internal: send a disconnect when key exchange fails

- DoKexDhInit() sends SSH_MSG_DISCONNECT with KEY_EXCHANGE_FAILED on
  WS_CRYPTO_FAILED and WS_PUBKEY_REJECTED_E, DoKexDhGexGroup() on
  WS_CRYPTO_FAILED and WS_DH_SIZE_E.
- DoKexDhReply() sends KEY_EXCHANGE_FAILED on WS_CRYPTO_FAILED and
  HOST_KEY_NOT_VERIFIABLE on WS_PUBKEY_REJECTED_E.
- DuplexEndpoint records the reason code of a plaintext outbound
  disconnect, and InitKexReplyHarnessKex() takes an explicit KEX
  algorithm.
- New mutator modes shorten f and e, write a zero-length e, cut the
  GEX prime below the requested floor and set the GEX generator to 1;
  LocateSinglePacketPayload() finds the payload for the single-packet
  rewriters.
- Tests assert the reason code on the wire for each new mode and for
  host key rejection, and assert no disconnect on a successful
  handshake.

Issue: F-8838

Author: yosuke-wolfssl

src/internal.c

@@ -5936,6 +5936,12 @@ static int DoKexDhInit(WOLFSSH* ssh, byte* buf, word32 len, word32* idx)
         ret = SendKexDhReply(ssh);
     }
 
+    /* RFC 8731 sec. 3: a rejected key exchange input aborts with a
+     * disconnect */
+    if (ret == WS_CRYPTO_FAILED || ret == WS_PUBKEY_REJECTED_E) {
+        (void)SendDisconnect(ssh, WOLFSSH_DISCONNECT_KEY_EXCHANGE_FAILED);
+    }
+
     return ret;
 }
 
@@ -7619,6 +7625,15 @@ static int DoKexDhReply(WOLFSSH* ssh, byte* buf, word32 len, word32* idx)
 
     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,
+     * which is server authentication, so it gets its own reason. */
+    if (ret == WS_CRYPTO_FAILED) {
+        (void)SendDisconnect(ssh, WOLFSSH_DISCONNECT_KEY_EXCHANGE_FAILED);
+    }
+    else if (ret == WS_PUBKEY_REJECTED_E) {
+        (void)SendDisconnect(ssh,
+                WOLFSSH_DISCONNECT_HOST_KEY_NOT_VERIFIABLE);
+    }
     WLOG(WS_LOG_DEBUG, "Leaving DoKexDhReply(), ret = %d", ret);
     return ret;
 }
@@ -8019,6 +8034,12 @@ static int DoKexDhGexGroup(WOLFSSH* ssh,
         ret = SendKexDhInit(ssh);
     }
 
+    /* RFC 4419 sec. 3: a group outside the requested range fails the key
+     * exchange */
+    if (ret == WS_CRYPTO_FAILED || ret == WS_DH_SIZE_E) {
+        (void)SendDisconnect(ssh, WOLFSSH_DISCONNECT_KEY_EXCHANGE_FAILED);
+    }
+
     return ret;
 }