Ssh "security hole": proposed fix

[David Mazieres]

Well, a while ago I suggested the following as simplified (and
therefore easier to find bugs in) ssh protocol:

> PREAMBLE:
> 
> (1)  A -> B: A
> (2)  B -> A: Cb, PKsb, PKb
> (3)  A -> B: {Kab}_PKsb, {A, B, Cb}_Kab
> (4)  B -> A: {{A, B, Cb, Kab, PKsb}_SKb}_Kab
> 
> SSH_AUTH_RHOSTS:
> 
> (5)  A -> B:  0
> 
> SSH_AUTH_RHOSTS_RSA:
> SSH_AUTH_RSA:
> 
> (5)  A -> B: {{A, B, Cb, Kab}_SKa, PKa}_Kab
> 
> SSH_AUTH_PASSWORD:
> 
> (5)  A -> B: {Ka}_Kab

I was assuming that you would basically have to tolerate
man-in-the-middle style attacks if A did not know PKb before the
exchange.  However, I have since realized that it is easy to have
one's public key in an NFS mounted home directory when talking to a
host one has never talked to before.  Thus, B can know PKa even if A
does not know PKb.  This is enough to prevent man in the middle
attacks if we modify the protocol slightly:

PREAMBLE:

(1)  A -> B: A
(2)  B -> A: Cb, PKsb, PKb
(3)  A -> B: {Kab}_PKsb, {A, B, Cb}_Kab
(4)  B -> A: {{A, B, Cb, Kab, PKsb}_SKb}_Kab

SSH_AUTH_RHOSTS:
SSH_AUTH_PASSWORD:

(5)  A -> B:  0

SSH_AUTH_RHOSTS_RSA:
SSH_AUTH_RSA:

(5)  A -> B: {{A, B, Cb, Kab, PKsb}_SKa, PKa}_Kab
                              ^^^^
IF AUTHENTICATION NOT SUFFICIENT:

(6)  B -> A: {"Passwd:"}_Kab
(7)  A -> B: {Ka}_Kab                           ; Ka is A's password

David