Based on this question, I'm interested in any rational argument that would support the idea of maintaining separate signing and authentication certificates.
Conventional logic & wisdom says there is no need to separate an Identity and Signing certificate, but I'm looking for any rational reason to the contrary (devil's advocate).
Here are a few imaginary examples that might have a basis in IT operations, risk management, or separation of concerns. I hope that someone will validate any of these reasons (or will come up with their own) that justify separate keys:
- Is there any need to have different lifetimes/expiration of authentication and signing certs?
- Is there ever a need to have different assurance levels of each certificate? (different CAs?)
- Is it plausible that I need to revoke one, and not the other? Why?
- Are there different software workflows, based on key usage (EKU) that would discourage automated operation? (preventing viruses from authenticating or signing as you)
- Should I defend from Non-technically savvy legislation that may require such separation in the future? (Security theatre)
- Do they have different storage requirements (smart card, trusted device, DPAPI, etc) for multiple devices? (Do I need the same or different cert on a Phone, PC, iPad)
- If multiple valid signing and identity certs are "active" at a time, does that help isolate and partition risk of misuse (all offending usages can be contained)?
These are a few ideas that I can think of that might justify the need to maintain different signing keys, but I'm unsure of how realistic they are (including stuff I haven't thought of).
From a technical perspective I can imagine needing separate keys for these (and possibly other reasons)
- Limitations on algorithm RSA vs ECC
- key length
- Internal compatibility (SHA1/SHA2/SHA3)