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What is the difference between .pfx and .cert certificate files?

Do we distribute .pfx or .cert for client authentication?

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2 Answers

up vote 18 down vote accepted

There are two objects: the private key, which is what the server owns, keeps secret, and uses to receive new SSL connections; and the public key which is mathematically linked to the private key, and made "public": it is sent to every client as part of the initial steps of the connection.

The certificate is, nominally, a container for the public key. It includes the public key, the server name, some extra information about the server, and a signature computed by a certification authority. When the server sends its public key to a client, it actually sends its certificate, with a few other certificates (the certificate which contains the public key of the CA which signed its certificate, and the certificate for the CA which signed the CA's certificate, and so on). Certificates are intrinsically public objects.

Some people use the term "certificate" to designate both the certificate and the private key; this is a common source of confusion. I personally stick to the strict definition for which the certificate is the signed container for the public key only.

A ".pfx" file is a PKCS#12 archive: a bag which can contain a lot of objects with optional password protection; but, usually, a PKCS#12 archive contains a certificate (possibly with its assorted set of CA certificates) and the corresponding private key.

On the other hand, a ".cert" (or ".cer" or ".crt") file usually contains a single certificate, alone and without any wrapping (no private key, no password protection, just the certificate).

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While doing client authentication, we require ssl client certificate to be installed on client browser. Is this .pfx file or .cert file? –  Xsecure123 Jan 21 '13 at 12:31
    
Certificates are public data; everybody has them. But client authentication is about having the client do something that only that client can do; so the client must know something which is not public, and that's the private key. Thus, the client must have a private key along with its certificate; if the key was generated out of the client browser, then the expected setup is to import it into the client along with the certificate. Therefore, a .pfx file. –  Thomas Pornin Jan 21 '13 at 13:26
    
I have got .pfx file from IIS server where my certificate is installed. Is this the .pfx file which should be distributed? Since CA provided .cert file including keys which was installed on server. –  Xsecure123 Jan 22 '13 at 11:13
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I know this is a year-old thread, but for future readers, as mentioned above, no you do not distribute the .pfx file because that is the file containing the private key. You can extract and distribute the certificate (which is public) from the .pfx file via the method described here: http://stackoverflow.com/questions/403174/convert-pfx-to-cer

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