Wipe the KEX shared secret once keys are derived

DoKexDhReply() and SendKexDhReply() zero all of ssh->k and reset kSz
on every exit, so K does not outlive the key exchange. GenerateKeys()
zeroes both handshake key sets when a derivation fails.

- the full-buffer wipe clears CreateMpint and ML-KEM hybrid leftovers
- tests/regress.c checks K after each KEX, the hybrids included
- tests/regress.c checks a failed derivation leaves no keys

Issue: F-8845, F-11680, F-11681, F-8843, F-11682, F-14001

Author: ejohnstown

src/internal.c

@@ -3700,6 +3700,12 @@ static int GenerateKeys(WOLFSSH* ssh, byte hashId, byte doKeyPad)
     }
 #endif /* SHOW_SECRETS */
 
+    /* Do not keep a partial key set from a failed derivation. */
+    if (ret != WS_SUCCESS && cK != NULL) {
+        WS_FORCEZERO(cK, sizeof(Keys));
+        WS_FORCEZERO(sK, sizeof(Keys));
+    }
+
     return ret;
 }
 
@@ -8914,6 +8920,10 @@ static int DoKexDhReply(WOLFSSH* ssh, byte* buf, word32 len, word32* idx)
         ret = SendNewKeys(ssh);
     }
 
+    /* K is only needed to derive the keys. Wipe all of it, success or not. */
+    WS_FORCEZERO(ssh->k, sizeof(ssh->k));
+    ssh->kSz = 0;
+
     if (sigKeyBlock_ptr)
         WFREE(sigKeyBlock_ptr, ssh->ctx->heap, DYNTYPE_PRIVKEY);
     /* RFC 4253 11.1: WS_PUBKEY_REJECTED_E here is only the host key check,
@@ -18675,6 +18685,12 @@ int SendKexDhReply(WOLFSSH* ssh)
     if (ret != WS_WANT_WRITE && ret != WS_SUCCESS)
         PurgePacket(ssh);
 
+    /* K is only needed to derive the keys. Wipe all of it, success or not. */
+    if (ssh != NULL) {
+        WS_FORCEZERO(ssh->k, sizeof(ssh->k));
+        ssh->kSz = 0;
+    }
+
     WLOG(WS_LOG_DEBUG, "Leaving SendKexDhReply(), ret = %d", ret);
     if (sigKeyBlock_ptr) {
         WS_FORCEZERO(sigKeyBlock_ptr, sizeof(struct wolfSSH_sigKeyBlockFull));

tests/regress.c

@@ -1500,6 +1500,17 @@ static void RunKexReplyHandshake(KexReplyHarness* harness,
     result->steps = REGRESS_MAX_HANDSHAKE_STEPS;
 }
 
+/* The shared secret K does not outlive the key exchange. */
+static void AssertKexSecretWiped(const WOLFSSH* ssh)
+{
+    word32 i;
+
+    AssertIntEQ(ssh->kSz, 0);
+    for (i = 0; i < (word32)sizeof(ssh->k); i++) {
+        AssertIntEQ(ssh->k[i], 0);
+    }
+}
+
 static void AssertHandshakeSucceeds(const char* keyAlgo, const char* keyPath)
 {
     KexReplyHarness harness;
@@ -1511,6 +1522,8 @@ static void AssertHandshakeSucceeds(const char* keyAlgo, const char* keyPath)
 
     AssertTrue(result.clientSuccess);
     AssertTrue(result.serverSuccess);
+    AssertKexSecretWiped(harness.client);
+    AssertKexSecretWiped(harness.server);
     AssertIntEQ(harness.mutator.mutatedPackets, 0);
     AssertIntEQ(harness.client->connectState, CONNECT_SERVER_CHANNEL_REQUEST_DONE);
     AssertIntEQ(harness.server->acceptState, ACCEPT_CLIENT_SESSION_ESTABLISHED);
@@ -1919,10 +1932,55 @@ static void AssertHandshakeRejectsCorruptedSig(const char* keyAlgo,
     AssertTrue(result.clientErr != WS_WANT_READ &&
             result.clientErr != WS_WANT_WRITE);
     AssertIntEQ(result.clientErr, expectedErr);
+    /* K was agreed before the verify failed. */
+    AssertKexSecretWiped(harness.client);
 
