placeholder, details to be implemented */
-	uint32_t nonce[FP_CONTEXT_NONCE_WORDS];
-};
-
-struct __ec_align4 ec_response_fp_context {
-	uint32_t nonce[FP_CONTEXT_NONCE_WORDS];
 };
 
 #define EC_CMD_FP_STATS 0x0407