VNC Passwords

[Scott C. Best]

Heyaz. I had a question about VNC passwords. I just got this
snippet from an email newsletter from "***@TridiaVNC" wherein it
states:

| VNC PASSWORDS ARE SENT IN CLEAR TEXT
|
| When you type your VNC password to view a remote PC, it is sent in
| clear, unencrypted text. This means ANYONE with common network
| sniffing software can see your password. It will take someone less
| than 2 seconds to take control of any PC on your network that uses
| that password and you won't have a clue they're doing it.
|
| TridiaVNC Pro encrypts all connections so that all data, including
| passwords, can't be seen by anyone.

While I agree that good encryption of otherwise plaintext
passwords is a Good Idea, I was wondering if something has changed in
the basic VNC platform while I wasn't looking? That is, my understanding
has been that a VNC Viewer doesn't actually transmit a VNC Password
across the network when I start a VNC session. Rather, it uses a form
of "challenge-response passwords" such as:

* VNC Viewer connects with VNC Server
* VNC Server creates a random number and transmits it to the VNC Viewer
* I type my password into the VNC Viewer, and it encrypts the received
random number with the text I type, sending the resulting ciphertext
to the VNC Server.
* VNC Server encrypts the same random number with the stored VNC password,
and then compares the ciphertext it generates with the ciphertext I
transmitted to it.
* If the ciphertexts match, I must have typed in the same password the
VNC Server has stored, and access is granted.

In such a scheme, the only text crossing the network is the
random number and the ciphertext. And if the encryption is a strong
"one-way function", it should be arbitrarily difficult for someone to
sniff these two values and calculate in reverse the password I typed in.

So I guess my question are:

* Do I or does TridiaVNC have this misunderstood?

* If I type in the wrong password, does the VNC Server generate a new
random number for my next attempt, or do I keep "re-using" the first
one?

* What encryption algorithm is used and how "strong" is it to being
reversed?

* Would it be at all worthwhile to (similar to SSH) use a Diffie-Hellman
transaction so that the initial random challenge was at least encrypted
and not transmitted in plaintext?

Thanks in advance for any thoughts on the above.

cheers,
Scott

[William Hooper]

Post by Scott C. Best
Heyaz. I had a question about VNC passwords. I just got this
| VNC PASSWORDS ARE SENT IN CLEAR TEXT
|
| When you type your VNC password to view a remote PC, it is sent in
| clear, unencrypted text.

Looks like they need to get their marketing on the same page. Their web site (http://www.tridiavnc.com/) list both "raw" vnc and their product as having "password protection".

Post by Scott C. Best
* Would it be at all worthwhile to (similar to SSH) use a Diffie-Hellman
transaction so that the initial random challenge was at least encrypted and
not transmitted in plaintext?

No, it makes it worthwhile to tunnel VNC over an encrypted tunnel when on an untrusted network.

[James Weatherall]

Scott,

The "email newsletter" from Tridia is incorrect. VNC has always used a
challenge-response protocol for verifying the user's password, and so
has never sent it in the clear. This simply sounds like their
marketting team has misunderstood the point of encrypting the rest of
the session.

Wez @ RealVNC Ltd.