openssh unix dev - Jan 2022 - Host names hashing

On 05.01.22 15:06, Blumenthal, Uri - 0553 - MITLL wrote:
> What are the cryptographic consequences of host name hah collision?
None, I would guess. Someone crafting a server name/IP that causes a 
hash collision with an existing hashed known_hosts entry is likely to be 
a nuisance to or a source of confusion of the user (ssh will complain 
about the existing entry for a different host keypair), not, per se, a 
failure of security.

The hashing is meant to obscure info about what host it matches, so the 
relevant failure mode is if the hash algo would become *reversible*.

One question about the real-world use cases, however: How many people 
are there hashing known_hosts entries *autogenerated by client use out 
of the very same account*? If I were to find that someone got ahold of 
the known_hosts file off my workplace machine, I'd be worried about 
disclosure of my *user keypairs*, while the info about the servers I 
access would probably be a lost cause, anyway (that info is available in 
cleartext in the config file(s), and/or the saved shell history).

A much more understandable use case would be if an organization 
distributes a centrally administered/collected known_hosts file to users 
but doesn't want marginally-trusted users to immediately see where to 
find the company LAN's crown jewels. However, in order to make a 
successful algo migration in *that* scenario, we'd have to address the 
possibility that users and the central repository could end up 
supporting *different* sets of algos for some length of time ...

Regards,
-- 
Jochen Bern
Systemingenieur

Binect GmbH
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On Wed, Jan 5, 2022 at 10:38 AM Jochen Bern <Jochen.Bern at binect.de>
wrote:
> The hashing is meant to obscure info about what host it matches, so the
> relevant failure mode is if the hash algo would become *reversible*.
And normally, it's the opposite of helpful. The known_hosts is useful
for casual review and for tuning .ssh/config as desired for more
specific uses, and the hashing obscures the commonly used SSH targets.
> One question about the real-world use cases, however: How many people
> are there hashing known_hosts entries *autogenerated by client use out
> of the very same account*? If I were to find that someone got ahold of
Many simply turn off known_hosts, because it is a burden to approve
new hostkeys and very awkward if there is an IP address overlap in a
DHCP or externally configured target. The .ssh/config settings for
this are quite old, and well documented:

Host *
     UserKnownHostsFile /dev/null
     StrictHostKeyChecking no
     LogLevel ERROR
> A much more understandable use case would be if an organization
> distributes a centrally administered/collected known_hosts file to users
> but doesn't want marginally-trusted users to immediately see where to
> find the company LAN's crown jewels. However, in order to make a
> successful algo migration in *that* scenario, we'd have to address the
> possibility that users and the central repository could end up
> supporting *different* sets of algos for some length of time ...
The burden is why I disable it in setups like, say, ansible server
setups to control wide varieties of VM's in a multi-purpose VLAN.