common,agent: Factor out KEM functions into common/kem.c.

* common/util.h (compute_kmac256): Remove.
(gnupg_ecc_kem_kdf, gnupg_kem_combiner): New.
* common/kmac.c (compute_kmac256): Don't expose.
(gnupg_ecc_kem_kdf, gnupg_kem_combiner): New.
* agent/pkdecrypt.c (agent_hybrid_pgp_kem_decrypt): Use
gnupg_ecc_kem_kdf and gnupg_kem_combiner.

--

Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>

common/kmac.c

@@ -1,4 +1,4 @@
-/* kmac.c -  Keccak based MAC
+/* kem.c -  KEM helper functions
  * Copyright (C) 2024  g10 Code GmbH.
  *
  * This file is part of GnuPG.
@@ -36,7 +36,7 @@
 #include "mischelp.h"
 
 #define KECCAK512_BLOCKSIZE 136
-gpg_error_t
+static gpg_error_t
 compute_kmac256 (void *digest, size_t digestlen,
                  const void *key, size_t keylen,
                  const void *custom, size_t customlen,
@@ -134,3 +134,76 @@ compute_kmac256 (void *digest, size_t digestlen,
   return gpg_error (GPG_ERR_NOT_IMPLEMENTED);
 #endif
 }
+
+/* Compute KEK (shared secret) for ECC with HASHALGO, ECDH result,
+   ciphertext in ECC_CT, public key in ECC_PK.  */
+gpg_error_t
+gnupg_ecc_kem_kdf (void *kek, size_t kek_len,
+                   int hashalgo, const void *ecdh, size_t ecdh_len,
+                   const void *ecc_ct, size_t ecc_ct_len,
+                   const void *ecc_pk, size_t ecc_pk_len)
+{
+  gcry_buffer_t iov[3];
+  unsigned int dlen;
+
+  dlen = gcry_md_get_algo_dlen (hashalgo);
+  if (kek_len != dlen)
+    return gpg_error (GPG_ERR_INV_LENGTH);
+
+  memset (iov, 0, sizeof (iov));
+
+  iov[0].data = (unsigned char *)ecdh;
+  iov[0].len = ecdh_len;
+  iov[1].data = (unsigned char *)ecc_ct;
+  iov[1].len = ecc_ct_len;
+  iov[2].data = (unsigned char *)ecc_pk;
+  iov[2].len = ecc_pk_len;
+  gcry_md_hash_buffers (hashalgo, 0, kek, iov, 3);
+
+  return 0;
+}
+
+
+/* domSeperation */
+#define KMAC_KEY "OpenPGPCompositeKeyDerivationFunction"
+
+/* customizationString */
+#define KMAC_CUSTOM "KDF"
+
+/* Compute KEK by combining two KEMs.  */
+gpg_error_t
+gnupg_kem_combiner (void *kek, size_t kek_len,
+                    const void *ecc_ss, size_t ecc_ss_len,
+                    const void *ecc_ct, size_t ecc_ct_len,
+                    const void *mlkem_ss, size_t mlkem_ss_len,
+                    const void *mlkem_ct, size_t mlkem_ct_len,
+                    const void *fixedinfo, size_t fixedinfo_len)
+{
+  gpg_error_t err;
+  gcry_buffer_t iov[6];
+
+  memset (iov, 0, sizeof (iov));
+
+  iov[0].data = "\x00\x00\x00\x01"; /* Counter */
+  iov[0].len = 4;
+
+  iov[1].data = (unsigned char *)ecc_ss;
+  iov[1].len = ecc_ss_len;
+
+  iov[2].data = (unsigned char *)ecc_ct;
+  iov[2].len = ecc_ct_len;
+
+  iov[3].data = (unsigned char *)mlkem_ss;
+  iov[3].len = mlkem_ss_len;
+
+  iov[4].data = (unsigned char *)mlkem_ct;
+  iov[4].len = mlkem_ct_len;
+
+  iov[5].data = (unsigned char *)fixedinfo;
+  iov[5].len = fixedinfo_len;
+
+  err = compute_kmac256 (kek, kek_len,
+                         KMAC_KEY, strlen (KMAC_KEY),
+                         KMAC_CUSTOM, strlen (KMAC_CUSTOM), iov, 6);
+  return err;
+}

common/util.h

@@ -299,11 +299,18 @@ char *gnupg_get_help_string (const char *key, int only_current_locale);
 /*-- localename.c --*/
 const char *gnupg_messages_locale_name (void);
 
-/*-- kmac.c --*/
-gpg_error_t compute_kmac256 (void *digest, size_t digestlen,
-                             const void *key, size_t keylen,
-                             const void *custom, size_t customlen,
-                             gcry_buffer_t *data_iov, int data_iovlen);
+/*-- kem.c --*/
+gpg_error_t gnupg_ecc_kem_kdf (void *kek, size_t kek_len,
+                               int hashalgo, const void *ecdh, size_t ecdh_len,
+                               const void *ecc_ct, size_t ecc_ct_len,
+                               const void *ecc_pk, size_t ecc_pk_len);
+
+gpg_error_t gnupg_kem_combiner  (void *kek, size_t kek_len,
+                                 const void *ecc_ss, size_t ecc_ss_len,
+                                 const void *ecc_ct, size_t ecc_ct_len,
+                                 const void *mlkem_ss, size_t mlkem_ss_len,
+                                 const void *mlkem_ct, size_t mlkem_ct_len,
+                                 const void *fixedinfo, size_t fixedinfo_len);
 
 /*-- miscellaneous.c --*/