Question: I just ordered a cheap Comodo PositiveSSL Certificate via a UK reseller, and I was rather surprised to find that the following files were emailed to me automatically, in a zip file:

* Root CA Certificate - AddTrustExternalCARoot.crt
* Intermediate CA Certificate - COMODORSAAddTrustCA.crt
* Intermediate CA Certificate - COMODORSADomainValidationSecureServerCA.crt
* Your PositiveSSL Certificate - domain\_name.crt

Additionally the cert itself (the last file) is added in text form at the end of the email.

It's for a site that does not need a lot of security - it does not handle credit cards or other highly confidential information. I set up a strong passphrase on the associated private key.

Am I right in assuming this cert is useless without the private key and passphrase? Or, given that email can be considered compromised, would an attacker wishing to decrypt my site traffic be at an advantage if they have these files?

I am minded to re-generate the certificate immediately, but I worry that Comodo will just "helpfully" send me a new zip file. I would much rather download all these files from the reseller's SSL website.

Answer: 
You are right assuming the certificate is useless without the private key, so sending it in the mail is no big security risk and is common practice actually. The certificate is supposed to be public, connecting to your website would also provide me with your certificate, so no need to hack your email there.

**edit**

When starting the connection the server sends the certificate which incorporates the public key. The client will generate a (symmetric) session key used for encrypting the rest of the communication and encrypt this with the public key. Now only the server with the corresponding private key can decrypt this session key and use it to decrypt and encrypt the following data.

This way it doesn't matter if someone else has your certificate, as long as they don't have the private key belonging to it, they won't be able to decrypt the session key and won't be able to impersonate your server.
Yes, what you are getting in the zip file is exactly what every visitor to your site would get every time they start a TLS session - the public keys with certifying information. The private key is the only thing that should be kept hidden from unauthorized access.