update TODO comments for Chacha20 implementation regarding block length and nonce handling

Author: publicvoid1337

pkg/driver_cryptocell_310/psa_cryptocell_310/cipher_chacha20.c

@@ -156,6 +156,7 @@ psa_status_t psa_cipher_chacha20_update(psa_cipher_chacha20_ctx_t *ctx,
         CRYS_CHACHA_BLOCK_SIZE_IN_BYTES) * CRYS_CHACHA_BLOCK_SIZE_IN_BYTES;
     /* xcrypt full blocks */
 
+    /* IoT-TODO: if blocks_length == 0: CRYS_CHACHA_Block returns PSA_INVALID_ARGUMENT (CRYS_CHACHA_DATA_IN_SIZE_ILLEGAL) */
     if(blocks_length > 0) {
         periph_status = CRYS_CHACHA_Block(  &ctx->ctx.post_setup,
                                         (uint8_t *)&input[input_index],

sys/crypto/psa_riot_cipher/chacha20.c

@@ -63,7 +63,7 @@ psa_status_t psa_cipher_chacha20_set_iv(psa_cipher_chacha20_ctx_t *ctx,
     /* See PSA Specification */
     switch (iv_length) {
         case 8:
-            /* IoT-TODO: have to implement this? */
+            /* IoT-TODO: Chacha20 implementation only handles 12byte nonce */
             /* 8 bytes: the cipher operation uses the original [CHACHA20] definition
             of ChaCha20: the provided IV is used as the 64-bit nonce, and the 64-bit
             counter value is set to zero. */