I have walked through the different fields of x509 certificates and came across some fields which enable to limit the usage of the certificate (for example for certain domains). Is it possible to limit which public key sizes and signature algorithm of the next certificates in the certificate chain are allowed?


1 Answer 1


These kinds of policies come from the Certificate Practise Statement of the CA signing a request and should not live in the fields of a CA certificate.

  • If you don't want e.g. < 3000 bit keys in child certs, the correct approach would be to refuse to sign a request that does not comply with minimums rather than expect the client to interperet a policy encoded in the CA's certificate.
  • In contrast, the signature algorithm and hash size are set by the CA when the signature is generated, so this is entirely down to the configuration of the signing process used.
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    umm I see, but about the second dot, does the ca rule the signature algorithm of its childs? (by setting its own to certain algorithm)
    – adi
    Commented May 22, 2018 at 10:34
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    The root CA can't control much beyond the depth of the child chain - again it should only be signing child CAs that conform to its' own rules, in this case that it has a CPS it approves of and will actually implement. If the child CAs want to disobey those, the solution is not a technical one but an administrative one. Commented May 22, 2018 at 10:38
  • I see, so it is not possible to automatically refuse to those certificates (meaning some human factor has to examine those certificate and decide whether or not to sign them )?
    – adi
    Commented May 27, 2018 at 16:25
  • That's why I referred to the process, not the parameters - you can easily script a test for the relevant conditions before signing so remove the human factor. Commented May 28, 2018 at 8:23

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