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sntrup: Remove needless conversion of shared secret to bignum

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The derived shared secret in SNTRUP761 is converted into a bignum,
only to be converted back to binary during use in kex.c.
Instead use field 'hybrid_shared_secret' in ssh_crypto_struct
to store it, just like the Hybrid MLKEM implementation.

Fixes #338

Signed-off-by: Shiva Kiran Koninty <shiva_kr@riseup.net>
Reviewed-by: Pavol Žáčik <pzacik@redhat.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
This commit is contained in:
Shiva Kiran Koninty
2026-02-23 10:42:56 +05:30
committed by Jakub Jelen
parent 90b07e2c18
commit d680b8ea8a
2 changed files with 12 additions and 16 deletions
Download Patch File Download Diff File Expand all files Collapse all files

8
src/kex.c
View File

@@ -1688,11 +1688,6 @@ int ssh_make_sessionid(ssh_session session)
switch (session->next_crypto->kex_type) {
case SSH_KEX_SNTRUP761X25519_SHA512:
case SSH_KEX_SNTRUP761X25519_SHA512_OPENSSH_COM:
rc = ssh_buffer_pack(buf,
"F",
session->next_crypto->shared_secret,
SHA512_DIGEST_LEN);
break;
case SSH_KEX_MLKEM768X25519_SHA256:
case SSH_KEX_MLKEM768NISTP256_SHA256:
#ifdef HAVE_MLKEM1024
@@ -1919,9 +1914,6 @@ int ssh_generate_session_keys(ssh_session session)
switch (session->next_crypto->kex_type) {
case SSH_KEX_SNTRUP761X25519_SHA512:
case SSH_KEX_SNTRUP761X25519_SHA512_OPENSSH_COM:
k_string = ssh_make_padded_bignum_string(crypto->shared_secret,
crypto->digest_len);
break;
case SSH_KEX_MLKEM768X25519_SHA256:
case SSH_KEX_MLKEM768NISTP256_SHA256:
#ifdef HAVE_MLKEM1024

20
src/sntrup761.c
View File

@@ -28,7 +28,6 @@
#include "libssh/sntrup761.h"
#ifdef HAVE_SNTRUP761
#include "libssh/bignum.h"
#include "libssh/buffer.h"
#include "libssh/crypto.h"
#include "libssh/dh.h"
@@ -141,7 +140,7 @@ static int ssh_sntrup761x25519_build_k(ssh_session session)
{
unsigned char ssk[SNTRUP761_SIZE + CURVE25519_PUBKEY_SIZE];
unsigned char *k = ssk + SNTRUP761_SIZE;
unsigned char hss[SHA512_DIGEST_LEN];
void *shared_secret_data = NULL;
int rc;
rc = ssh_curve25519_create_k(session, k);
@@ -216,22 +215,27 @@ static int ssh_sntrup761x25519_build_k(ssh_session session)
ssh_log_hexdump("kem key", ssk, SNTRUP761_SIZE);
#endif
sha512(ssk, sizeof ssk, hss);
bignum_bin2bn(hss, sizeof hss, &session->next_crypto->shared_secret);
if (session->next_crypto->shared_secret == NULL) {
ssh_string_burn(session->next_crypto->hybrid_shared_secret);
ssh_string_free(session->next_crypto->hybrid_shared_secret);
session->next_crypto->hybrid_shared_secret =
ssh_string_new(SHA512_DIGEST_LEN);
if (session->next_crypto->hybrid_shared_secret == NULL) {
ssh_set_error_oom(session);
rc = SSH_ERROR;
goto cleanup;
}
shared_secret_data =
ssh_string_data(session->next_crypto->hybrid_shared_secret);
sha512(ssk, sizeof ssk, shared_secret_data);
#ifdef DEBUG_CRYPTO
ssh_print_bignum("Shared secret key", session->next_crypto->shared_secret);
ssh_log_hexdump("Shared secret key", shared_secret_data, SHA512_DIGEST_LEN);
#endif
return 0;
cleanup:
ssh_burn(ssk, sizeof ssk);
ssh_burn(hss, sizeof hss);
return rc;
}