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Suppose I want to change the hash algorithm used for a certificate signature (ex: SHA1 to SHA2).

Is it better to revoke the previous certificate and issue a new one with expected hash algorithm ? Or is it possible to just push a new CSR with the same To-Be-Signed certificate, but expecting that another algo used ? What about Root or intermediate CA ?

I guess that even if it is feasible, it may have security issues around. Are there best practices concerning this particular case ?

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Some CAs will not allow two certificates with the same SubjectDN, so the new one would supersede the old one. If there is no reason to believe the previous certificate had its PrKey compromised, it does not need to be revoked prior to signing and installing the replacement. Speak to the operators of your CA, especially the personnel acting as an RA, since they would be best apprised of their policy and the BCP.

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  • Thanks for your answer, that seems to be a big issue. How to maintain a old hierarchy (to avoid massive revocations for hundreds of certificates) and introducing a new one ("SHA256 chain") along during a sliding period ?
    – crypto-learner
    Commented Dec 4, 2014 at 14:14
  • Run both CAs concurrently, issue from the new one when you renew, and plan long-term (at least 1.5 times your maximum certificate life). Issue new assets from the new CA, unless there is a technical impediment that would prevent using a certificate signed using SHA256RSA.
    – DTK
    Commented Dec 5, 2014 at 4:57

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