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hmac: add comments explaining why each buffer needs erasing

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As requested in review: clarify that the padding arrays carry
key material (key XOR fixed constant, trivially reversible),
not just padding bytes.
This commit is contained in:
Michal Rybecky
2026-04-01 15:12:46 +02:00
committed by Zbigniew Jędrzejewski-Szmek
parent 0ffefb8a4d
commit b675582861
1 changed files with 4 additions and 4 deletions
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src/basic/hmac.c
+4 -4
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@@ -17,13 +17,13 @@ void hmac_sha256(const void *key,
uint8_t res[static SHA256_DIGEST_SIZE]) {
uint8_t inner_padding[HMAC_BLOCK_SIZE] = { };
CLEANUP_ERASE(inner_padding);
CLEANUP_ERASE(inner_padding); /* carries key ^ 0x36, trivially reversible to the original key */
uint8_t outer_padding[HMAC_BLOCK_SIZE] = { };
CLEANUP_ERASE(outer_padding);
CLEANUP_ERASE(outer_padding); /* carries key ^ 0x5c, trivially reversible to the original key */
uint8_t replacement_key[SHA256_DIGEST_SIZE];
CLEANUP_ERASE(replacement_key);
CLEANUP_ERASE(replacement_key); /* SHA-256 of the key when key_size > block size */
struct sha256_ctx hash;
CLEANUP_ERASE(hash);
CLEANUP_ERASE(hash); /* intermediate state derived from key material */
assert(key);
assert(key_size > 0);