Please help test recent changes

Fabian Stelzer
Mon, 10 Jan 2022 12:47:31 -0000

On 09.01.2022 15:56, ?ngel wrote:
>On 2022-01-07 at 18:51 +0000, Morgan, Iain (ARC-TN)[InuTeq, LLC] wrote:
>> What we would want is some way of associating the restrictions with a
>> key so that they are automatically applied when the key is loaded
>> into the agent, either by ssh-add or when triggered by
>> AddKeysToAgent. That would either mean adding the restrictions to the
>> private key, or adding another file that is associated with the key
>> which contains the restrictions. The latter would be easier to
>> implement from a backwards-compatibility standpoint, but I'm not
>> particularly happy about adding another file type.
>>
>> The file containing the options would be assumed to be in the same
>> directory as the private key, and would be formed by adding an
>> extension such as ".opts" or ".cfg" to the name of
the private key.
>>
>> I don't have any specific ideas as to how to express the
>> restrictions, but we would probably want one line for each allowed
>> path. Also, the ssh-add command line should be able to override the
>> restrictions. Perhaps the syntax might be something like:
>>
>>         AllowedPath: foo.example.com > bar.example.com ...
>>
>> Alternatively, perhaps it would be possible to include the
>> restrictions as plain-text in the private key, before the "-----
>> BEGIN" line. That assumes that any text outside of the BEGIN ...
END
>> sequence is ignored by existing implementations.
>>
>> As an aside, either approach should allow for adding support for
>> other kinds of restrictions. For example, it might be convenient to
>> add a Timeout parameter that would correspond to the -t option with
>> ssh-add.
>>
>> --
>> Iain
>
>Good point. A key should probably always have the same restrictions
>(after some testing to get those right).
>
>I would include them as armor headers, though:
>
>-----BEGIN ALGO PRIVATE KEY-----
>Proc-Type: 4,ENCRYPTED
>DEK-Info: AES-128-CBC,AABBCC...
>Destination-constraint: perseus at cetus.example.org
>Destination-constraint: scylla.example.org
>Destination-constraint: scylla.example.org>medea at
charybdis.example.orgscylla.example.org
>
>SGVsbG8gd29ybGQhCg...
>

I think the trend is (and should be) going towards keeping private keys in
hardware tokens and no longer in files on disk. FIDO, PIV Smartcards, GPG
Smartcards, HSMs, TPMs all allow for very safe private key storage. If you
allow these constraints to be specified in a config file either referenced
under a key fingerprint (probably easiest):

Key SHA:256INGERPRINT:
DestinationConstraint: perseus at cetus.example.org
DestinationConstraint: scylla.example.org
DestinationConstraint:
scylla.example.org>medea at charybdis.example.orgscylla.example.org

Or maybe under a Host section:
Host cetus.example.org
     ForwardAgent yes
     ForwardKey SHA256:xxx
     ForwardKeyConstraint scylla.example.org

then you can still auto load these instead of writing ssh-add scripts. I
think most ssh users who would use this feature probably already have some
mechanism of managing their .ssh/config.

>
>I don't think backwards compatibility of the private key is a concern
>here. You already need an updated openssh stack to make use of the new
>features, and copying private keys between systems isn't generally a
>good idea (it's better to have a different key per source host).
>
>Nonetheless, by placing these headers at the end, they seem to be
>ignored by prior ssh-add versions (placing them before Proc-Type or
>DEK-Info cause "invalid format" errors).
>
>
>Best regards
>
>
>
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