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kex: clear buffers holding secrets

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- kex: clear `k_value` buffer in `kex_*_state_cleanup()` functions.
  Reported-by: Ravikanth
  Fixes #2653

- kex: clear shared secret buffer after use in
  `kex_mlkem768x25519_sha256()`.
  Reported-by: Ravikanth
  Fixes #2654
  Follow-up to 3ba252e2a6 #1644

- kex: clear shared secret buffer after use in `kex_mlkem_nistp()`.
  Follow-up to 3ba252e2a6 #1644

Closes #2655
This commit is contained in:
Viktor Szakats
2026-10-06 04:41:53 +02:00
parent 8f80526c49
commit 826fdb283d
1 changed files with 40 additions and 15 deletions
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55
src/kex.c
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@@ -375,8 +375,11 @@ static void kex_diffie_hellman_state_cleanup(
SSH2_SAFEFREE(session, exchange_state->e_packet);
if(exchange_state->s_packet)
SSH2_SAFEFREE(session, exchange_state->s_packet);
if(exchange_state->k_value)
SSH2_SAFEFREE(session, exchange_state->k_value);
if(exchange_state->k_value) {
ssh2_zero_free(session, exchange_state->k_value,
exchange_state->k_value_len);
exchange_state->k_value = NULL;
}
exchange_state->state = ssh2_NB_state_idle;
}
@@ -1490,8 +1493,11 @@ static void kex_ecdh_exchange_state_cleanup(
ssh2_bn_free(exchange_state->k);
exchange_state->k = NULL;
if(exchange_state->k_value)
SSH2_SAFEFREE(session, exchange_state->k_value);
if(exchange_state->k_value) {
ssh2_zero_free(session, exchange_state->k_value,
exchange_state->k_value_len);
exchange_state->k_value = NULL;
}
exchange_state->state = ssh2_NB_state_idle;
}
@@ -1795,8 +1801,11 @@ static void kex_mlkem_nistp_exchange_state_cleanup(
ssh2_bn_free(exchange_state->k);
exchange_state->k = NULL;
if(exchange_state->k_value)
SSH2_SAFEFREE(session, exchange_state->k_value);
if(exchange_state->k_value) {
ssh2_zero_free(session, exchange_state->k_value,
exchange_state->k_value_len);
exchange_state->k_value = NULL;
}
exchange_state->state = ssh2_NB_state_idle;
}
@@ -1954,6 +1963,7 @@ static int kex_mlkem_nistp(LIBSSH2_SESSION *session,
rc = ssh2_mlkem_get_sk(shared_secret, mlkem_size, private_pq_key,
server_public_key);
if(rc) {
ssh2_explicit_zero(shared_secret, shared_secret_len);
ret = ssh2_err(session, LIBSSH2_ERROR_KEX_FAILURE,
"Unable to create mlkem shared secret");
goto clean_exit;
@@ -1961,14 +1971,17 @@ static int kex_mlkem_nistp(LIBSSH2_SESSION *session,
if(ssh2_bn_to_bin(exchange_state->k,
shared_secret + SSH2_MLKEM_SHARED_SECRET_LEN)) {
ssh2_explicit_zero(shared_secret, shared_secret_len);
ret = ssh2_err(session, LIBSSH2_ERROR_OUT_OF_BOUNDARY,
"Cannot write shared secret");
goto clean_exit;
}
/* verify hash */
if(!ssh2_hash(hash_alg, shared_secret, shared_secret_len,
exchange_state->k_value + 4, digest_len)) {
rc = ssh2_hash(hash_alg, shared_secret, shared_secret_len,
exchange_state->k_value + 4, digest_len);
ssh2_explicit_zero(shared_secret, shared_secret_len);
if(!rc) {
ret = ssh2_err(session, LIBSSH2_ERROR_HASH_CALC,
"kex: failed to calculate hash");
goto clean_exit;
@@ -2147,8 +2160,11 @@ static void kex_curve25519_exchange_state_cleanup(
ssh2_bn_free(exchange_state->k);
exchange_state->k = NULL;
if(exchange_state->k_value)
SSH2_SAFEFREE(session, exchange_state->k_value);
if(exchange_state->k_value) {
ssh2_zero_free(session, exchange_state->k_value,
exchange_state->k_value_len);
exchange_state->k_value = NULL;
}
exchange_state->state = ssh2_NB_state_idle;
}
@@ -2407,8 +2423,11 @@ static void kex_mlkem768x25519_exchange_state_cleanup(
ssh2_bn_free(exchange_state->k);
exchange_state->k = NULL;
if(exchange_state->k_value)
SSH2_SAFEFREE(session, exchange_state->k_value);
if(exchange_state->k_value) {
ssh2_zero_free(session, exchange_state->k_value,
exchange_state->k_value_len);
exchange_state->k_value = NULL;
}
exchange_state->state = ssh2_NB_state_idle;
}
@@ -2523,6 +2542,7 @@ static int kex_mlkem768x25519_sha256(
rc = ssh2_mlkem_get_sk(shared_secret, 768,
private_pq_key, server_public_key);
if(rc) {
ssh2_explicit_zero(shared_secret, sizeof(shared_secret));
ret = ssh2_err(session, LIBSSH2_ERROR_KEX_FAILURE,
"Unable to create mlkem shared secret");
goto clean_exit;
@@ -2533,6 +2553,7 @@ static int kex_mlkem768x25519_sha256(
server_public_key +
SSH2_MLKEM_768_CIPHERTEXT);
if(rc) {
ssh2_explicit_zero(shared_secret, sizeof(shared_secret));
ret = ssh2_err(session, LIBSSH2_ERROR_KEX_FAILURE,
"Unable to create curve25519 shared secret");
goto clean_exit;
@@ -2545,6 +2566,7 @@ static int kex_mlkem768x25519_sha256(
uninitialized. */
x25519_len = ssh2_bn_bytes(exchange_state->k);
if(x25519_len == 0 || x25519_len > SSH2_ED25519_KEY_LEN) {
ssh2_explicit_zero(shared_secret, sizeof(shared_secret));
ret = ssh2_err(session, LIBSSH2_ERROR_OUT_OF_BOUNDARY,
"Invalid curve25519 shared secret length");
goto clean_exit;
@@ -2554,15 +2576,18 @@ static int kex_mlkem768x25519_sha256(
if(ssh2_bn_to_bin(exchange_state->k,
shared_secret + SSH2_MLKEM_SHARED_SECRET_LEN +
SSH2_ED25519_KEY_LEN - x25519_len)) {
ssh2_explicit_zero(shared_secret, sizeof(shared_secret));
ret = ssh2_err(session, LIBSSH2_ERROR_OUT_OF_BOUNDARY,
"Cannot write shared secret");
goto clean_exit;
}
/* verify hash */
if(!ssh2_hash(SSH2_SHA256_ALG, shared_secret,
SSH2_MLKEM_SHARED_SECRET_LEN + SSH2_ED25519_KEY_LEN,
exchange_state->k_value + 4, SSH2_SHA256_DIG_LEN)) {
rc = ssh2_hash(SSH2_SHA256_ALG, shared_secret,
SSH2_MLKEM_SHARED_SECRET_LEN + SSH2_ED25519_KEY_LEN,
exchange_state->k_value + 4, SSH2_SHA256_DIG_LEN);
ssh2_explicit_zero(shared_secret, sizeof(shared_secret));
if(!rc) {
ret = ssh2_err(session, LIBSSH2_ERROR_HASH_CALC,
"kex: failed to calculate hash");
goto clean_exit;