[dm-crypt] Remote unlock security

[dm-crypt] Remote unlock security

[David Jacquet]

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Hi,

I am trying to configure my server to be able to be unlocked via ssh and
dropbear. From the README.gz
I understand that I can issue the command:

ssh -o "UserKnownHostsFile=~/.ssh/known_hosts.initramfs" \
-i "~/id_rsa.initramfs" root@initramfshost.example.com \
"echo -ne \"secret\" >/lib/cryptsetup/passfifo"

What exactly will happen with the "secret" string? Will it be written to an
unprotected part of a hard drive. If so
it may be retrieved by a careful investigation of that drive. From my non
expert and humble opinion, a key (as
the "secret") should only be stored on RAM (and erased even from the RAM as
soon as possible).

Even if only stored in the RAM, I guess that the "secret" string will be
stored in the .bash_history file on the
computer from which the ssh-command was issued. I guess it is more
recommended to log into the remote
computer and then issue ( cat > /lib/cryptsetup/passfifo --> "secret" -->
CTRL+D, will that work?)

Best Regards
David

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Re: [dm-crypt] Remote unlock security

[Arno Wagner]

Hi David,
On Mon, Dec 20, 2010 at 08:05:25PM +0100, David Jacquet wrote:
> Hi,
> 
> I am trying to configure my server to be able to be unlocked via ssh and
> dropbear. From the README.gz
> I understand that I can issue the command:
> 
> ssh -o "UserKnownHostsFile=~/.ssh/known_hosts.initramfs" \
> -i "~/id_rsa.initramfs" root@initramfshost.example.com \
> "echo -ne \"secret\" >/lib/cryptsetup/passfifo"
> 
> What exactly will happen with the "secret" string? Will it be written to an
> unprotected part of a hard drive. 

I do not understand what "passfifo" is suppoded to do, you 
should probably do something like this instead:

ssh "cat <file-with-secret> | cryptsetyp --key-file - <other options>"

> If so
> it may be retrieved by a careful investigation of that drive. From my non
> expert and humble opinion, a key (as
> the "secret") should only be stored on RAM (and erased even from the RAM as
> soon as possible).

Indeed. However "as soon as possible" is on device removal from
LUKS/dm-crypt control.
 
> Even if only stored in the RAM, I guess that the "secret" string will be
> stored in the .bash_history file on the
> computer from which the ssh-command was issued. 

Therefore never show it to bash.

> I guess it is more
> recommended to log into the remote
> computer and then issue ( cat > /lib/cryptsetup/passfifo --> "secret" -->
> CTRL+D, will that work?)

Still don't get what "passfifo" is for. Is this some contruction
like this?

  mkfifo passfifo
  cryptsetup --key-file passfifo

Arno
-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
----
Cuddly UI's are the manifestation of wishful thinking. -- Dylan Evans

If it's in the news, don't worry about it.  The very definition of 
"news" is "something that hardly ever happens." -- Bruce Schneier 

Re: [dm-crypt] Remote unlock security

[epvdm]

On Tue, Dec 21, 2010 at 06:27:41AM +0100, Arno Wagner wrote:
> 
> Hi David,
> On Mon, Dec 20, 2010 at 08:05:25PM +0100, David Jacquet wrote:
> > Hi,
> > 
> > I am trying to configure my server to be able to be unlocked via ssh and
> > dropbear. From the README.gz
> > I understand that I can issue the command:
> > 
> > ssh -o "UserKnownHostsFile=~/.ssh/known_hosts.initramfs" \
> > -i "~/id_rsa.initramfs" root@initramfshost.example.com \
> > "echo -ne \"secret\" >/lib/cryptsetup/passfifo"
> > 
> > What exactly will happen with the "secret" string? Will it be written to an
> > unprotected part of a hard drive. 
> 
> I do not understand what "passfifo" is suppoded to do, you 
> should probably do something like this instead:
> 
> ssh "cat <file-with-secret> | cryptsetyp --key-file - <other options>"
> 
> > If so
> > it may be retrieved by a careful investigation of that drive. From my non
> > expert and humble opinion, a key (as
> > the "secret") should only be stored on RAM (and erased even from the RAM as
> > soon as possible).
> 
> Indeed. However "as soon as possible" is on device removal from
> LUKS/dm-crypt control.
>  
> > Even if only stored in the RAM, I guess that the "secret" string will be
> > stored in the .bash_history file on the
> > computer from which the ssh-command was issued. 
> 
> Therefore never show it to bash.
> 
> > I guess it is more
> > recommended to log into the remote
> > computer and then issue ( cat > /lib/cryptsetup/passfifo --> "secret" -->
> > CTRL+D, will that work?)
> 
> Still don't get what "passfifo" is for. Is this some contruction
> like this?
> 
>   mkfifo passfifo
>   cryptsetup --key-file passfifo
> 
> Arno


apparently in some linux distributions the initrd cryptdisk setup reads the
password from a fifo. 

regardless when setting up something like this, it's important to realize that
if you care about the actual security of the data and your passphrase, this is
not a safe arrangement unless you can independently confirm that the machine
you're trying to unlock hasn't been tampered with. For instance, an attacker
could modify the (unencrypted) initramfs to save whatever passphrase you type
remotely. 

Of course if you just use encrypted disks for convenience, e.g., so you don't
have to worry about scrubbing them before discarding old equipment, it doesn't
matter. 

eric

Re: [dm-crypt] Remote unlock security

[David Jacquet]

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Hi,

thanks for clears answers. Some thoughts: first of all it is great that the
"secret"
string is not written temporarily to a disc, that was my primary concern.
Therefore I think
that the added insecurity in this case comes from the computer being stored
remotely,
not unlocked remotely. I don't see why the computer couldn't be tampered (by
hardware
or software methods) with even if it was unlock by classic means: human on
site
entering secret key.

As for the method passfifo itself, I do not exactly know what is happening.
I am running
Ubuntu server 10.04, and there is some partly binary, partly text, script
file called

 /lib/cryptsetup/askpass

which I _guess_ is constructed for the sole purpose of remote ssh unlock,
but I am having
difficulties getting any documentation on this file. With the Ubuntu Plymoth
startup it seems
non trivial actually getting this to work (there are some bug on this on
launchpad), but I wanted
to find out the security issues before trying the actual solution.

David

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Re: [dm-crypt] Remote unlock security

[Jonas Meurer]

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Hey David,

On 21/12/2010 David Jacquet wrote:
> thanks for clears answers. Some thoughts: first of all it is great that the
> "secret"
> string is not written temporarily to a disc, that was my primary concern.
> Therefore I think
> that the added insecurity in this case comes from the computer being stored
> remotely,
> not unlocked remotely. I don't see why the computer couldn't be tampered (by
> hardware
> or software methods) with even if it was unlock by classic means: human on
> site
> entering secret key.
> 
> As for the method passfifo itself, I do not exactly know what is happening.
> I am running
> Ubuntu server 10.04, and there is some partly binary, partly text, script
> file called
> 
>  /lib/cryptsetup/askpass
> 
> which I _guess_ is constructed for the sole purpose of remote ssh unlock,
> but I am having
> difficulties getting any documentation on this file. With the Ubuntu Plymoth
> startup it seems
> non trivial actually getting this to work (there are some bug on this on
> launchpad), but I wanted
> to find out the security issues before trying the actual solution.

askpass is a helper utility, intended to be used for passphrase
prompting in the initramfs script. usplash and splashy support, the fifo
file you're talking about, and console passphrase prompt are the main
features of this utility.
it's shipped with the debian and ubuntu cryptsetup package.

greetings,
 jonas

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