creating a symmetric public/private key

Hi there,

can I use this lib also to generate a certificate with a private key that was created?
Something like:
```
let keypair = try CryptorRSA.makeKeyPair(CryptorRSA.RSAKey.KeySize.bits2048)
var cert = createCertificate()
cert.publicKey = keypair.publicKey
cert.sign(keypair.privateKey, sha256)
```
is it possible?
thanks