Using GnuPG subkeys at two insecure locations

[Raphaël Poss]

Hi all,

I have read Adrian's tutorial at http://fortytwo.ch/gpg/subkeys and 
related documents, and I am yet unsure how to apply this technique to my 
situation where I have _two_ "insecure" locations instead of one.

You will find below: description of situation, my needs, how I did 
achieve something using Adrian's technique for my needs, tests, and 
additional questions. You can jump straight to the end if you are not 
interested by the details.


Here is the situation:

- laptop as main personal computer, carried around and therefore 
not-so-secure,

- windows workstation at work, not-so-secure as well,

- some other computer in "hidden" place, secure and not connected to the 
internet.


What I would like to:

- send encrypted email with the insecure computers,

- sign email with both insecure computers,

- send encrypted-to-self email from the windows workstation, that can be 
decrypted on the laptop

- sign other keys with the laptop

- read encrypted emails sent by other people to me on the laptop

- no private encryption key on the windows workstation

- have a subkey for "secure" communication attached to my primary key, 
so that people can send me stuff that can be read only on my secure computer


Here is what I did on the secure computer:

1. create a primary DSA key id# 0x5024FAE3

2. create a DSA signing subkey id# 0x66808804 for use on the windows 
workstation

3. create a DSA signing subkey id# 0xACD488B7 for use on the laptop

4. create an EL-Gamal encryption subkey id# 0xB8838617 for use on the laptop

5. create an El-Gamal encryption subkey id# 0x7FEFD6B8 for use on the 
secure computer

6. export public keys to file

7. backup everything

8. delete 0xACD488B7 0xB8838617 0x7FEFD6B8

9. export secret subkeys 0x66808804, import them on windows workstation 
with public keys

10. restore everything

11. mark 0xACD488B7 as able to sign other keys

12. delete  0x66808804  0x7FEFD6B8

13. export secret subkeys 0xACD488B7 0xB8838617, import them on laptop 
with public keys

14. restore everything


Here are the tests I did:

- send signed mail and verify them on either insecure computer or 
3rd-party with access to only public keys

- send encrypted mail with any computer and decrypt it on the laptop

- encrypt data using the secure key, decrypt it on the secure computer

Here are the tests I did not do yet (as of today, I don't have access to 
the secure computer):

- sign other keys, check signatures, etc.

- sign other keys on the laptop without access to the primary key,

- test everything with PGP




Now, questions:

Q1. how do you think other software (PGP, old GPG, ...) behave when they 
see multiple encryption public subkeys?

Q2. will signatures on other keys made with the laptop be recognised by 
other software? Is there anything I should care for w.r.t trust when I 
sign keys?

Q3. do you think it is better I do not entrust the laptop subkey to sign 
other keys?

For that last question I have to state the difference between the 
windows workstation and the laptop: the laptop is "more" secure than the 
workstation. If the laptop is compromised I would know about it 
immediately, and issue any relevant revocation certificates straight 
away. Any encrypted data on the laptop is deleted securely after I have 
decrypted it.

The windows workstation is quite secure (read: well-maintained 
configuration, good IT staff and workplace access control) but could be 
compromised at any time *by the company* without me knowing it, and data 
backups including my secret keyring will float around for several years. 
I will probably revocate company-related keys only when I leave the 
employment. Or maybe on a yearly basis. I don't know yet.

Therefore, any signature made with the laptop before the key is revoked 
should be trusted (as much as a signature made with the primary key), 
whereas any signature made with the windows workstation is valid only 
inside the company.

Q4. How can I mark my level of trust for the different subkeys using gnupg?


Thanks in advance for any enlightenment,

-- 
Raphaël Poss







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