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I need to simplify (at the cost of security features) an SMIME deployment.

How can I generate a CSR that reuses the same public and private key for more than one email address?

In other words, I have N email addresses, and want to generate N CSRs (and resulting certificates); however the crypto will be the same. (public and private key). Since I'm using a different subject name, I'm assuming the SerialNumber/Thumbprint/Hash will be different among keys, and this is OK for client software.

I intend to use this for signing and encryption key usages, so preferably the CSR key usage will include both usages (and assume the server will approve of such a request)

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  • What key management problem are you trying to solve?
    – c-US
    Commented Jul 7, 2014 at 4:45
  • @c-US The problem I'm trying to solve is to make signing and encryption easier when using multiple, different email accounts that deliver to the same inbox. For example [email protected] [email protected] and [email protected] have all been acquired by Microsoft. We want an auditor to only need one key to decrypt all 3 of those messages.
    – makerofthings7
    Commented Jul 7, 2014 at 11:26

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How to generate a CSR using an existing keypair depends largely on the software you are using. There is however technically speaking no reason it should not work. May I ask why you want to reuse the same keypair for all certificates?

With openssl you can specify which private key to use so if you generate a private key one and use the same private key for all CSR requests then all requests will use the same keypair.

openssl req -new -key privkey.pem

It might be that the CA however does not allow keypairs to be reused. For example EJBCA has the option 'Enforce unique public keys' which only allows a key to be reused for the same person.

Whether or not you get into problems reusing a private key for different certificates depends on how you are going to use the certificates. Most (if not all) S/MIME clients search for the private key to decrypt with using the issuer/serial number pair. In other words, even though the private keys of the certificates are the same, you cannot decrypt the email with for example Outlook if you use the certificate of someone else (in principle it should be possible since the private key is the same, but since the serial number is different, Outlook won't find the correct certificate).

Security wise, it might not be the best decision to reuse the private key (if one certificate must be revoked you need to revoke them all). Whether or not it will work, depends on your use case.

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You can't do this as private key crypto exists as the key pair generated uses the input (which varies). You'd need an algorithm (logrithmic?) That produces the same output each time, something that disregarded the input.

You lose the very thing you're trying to accomplish, "signing" or the ability to provide non - repudiation.

Why on earth would this ever be ok? I think what you're saying is that you want it to look secure but in reality the keys don't matter.

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You can't do this as private key crypto exists as the key pair generated uses the input (which varies). You'd need an algorithm (logrithmic?) That produces the same output each time, something that disregarded the input.

You lose the very thing you're trying to accomplish, "signing" or the ability to provide non - repudiation.

Why on earth would this ever be ok? I think what you're saying is that you want it to look secure but in reality the keys don't matter.

EDIT

Or are you asking how to exploit that, hm? 2.4.1 https://www.rfc-editor.org/rfc/rfc5751

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