known device/sram data persistence working

libusbhost_stm32f4.ld

@@ -33,6 +33,17 @@ MEMORY
 /* Include the common ld script. */
 INCLUDE libopencm3_stm32f4.ld
 
+/* Extra stuff */
+SECTIONS
+{
+    .backup_sram : {
+        . = ALIGN(4);
+        __backup_sram_start = .;
+        *(.backup_sram)
+        __backup_sram_end = .;
+    } >backup
+}
+
 PROVIDE(_ram_start = ORIGIN(ram));
 PROVIDE(_ram_end = ORIGIN(ram) + LENGTH(ram));
 PROVIDE(_rom_start = ORIGIN(rom));

pairing.py

@@ -3,6 +3,7 @@ import threading
 import binascii
 import re
 import os
+import time
 
 import serial
 import gi
@@ -136,11 +137,11 @@ def run_pairing_gui(port, baudrate, debug=False):
             raise SystemError('Unknown noise error')
 
         with open(known_devices_file, 'a') as f:
-            f.write(noise.remote_fingerprint)
+            f.write(f'{noise.remote_fingerprint} # added {time.ctime()}\n')
 
     else:
         with open(known_devices_file) as f:
-            known_devices = [ l.strip() for l in f.readlines() if not l[0] == '#' ]
+            known_devices = [ l.strip().partition('#')[0].strip() for l in f.readlines() if not l[0] == '#' ]
 
         if noise.remote_fingerprint not in known_devices:
             raise ValueError('Remote host is untrusted but seems to trust us.')

src/demo.c

@@ -38,6 +38,7 @@
 #include <libopencm3/stm32/timer.h>
 #include <libopencm3/stm32/otg_hs.h>
 #include <libopencm3/stm32/otg_fs.h>
+#include <libopencm3/stm32/pwr.h>
 #include <libopencm3/stm32/dma.h>
 #include <libopencm3/cm3/nvic.h>
 #include <libopencmsis/core_cm3.h>
@@ -56,7 +57,9 @@
 
 
 static struct NoiseState noise_state;
-static uint8_t remote_key_reference[CURVE25519_KEY_LEN];
+static uint8_t remote_key_reference[BLAKE2S_HASH_SIZE] __attribute__((section(".backup_sram")));
+static uint8_t local_key[CURVE25519_KEY_LEN] __attribute__((section(".backup_sram")));
+static uint8_t identity_key_valid __attribute__((section(".backup_sram"))) = 0;
 
 
 void _fini(void);
@@ -81,6 +84,9 @@ static void clock_setup(void) {
 	rcc_periph_clock_enable(RCC_DMA2);
 	rcc_periph_clock_enable(RCC_DMA1);
 
+	rcc_periph_clock_enable(RCC_PWR);
+	rcc_periph_clock_enable(RCC_BKPSRAM);
+
 	rcc_periph_clock_enable(RCC_RNG);
 }
 
@@ -413,6 +419,8 @@ int main(void)
 {
 	clock_setup();
 	gpio_setup();
+    pwr_disable_backup_domain_write_protect();
+    PWR_CSR |= PWR_CSR_BRE; /* Enable backup SRAM battery power regulator */
 
 	/* provides time_curr_us to usbh_poll function */
 	tim6_setup();
@@ -447,12 +455,17 @@ int main(void)
 	LOG_PRINTF("Initializing RNG...\n");
     rand_init();
 
-    noise_state_init(&noise_state, remote_key_reference);
+    noise_state_init(&noise_state, remote_key_reference, local_key);
     /* FIXME load remote key from backup memory */
     /* FIXME only run this on first boot and persist key in backup sram. Allow reset via jumper-triggered factory reset function. */
-    LOG_PRINTF("Generating identity key...\n");
+    if (!identity_key_valid) {
-    if (generate_identity_key(&noise_state))
+        LOG_PRINTF("Generating identity key...\n");
-        LOG_PRINTF("Error generating identiy key\n");
+        if (generate_identity_key(&noise_state)) {
+            LOG_PRINTF("Error generating identiy key\n");
+        } else {
+            identity_key_valid = 1;
+        }
+    }
 
     int poll_ctr = 0;
 	while (23) {

src/noise.c

@@ -3,6 +3,9 @@
 
 #include "noise.h"
 #include "packet_interface.h"
+#include "rand_stm32.h"
+
+#include "crypto/noise-c/src/crypto/blake2/blake2s.h"
 
 
 #define HANDLE_NOISE_ERROR(x, msg) do { \
@@ -20,12 +23,13 @@ volatile uint8_t host_packet_buf[MAX_HOST_PACKET_SIZE];
 volatile int host_packet_length = 0;
 
