add support for persistent srk

[novag]

• Persisting the SRK allows the creation and use of individual keys without specifying the owner password

Example:

1. Set an owner password: tpm2_changeauth -c owner myPass
2. Create SSH key and save SRK to handle ID 0x81000002: ssh-tpm-keygen -o -s 0x81000002
3. Create another SSH key without entering the owner password: ssh-tpm-keygen -s 0x81000002
4. Start the SSH agent without entering the owner password: /ssh-tpm-agent -s 0x81000002 -l /var/tmp/tpm.sock

Based on #37

[Foxboron]

This lacks a test. You should be able to recreate the flow as you describe through the TPM simulator.

[Foxboron]

This is probably not going to work out in it's current form. We need to extend the SRK to the key format and I'm changing this to the ASN.1 key definition.

https://github.com/Foxboron/go-tpm-keyfiles

It should allow us to embed the parent handle into the keyfiles which should offer more flexibility.

[Foxboron]

@novag

Yo, thanks for working on this.

I've been tidying up go-tpm-keyfiles with the intention of moving most of the keyfile handling and TPM stuff around these to this library. I think your time is better spent making sure your use-case of persistent SRKs is handeled in this library as it should be much easier to support this in ssh-tpm-agent as a result.

I think it should work as-is, but I have not yet written any tests for it. parent should point at a PERSISTENT handle and ReadPublic from it.

https://github.com/Foxboron/go-tpm-keyfiles

Relevant code is here: https://github.com/Foxboron/go-tpm-keyfiles/blob/master/tpm.go#L458

[Foxboron]

Yo,

I'm sorry for making this PR difficult to merge for you. I've added a --parent-handle flag to ssh-tpm-keygen which might do what you want to do as it just hooks directly into go-tpm-keyfiles.

I believe there is some flag parsing missing for specifying static handles instead of a hierarchy, but I think this is going to be easier to implement.

If you don't want to work on this anymore I can salvage the interesting bits from this PR and work it out for you :)