Can a CSR be created in OpenSSL with SHA2? If so, what would the command be and what does this tell the CA, if anything? If you could create a CSR using SHA2 would it "tell" the signing CA to use it on the entire cert chain?

  • For what it's worth, I don't remember the details, but I've had no problems going SHA-2 with my CAs. They offer SHA-1 or SHA-2 as a choice when ordering or take the hint from the CSR; and they provide a fully SHA-2 chain, by default or at least as an alternate download. Commented Aug 13, 2014 at 4:28

1 Answer 1


You can add, for example the -sha256 flag to the OpenSSL command line when generating the CSR. I don't believe any CA will change how they sign your CSR based on this, and it certainly won't affect the certificate chain. They're not resigning the cert chain for each key, the only signature operation they do is on your CSR itself. Any intermediate/root CAs provided will be exactly the same for all customers.

  • 5
    Yes, I was able to use the command openssl req -sha256 -new -key fd.key -out fd.csr to get a SHA2 CSR. If it has no bearing on how the CA signs the cert, then what are the use cases for creating a CSR with SHA2-256/384/512? I'm not clear on why its used.
    – user53029
    Commented Aug 13, 2014 at 4:17
  • I suppose its hashed to prevent attackers from extracting the private key from it?
    – user53029
    Commented Aug 13, 2014 at 4:27
  • 1
    No, RSA keeps your private key safe by nature. That's what it's for. :-) RSA can only sign relatively small things, so you hash the data and sign the conveniently-sized hash. Commented Aug 13, 2014 at 5:23
  • 15
    StartSSL certificates apparently respect the CSR SHA type, as I just had to resubmit a CSR but generated with -sha256 in order to get the SHA2 cert back. Commented Oct 16, 2014 at 5:40
  • Not an available flag if you generate the CSR from an existing cert using openssl x509 -x509toreq -in my.crt -signkey my.key -out my.csr
    – Snekse
    Commented Sep 17, 2015 at 18:16

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