0

I've heard that companies store their private keys offline "in a safe that will never be opened" and stuff like that (the context for this is a console developer like Microsoft keeping the Xbox private keys offline in a safe).

How would they still sign code with their private keys if they're fully locked away? Does someone go and physically get the keys to sign the code? Is this a figure of speech that I don't understand?

  • As you said, they physically go in, get the keys and sign the code. – Zavior Apr 18 at 18:00
3

By having a chain of trust:

  • The master key itself is kept in a safe, airgapped from the evil Internet.
  • Once in a blue moon, that key is taken out of the safe and used to sign an intermediate key.
  • That intermediate key is what is used to sign code on a day-to-day basis.

Validation of the code on an end user's device is then a matter of asking "has this code been correctly signed with a key which itself was signed by the master key?" The advantage of all this is that if an intermediate key is compromised, it can easily be revoked and a new one generated without needing to change the end user devices - all they care about is the root of trust, the master key.

(Small disclaimer: I don't actually know how big software houses do this, but this is how the SSL CAs work, and they're effectively solving the same problem).

  • Great answer. Also, you can put the intermediate key on a smart card where it can be used but not copied. – Karl Bielefeldt Apr 19 at 12:58
  • @KarlBielefeldt Or even better in a dedicated HSM. – Philip Kendall Apr 19 at 15:48
1

The "safe that will never be opened" is probably a figure of speech.

On the other hand, such private keys are of very great (economic) value to a company like Microsoft, so they will be protected very carefully. I can fully imagine that those keys are really stored in a safe that is only accessible to less than a handful of people who are trusted enough to sign the released code.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.