     FreeKexReplyHarness(&harness);
 }
 
+/* Every key agreement, the ML-KEM hybrids included, wipes K on both sides. */
+static void TestKexSecretWipedPerKex(void)
+{
+    static const char* kexAlgos[] = {
+#ifndef WOLFSSH_NO_DH_GROUP14_SHA256
+        "diffie-hellman-group14-sha256",
+#endif
+#ifndef WOLFSSH_NO_ECDH_SHA2_NISTP256
+        "ecdh-sha2-nistp256",
+#endif
+#ifndef WOLFSSH_NO_CURVE25519_SHA256
+        "curve25519-sha256",
+#endif
+#ifndef WOLFSSH_NO_NISTP256_MLKEM768_SHA256
+        "mlkem768nistp256-sha256",
+#endif
+#ifndef WOLFSSH_NO_NISTP384_MLKEM1024_SHA384
+        "mlkem1024nistp384-sha384",
+#endif
+#ifndef WOLFSSH_NO_CURVE25519_MLKEM768_SHA256
+        "mlkem768x25519-sha256",
+#endif
+        NULL
+    };
+    KexReplyHarness harness;
+    KexReplyRunResult result;
+    word32 i;
+
+    for (i = 0; kexAlgos[i] != NULL; i++) {
+        InitKexReplyHarnessKex(&harness, kexAlgos[i],
+                REGRESS_DEFAULT_KEY_ALGO, REGRESS_DEFAULT_KEY_PATH, 0,
+                REGRESS_MUTATE_SIG_NAME, NULL, 0);
+        RunKexReplyHandshake(&harness, &result);
+
+        AssertTrue(result.clientSuccess);
+        AssertTrue(result.serverSuccess);
+        AssertKexSecretWiped(harness.client);
+        AssertKexSecretWiped(harness.server);
+
+        FreeKexReplyHarness(&harness);
+    }
+}
+
 #ifndef WOLFSSH_NO_RSA_SHA2_256
 static void TestKexDhReplyRejectsRsaSha2_256CorruptSig(void)
 {
@@ -15674,6 +15732,7 @@ static void TestGenerateKeysSplit(void)
     word32 payloadSz;
     word32 idx;
     byte zeros[AES_256_KEY_SIZE];
+    Keys zeroKeys;
 
     WMEMSET(zeros, 0, sizeof(zeros));
 
@@ -15741,6 +15800,13 @@ static void TestGenerateKeysSplit(void)
     AssertTrue(WMEMCMP(ssh->handshake->peerKeys.encKey,
                        ssh->handshake->keys.encKey, AES_128_KEY_SIZE) != 0);
 
+    /* A failed derivation leaves no key material behind. */
+    AssertTrue(wolfSSH_TestGenerateKeys(ssh, WC_HASH_TYPE_NONE) != WS_SUCCESS);
+    WMEMSET(&zeroKeys, 0, sizeof(zeroKeys));
+    AssertTrue(WMEMCMP(&ssh->handshake->keys, &zeroKeys, sizeof(Keys)) == 0);
+    AssertTrue(WMEMCMP(&ssh->handshake->peerKeys, &zeroKeys,
+                       sizeof(Keys)) == 0);
+
     wolfSSH_free(ssh);
 
 #ifndef WOLFSSH_NO_AES_GCM
@@ -17406,6 +17472,7 @@ int main(int argc, char** argv)
     TestKexDhReplyRejectsEd25519CorruptSig();
     #endif
     TestKexDhReplyRejectsSigNameOverrun();
+    TestKexSecretWipedPerKex();
     #ifdef REGRESS_TRUNC_KEX_ALGO
     TestKexDhReplyTruncatedFSendsDisconnect();
     TestKexDhInitTruncatedESendsDisconnect();