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With regard to this answer, I have a question about Key Usage in an SSL certificate.

Most SSL server certificates have a RSA key which is not restricted through a Key Usage extension, so you can use both "RSA" and "DHE_RSA" key types.

  • How do I determine if my certificate has a key usage restriction?
  • Do all SSL Keys have an RSA key?
  • Is there only one key usage to look out for, or are there others?

Where should I learn more?

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up vote 2 down vote accepted

You should learn to use the "openssl" command-line tool (available for Windows, Linux, Mac OS X, etc.).

Type on the command line: (where mycert is your certificate)

openssl x509 -text -in mycert

This decodes all the fields in the certificate, and will list any restrictions.

No, it doesn't have to be RSA. You can select Diffie-Helmman instead when you create your certificate.

There are a wide variety of certificate uses. For example, you might want a certificate that can sign other certificates for the domain, so that you don't need to buy individual certificates from a CA, but can create them yourself.

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Not all certificates use RSA, but most do. A few months ago, I scanned a lot of random IP addresses to find SSL servers, and out of 10147 certificate chains (from 16027 servers -- there is considerable chain reuse), only 9 of them used anything else than RSA (6 with DSA keys, 3 with GOST keys; no Diffie-Hellman, no ECDSA). This is more than market dominance; RSA has an almost monopoly on cryptographic algorithms for SSL servers.

(I should write a Web page somewhere with all the stats.)

Apart from the Key Usage extension, a few other extensions can be relevant; see this answer for details.

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