From 9dd3219c3fd6a5160a604155639b156f967a4d09 Mon Sep 17 00:00:00 2001
From: PeterKietzmann <peter.kietzmann@haw-hamburg.de>
Date: Wed, 15 Jan 2020 17:19:54 +0100
Subject: [PATCH 1/2] sys/puf_sram: counter based reseed after soft reset

---
 sys/include/puf_sram.h  |  7 +++++++
 sys/puf_sram/puf_sram.c | 12 ++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/sys/include/puf_sram.h b/sys/include/puf_sram.h
index f8969192f..e7044cc7e 100644
--- a/sys/include/puf_sram.h
+++ b/sys/include/puf_sram.h
@@ -89,6 +89,13 @@ extern uint32_t puf_sram_seed;
  */
 extern uint32_t puf_sram_state;
 
+/**
+ * @brief Counter variable allocated in puf_sram.c. It is incremented
+          during each soft reset when no new PUF measurement was taken
+          and it gets reset to zero after a power cycle was detected.
+ */
+extern uint32_t puf_sram_softreset_cnt;
+
 /**
  * @brief checks source of reboot by @p puf_sram_softreset and conditionally
           calls @p puf_sram_generate
diff --git a/sys/puf_sram/puf_sram.c b/sys/puf_sram/puf_sram.c
index e2195ef12..63a3b4aa4 100644
--- a/sys/puf_sram/puf_sram.c
+++ b/sys/puf_sram/puf_sram.c
@@ -24,6 +24,9 @@ PUF_SRAM_ATTRIBUTES uint32_t puf_sram_seed;
 /* Allocation of the PUF seed state */
 PUF_SRAM_ATTRIBUTES uint32_t puf_sram_state;
 
+/* Allocation of the PUF soft reset conter*/
+PUF_SRAM_ATTRIBUTES uint32_t puf_sram_softreset_cnt;
+
 /* Allocation of the memory marker */
 PUF_SRAM_ATTRIBUTES uint32_t puf_sram_marker;
 
@@ -42,6 +45,8 @@ void puf_sram_generate(const uint8_t *ram, size_t len)
     puf_sram_marker = PUF_SRAM_MARKER;
     /* setting state to 0 means seed was generated from SRAM pattern */
     puf_sram_state = 0;
+    /* reset counter of detected soft resets */
+    puf_sram_softreset_cnt = 0;
 }
 
 bool puf_sram_softreset(void)
@@ -51,5 +56,12 @@ bool puf_sram_softreset(void)
         return 0;
     }
     puf_sram_state = 1;
+
+    /* increment number of detected soft resets */
+    puf_sram_softreset_cnt++;
+
+    /* generate alterntive seed value */
+    puf_sram_seed ^= puf_sram_softreset_cnt;
+    puf_sram_seed = dek_hash((uint8_t *)&puf_sram_seed, sizeof(puf_sram_seed));
     return 1;
 }

From ecc7ee8255416a43e6bb2ba5022ee37c00f2f702 Mon Sep 17 00:00:00 2001
From: PeterKietzmann <peter.kietzmann@haw-hamburg.de>
Date: Wed, 15 Jan 2020 18:10:07 +0100
Subject: [PATCH 2/2] tests/puf_sram: disable interactive sync

---
 tests/puf_sram/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tests/puf_sram/Makefile b/tests/puf_sram/Makefile
index e8f75c97f..5ab039b74 100644
--- a/tests/puf_sram/Makefile
+++ b/tests/puf_sram/Makefile
@@ -3,5 +3,6 @@ BOARD ?= nucleo-f411re
 include ../Makefile.tests_common
 
 USEMODULE += puf_sram
+DISABLE_MODULE += test_utils_interactive_sync
 
 include $(RIOTBASE)/Makefile.include