Improve readability of expansion size
Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/aesce.c b/library/aesce.c
index 356d0a3..e47665a 100644
--- a/library/aesce.c
+++ b/library/aesce.c
@@ -185,15 +185,17 @@
const unsigned char *key,
const size_t key_bit_length)
{
-
- const uint32_t key_len_in_words = key_bit_length / 32;
- const size_t round_key_len_in_words = 4;
static uint8_t const rcon[] = { 0x01, 0x02, 0x04, 0x08, 0x10,
0x20, 0x40, 0x80, 0x1b, 0x36 };
- /* Require max(key_len_in_words, round_key_len_len_in_words) + 7 */
- const size_t round_keys_needed = key_len_in_words + 7;
+ /* See https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.197.pdf
+ * - Section 5, Nr = Nk + 6
+ * - Section 5.2, the key expansion size is Nb*(Nr+1)
+ */
+ const uint32_t key_len_in_words = key_bit_length / 32; /* Nk */
+ const size_t round_key_len_in_words = 4; /* Nb */
+ const size_t round_keys_needed = key_len_in_words + 6; /* Nr */
const size_t key_expansion_size_in_words =
- round_keys_needed * round_key_len_in_words;
+ round_key_len_in_words * (round_keys_needed + 1); /* Nb*(Nr+1) */
const uint32_t *rko_end = (uint32_t *) rk + key_expansion_size_in_words;
memcpy(rk, key, key_len_in_words * 4);