 
-void noise_state_init(struct NoiseState *st, uint8_t *remote_key_reference) {
+void noise_state_init(struct NoiseState *st, uint8_t *remote_key_reference, uint8_t *local_key) {
     st->handshake_state = HANDSHAKE_UNINITIALIZED;
     st->handshake = NULL;
     st->tx_cipher = NULL;
     st->rx_cipher = NULL;
     st->remote_key_reference = remote_key_reference;
+    st->local_key = local_key;
     st->failed_handshakes = 0;
 }
 
@@ -51,7 +55,7 @@ int start_protocol_handshake(struct NoiseState *st) {
     HANDLE_NOISE_ERROR(noise_handshakestate_new_by_name(&handshake, "Noise_XX_25519_ChaChaPoly_BLAKE2s", NOISE_ROLE_RESPONDER), "instantiating handshake pattern");
 
     NoiseDHState *dh = noise_handshakestate_get_local_keypair_dh(handshake);
-    HANDLE_NOISE_ERROR(noise_dhstate_set_keypair_private(dh, st->local_key, sizeof(st->local_key)), "loading local private keys");
+    HANDLE_NOISE_ERROR(noise_dhstate_set_keypair_private(dh, st->local_key, CURVE25519_KEY_LEN), "loading local private keys");
 
     HANDLE_NOISE_ERROR(noise_handshakestate_start(handshake), "starting handshake");
 
@@ -74,7 +78,7 @@ int generate_identity_key(struct NoiseState *st) {
     uint8_t unused[CURVE25519_KEY_LEN]; /* the noise api is a bit bad here. */
     memset(st->local_key, 0, sizeof(st->local_key));
 
-    HANDLE_NOISE_ERROR(noise_dhstate_get_keypair(dh, st->local_key, sizeof(st->local_key), unused, sizeof(unused)), "saving key pair");
+    HANDLE_NOISE_ERROR(noise_dhstate_get_keypair(dh, st->local_key, CURVE25519_KEY_LEN, unused, sizeof(unused)), "saving key pair");
 
     return 0;
 errout:
@@ -146,7 +150,14 @@ int try_continue_noise_handshake(struct NoiseState *st, uint8_t *buf, size_t len
 
         HANDLE_NOISE_ERROR(noise_dhstate_get_public_key(remote_dh, st->remote_key, sizeof(st->remote_key)), "getting remote pubkey");
 
-        if (!memcmp(st->remote_key, st->remote_key_reference, sizeof(st->remote_key))) { /* keys match */
+        /* TODO support list of known remote hosts here instead of just one */
+        uint8_t remote_fp[BLAKE2S_HASH_SIZE];
+        BLAKE2s_context_t bc;
+        BLAKE2s_reset(&bc);
+        BLAKE2s_update(&bc, st->remote_key, sizeof(st->remote_key));
+        BLAKE2s_finish(&bc, remote_fp);
+
+        if (!memcmp(remote_fp, st->remote_key_reference, sizeof(remote_fp))) { /* keys match */
             uint8_t response = REPORT_PAIRING_SUCCESS;
             if (send_encrypted_message(st, &response, sizeof(response)))
                 LOG_PRINTF("Error sending pairing response packet\n");
@@ -177,7 +188,10 @@ errout:
 }
 
 void persist_remote_key(struct NoiseState *st) {
-    memcpy(st->remote_key_reference, st->remote_key, sizeof(st->remote_key));
+    BLAKE2s_context_t bc;
+    BLAKE2s_reset(&bc);
+    BLAKE2s_update(&bc, st->remote_key, sizeof(st->remote_key));
+    BLAKE2s_finish(&bc, st->remote_key_reference);
     st->handshake_state = HANDSHAKE_DONE_KNOWN_HOST;
 }
 

src/noise.h

@@ -30,7 +30,7 @@ struct NoiseState {
     NoiseHandshakeState *handshake;
     enum handshake_state handshake_state;
     NoiseCipherState *tx_cipher, *rx_cipher;
-    uint8_t local_key[CURVE25519_KEY_LEN];
+    uint8_t *local_key;
     uint8_t remote_key[CURVE25519_KEY_LEN];
     uint8_t *remote_key_reference;
     uint8_t handshake_hash[BLAKE2S_HASH_SIZE];
@@ -39,7 +39,7 @@ struct NoiseState {
 
 
 void uninit_handshake(struct NoiseState *st, enum handshake_state new_state);
-void noise_state_init(struct NoiseState *st, uint8_t *remote_key_reference);
+void noise_state_init(struct NoiseState *st, uint8_t *remote_key_reference, uint8_t *local_key);
 void persist_remote_key(struct NoiseState *st);
 int start_protocol_handshake(struct NoiseState *st);
 int reset_protocol_handshake(struct NoiseState *st);