Re: [PATCH 0/2] public key: IMA signer logging: Log public key of IMA Signature signer in IMA log

Ken Goldman
2019-05-17T14:42:05Z

On 5/16/2019 9:29 PM, Lakshmi wrote:
> On 5/16/19 7:34 AM, Ken Goldman wrote:
>
>>> But outside the client machine this key id is not sufficient to
>>> uniquely determine which key the signature corresponds to.
>>
>> Why is this not sufficient?
>>
>> In my implementation, I create a lookup table at the attestation
>> service that maps the 4-byte IMA log key identifier to the signing
>> public key.
>>
>> Are you concerned about collisions?  Something else?
>
> Yes - the concern is collision.
>
> The "Subject Key Identifier" (SKI) for no two certificate can be the
> same. But since we are using only the last 4 bytes of the SKI it can
> collide. That's mainly the reason I want to log the entire public key.
>

Since a platform typically uses only a few signing keys, 4 bytes makes
the chance of a collision quite small. The collision would have to be
within the same log, not global.

In that worst case, the verifier would have to try two keys. It's a
slight performance penalty, but does anything break?

A new template with a larger SKI, perhaps 8 bytes, might be safer. It
doesn't expand the log size nearly as much as having a full public key.

>> Are you suggesting that the client supply the verification public key
>> and that the verifier trust it?  Wouldn't that make the log self signed?
>>
>> How would the verifier determine that the key from the IMA log is
>> valid / trusted / not revoked from the log itself?
>
> IMA log is backed by the TPM. So if the public key is added to the IMA
> log the attestation service can validate the key information.
> I am not sure if that answers your question.

The TPM just ensures that the log has not been altered. It does nothing
for signature verification, right?

The verifier can check that the supplied signature matches the supplied
public key. However, how could it verify that the public key is trusted
to sign the code? Doesn't that have to be out of band?

E.g., an attacker could create a log with their own signatures and
public keys. The signature would verify, but it's the attacker's key.

It's essentially a self-signed file.

>> A minor question here.
>>
>> Are you proposing that the IMA log contain a single ima-sigkey entry
>> per public key followed by ima-sig entries?
>>
>> Or are you proposing that ima-sig be replaced by ima-sigkey, and that
>> each event would contain both the signature and the public key?
>>
>> If the latter, this could add 25M to a server's 100K log.  Would that
>> increase in size concern anyone?  Could it be a concern on the other
>> end, an IoT device with limited memory?
>
> Mimi had raised the same concern. I will update my implementation to
> include the certification information in the IMA log only once per key -
> when that key is added to the IMA or Platform keyring.

If you include the public key only once, don't you have the same
collision problem? Two log entries could (in theory) and the same SKI.
How would the verifier know which public key to use.

However, I think the fundamental question is whether the verifier can
accept public keys supplied by the untrusted client. I believe that the
verifier has to receive the public keys out of band, from a trusted
source - not the client.