Hi!
Working on a new two-tier PKI design, I have come across an uncertainty which I cannot quite google an answer to.
In the design, I wish to use CNG with SHA2 (SHA-256) and 4096-bit keys. The clients will mostly consist of Windows machines ranging from XP and Server 2003 to Windows 8.1 and Server 2012 R2, but there will also be UNIX/Linux based machines, as well as a few network devices from Cisco (newer version LAN Controllers, etc). An HSM will be used to secure CA private keys.
My question is: In the case I have to issue “Legacy” certificates, will it be possible to create a new subordinate CA with a lower key size and for example SHA1 algorithm, and issue “compatibility” certificates from it? Or is cross-certifying a separate hierarchy with a SHA1 and 2048-bit Root the only way to go in order to achieve full compatibility across platforms?
Any feedback or experience in the matter will be greatly appreciated!