You can actually create a certificate for the the Attestation Identity
keys, which are produced as a result of the ActivateIdentity. The resulting
key is not in a format that can be readily added to a certificate though.
You will need to parse it with TrouSerS calls: something like getModulus
and getExponent (the exact name is the the TSS spec), then construct the
AIK certificate that way. It's more work, but it could be done.
-- per aspera ad astra --
On Tue, Aug 6, 2013 at 7:07 PM, Segall, Ariel E <[email protected]> wrote:
> I'll admit to not being very familiar with the openssl-tpm-engine
> software, so hopefully someone else can jump in with that expertise.
>
> However, you're probably correct that a new key is being created, and
> that certificates can be a bit of a challenge with the TPM. Here's why:
>
> - All TPM keys (at least in the current TPMs) are restricted in their
> use. They can only be used for one of signing or decryption, to prevent
> certain attacks. They're further subdivided by the kind of information that
> they can sign or decrypt, for some other use cases you probably don't need
> to worry about right now. It sounds like what you're really trying to do
> here is sign data. (It's the equivalent operation to encrypt with private
> key, decrypt with public.)
>
> - The EK is a *very* limited-use key, because it is the TPM's unique
> lifetime identifier, and using it thus can create privacy concerns. The
> only command which lets you use the EK is something called
> ActivateIdentity, which is used for certifying identity keys; that decrypts
> data sent to the TPM in a very special format. It's really not what you
> want to use here at all.
>
> - Two kinds of TPM keys are suitable for signing data: Signing Keys, as
> the name implies, which can sign arbitrary user-provided data, and Identity
> Keys, which sign data created by the TPM itself. You probably want a
> signing key for this. (They come in three flavors; hopefully your package
> hides the choice from you, but if you have to pick, you want either SHA or
> DER depending on what you're signing.) Signing keys are also the *only*
> TPM keys which can create normal X.509 CSRs, because they're the only ones
> which can sign arbitrary data.
>
> - When you're seeing the package "add a new RSA key and encrypt it with
> the TPM", it's doing one of two things; I don't know which. One possibility
> is that it's creating a TPM key, as described above (all user-created TPM
> keys are encrypted with other TPM keys, usually the automatically-created
> Storage Root Key); in this case, it will have constrained use, but will
> never exist unencrypted outside of the TPM, resulting in a much tighter
> machine-key binding. Of course, the TPM can be a little slow. The other
> possibility is that it's creating an RSA key in software, and encrypting it
> with the TPM. In this case, you can use your RSA key for either signing or
> decryption, and you're working at software speeds; on the other hand, you
> only get the authentication that you're on the platform when you first
> decrypt the key, and it can then be removed from the platform by malware.
> Depending on your threat model, this might be a major problem, or a minor
> one.
>
> That was a bit of a long and complicated explanation. Let me know if any
> of this doesn't make sense, and hopefully someone else will jump in with
> the openssl-specific information.
>
> Ariel
>
>
> On Aug 6, 2013, at 4:27 PM, Pankaj <[email protected]>
> wrote:
>
> Hi,
>
> I want to use TPM for SSL authentication. I have also downloaded the
> openssl-tpm-engine.
>
> I am still not sure what this package is trying to do.
>
> Here is what I am thinking to use:
>
> - Use the RSA EK in the TPM for authentication
> - Get the Pubic EK and create a CSR to a CA (An internal manufacturing
> CA for the server with TPM)
> - Provide a unique Id with CSR(Product-ID and Serial Number probably)
> - The CA sends the signed Certificate and associated chain
> - Install the Certificate Chain in the server's hard drive
>
> During SSL authentication, use TPM for encrypting the data to be sent to
> peer with TPM Private EK. The peer can decrypt it with the Public EK which
> is part of the certificate.
>
> The tpm engine gets loaded with the openssl, but somehow the whole thing
> in openssl_tpm_engine is still not clear to me. I think that this package
> is adding a new RSA key and encrypting it with TPM. I also think that this
> will work only with self signed certificates.
>
> Can anyone please explain !
>
> Regards
> Pankaj Shukla
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