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Cryptography & TLS

#HKDF

c
#define FIO_HKDF
#include "fio-stl.h"

HMAC-based Key Derivation Function per RFC 5869. HKDF extracts entropy from input keying material into a pseudorandom key, then expands that key into one or more output keys.

Supports SHA-256 and SHA-384 as the underlying HMAC.

Note: HKDF needs fio_sha256_hmac and fio_sha512_hmac. Define FIO_SHA2 before FIO_HKDF, or use FIO_CRYPTO to pull in everything.

#Constants

#FIO_HKDF_SHA256_HASH_LEN

c
#define FIO_HKDF_SHA256_HASH_LEN 32

SHA-256 hash length in bytes.

#FIO_HKDF_SHA384_HASH_LEN

c
#define FIO_HKDF_SHA384_HASH_LEN 48

SHA-384 hash length in bytes.

#Functions

#fio_hkdf_extract

c
SFUNC void fio_hkdf_extract(void *restrict prk,
                            const void *restrict salt,
                            size_t salt_len,
                            const void *restrict ikm,
                            size_t ikm_len,
                            int use_sha384);

HKDF-Extract: PRK = HMAC-Hash(salt, IKM).

Parameters:

  • prk — output buffer (32 bytes for SHA-256, 48 for SHA-384).
  • salt — optional salt; if NULL, a zero string of hash length is used.
  • salt_len — salt length.
  • ikm — input keying material.
  • ikm_len — IKM length.
  • use_sha384 — non-zero for SHA-384, zero for SHA-256.

#fio_hkdf_expand

c
SFUNC void fio_hkdf_expand(void *restrict okm,
                           size_t okm_len,
                           const void *restrict prk,
                           size_t prk_len,
                           const void *restrict info,
                           size_t info_len,
                           int use_sha384);

HKDF-Expand: OKM = HKDF-Expand(PRK, info, L).

Parameters:

  • okm — output buffer.
  • okm_len — desired output length (max 255 * hash_len).
  • prk — pseudorandom key from Extract.
  • prk_len — 32 for SHA-256, 48 for SHA-384.
  • info — optional context/info string; may be NULL.
  • info_len — info length.
  • use_sha384 — non-zero for SHA-384, zero for SHA-256.

#fio_hkdf

c
SFUNC void fio_hkdf(void *restrict okm,
                    size_t okm_len,
                    const void *restrict salt,
                    size_t salt_len,
                    const void *restrict ikm,
                    size_t ikm_len,
                    const void *restrict info,
                    size_t info_len,
                    int use_sha384);

Combined HKDF: Extract followed by Expand in one call.

#Example

c
#define FIO_SHA2
#define FIO_HKDF
#include "fio-stl.h"

int main(void) {
  const char *ikm = "input keying material";
  const char *salt = "salt";
  const char *info = "app context";
  uint8_t key[32];

  fio_hkdf(key, sizeof(key), salt, strlen(salt),
           ikm, strlen(ikm), info, strlen(info), 0);
  return 0;
}