move attestion key to not be part of firmware

targets/stm32l442/merge_hex.py

@@ -4,21 +4,38 @@
 
 from intelhex import IntelHex
 import sys
+from binascii import unhexlify
 
 if len(sys.argv) < 3:
-    print('usage: %s <file1.hex> <file2.hex> [...] <output.hex>')
+    print('usage: %s <file1.hex> <file2.hex> [...] [-s <secret_attestation_key>] <output.hex>')
     sys.exit(1)
 
 def flash_addr(num):
     return 0x08000000 + num * 2048
 
+args = sys.argv[:]
+
+# generic / hacker attestation key
+secret_attestation_key = "cd67aa310d091ed16e7e9892aa070e1994fcd714ae7c408fb946b72e5fe75d30"
+
+# user supplied, optional
+for i,x in enumerate(args):
+    if x == '-s':
+        secret_attestation_key = args[i+1]
+        break
+
+if secret_attestation_key is not None:
+    args = args[:i] + args[i+2:]
+
+# TODO put definitions somewhere else
 PAGES = 128
 APPLICATION_END_PAGE = PAGES - 19
 AUTH_WORD_ADDR       = (flash_addr(APPLICATION_END_PAGE)-8)
+ATTEST_ADDR          = (flash_addr(PAGES - 15))
 
-first = IntelHex(sys.argv[1])
-for i in range(2, len(sys.argv)-1):
-    first.merge(IntelHex( sys.argv[i] ), overlap = 'replace')
+first = IntelHex(args[1])
+for i in range(2, len(args)-1):
+    first.merge(IntelHex( args[i] ), overlap = 'replace')
 
 first[AUTH_WORD_ADDR]   = 0
 first[AUTH_WORD_ADDR+1] = 0
@@ -30,4 +47,11 @@ first[AUTH_WORD_ADDR+5] = 0xff
 first[AUTH_WORD_ADDR+6] = 0xff
 first[AUTH_WORD_ADDR+7] = 0xff
 
-first.tofile(sys.argv[len(sys.argv)-1], format='hex')
+if secret_attestation_key is not None:
+    key = unhexlify(secret_attestation_key)
+    print('using key ',key)
+    for i,x in enumerate(key):
+        print(hex(ATTEST_ADDR + i))
+        first[ATTEST_ADDR + i] = x
+
+first.tofile(args[len(args)-1], format='hex')

targets/stm32l442/src/crypto.c

@@ -20,6 +20,7 @@
 #include "device.h"
 #include APP_CONFIG
 #include "log.h"
+#include "memory_layout.h"
 
 
 typedef enum
@@ -164,7 +165,9 @@ void crypto_ecc256_init()
 
 void crypto_ecc256_load_attestation_key()
 {
-    _signing_key = attestation_key;
+    static uint8_t _key [32];
+    memmove(_key, (uint8_t*)ATTESTATION_KEY_ADDR, 32);
+    _signing_key = _key;
     _key_len = 32;
 }
 

targets/stm32l442/src/memory_layout.h

@@ -17,7 +17,7 @@
 // Storage of FIDO2 resident keys
 #define RK_NUM_PAGES    10
 #define RK_START_PAGE   (PAGES - 14)
-#define RK_END_PAGE     (PAGES - 14 + RK_NUM_PAGES)
+#define RK_END_PAGE     (PAGES - 14 + RK_NUM_PAGES)     // not included
 
 // Start of application code
 #ifndef APPLICATION_START_PAGE
@@ -25,6 +25,10 @@
 #endif
 #define APPLICATION_START_ADDR	(0x08000000 + ((APPLICATION_START_PAGE)*PAGE_SIZE))
 
+// where attestation key is located
+#define ATTESTATION_KEY_PAGE    (PAGES - 15)
+#define ATTESTATION_KEY_ADDR    (0x08000000 + ATTESTATION_KEY_PAGE*PAGE_SIZE)
+
 // End of application code.  Leave some extra room for future data storage.
 // NOT included in application
 #define APPLICATION_END_PAGE	((PAGES - 19))