[cc20rng] Revamping the ChaCha20 seeding

- chacha20_prng_block() uses counter in the state struct

- chacha20_setup() replaces chacha20_prng_reseed() and fills the whole
  state struct, fixing a bug where only half of the key was being set;
  as a result of 'counter' being set, a state struct filled with
  entropy from the TRNG makes reseeding occur after a random number of
  rounds instead of after a fixed 2^32-1 rounds

- decrementing of the block counter is done in chacha20_prng_block()

- chacha output is copied to buf _after_ the interrupt driven
  transmission of buf to UART has finished, to stop the race between
  reading and refilling of buf

1 files changed, 13 insertions, 8 deletions

diff --git a/src/cc20rng/cc20_prng.h b/src/cc20rng/cc20_prng.h
index 7b597d0..08f78d7 100644
--- a/src/cc20rng/cc20_prng.h
+++ b/src/cc20rng/cc20_prng.h
@@ -3,17 +3,22 @@
 
 #include <stdint.h>
 
-#define CHACHA20_MAX_BLOCK_COUNTER 0xffffffff
-#define CHACHA20_NUM_WORDS 16
-#define CHACHA20_BLOCK_SIZE (CHACHA20_NUM_WORDS * 4)
+#define CHACHA20_BLOCK_SIZE_WORDS 16
+#define CHACHA20_BLOCK_SIZE (CHACHA20_BLOCK_SIZE_WORDS * 4)
 
 struct cc20_state {
-    uint32_t i[CHACHA20_NUM_WORDS];
+    union {
+        struct {
+            uint32_t constant[4];
+            uint32_t key[8];
+            uint32_t counter;
+            uint32_t nonce[3];
+        } s;
+        uint32_t i[CHACHA20_BLOCK_SIZE_WORDS];
+    };
 };
 
-extern void chacha20_prng_reseed(struct cc20_state *cc, uint32_t *entropy);
-extern void chacha20_prng_block(struct cc20_state *cc, uint32_t block_counter,
-                                struct cc20_state *out);
-extern int chacha20_prng_self_test();
+void chacha20_prng_block(struct cc20_state *cc, uint8_t *out);
+int chacha20_prng_self_test();
 
 #endif /* __STM32_CHACHA20_H */