[openssl-dev] [openssl-users] OpenSSL engine and TPM usage.

[James Bottomley]

On Thu, 2017-10-26 at 13:57 +0200, Richard Levitte wrote:
> In message <CALq8RvLJVQtPhxtQuJzK5oWpb9w79u02FeW1ZoN636va6=F7NA at mail.
> gmail.com> on Wed, 25 Oct 2017 22:36:45 +0530, Jayalakshmi bhat <bhat
> .jayalakshmi at gmail.com> said:
> 
> bhat.jayalakshmi> Our device uses TPM to protect certificate private
> bhat.jayalakshmi> keys. We have written engine interface to integrate
> bhat.jayalakshmi> TPM functionality into OpenSSL. Thus TPM gets
> loaded
> bhat.jayalakshmi> as an engine instance. Also we have mapped RSA
> bhat.jayalakshmi> operations to TPM APIS as like
> encryption/decryption
> bhat.jayalakshmi> etc.
> bhat.jayalakshmi> 
> bhat.jayalakshmi> Now we are into few issues. there are few
> bhat.jayalakshmi> applications that wants to use application specific
> bhat.jayalakshmi> identity certificate. In such cases RSA APIs should
> bhat.jayalakshmi> not get mapped to TPM APIs.
> bhat.jayalakshmi> 
> bhat.jayalakshmi> I wanted to know when we use engine instance for
> bhat.jayalakshmi> encyrption/decryption operation, can it be done
> bhat.jayalakshmi> selectively?
> 
> Yes, it should be possible.  However, this is up to the application,
> not to your engine.  The engine simply offers methods, the
> application
> chooses to use some or all of them as default algo implementations
> through the diverse ENGINE_set_default* functions.
> 
> For keys such as RSA ones, you can simply create the resulting
> fetched
> key with RSA_new_method(e), where |e| is your TPM engine, and your
> engine's RSA method will be attached to that key rather than the
> default.

Unless you have a Firmware TPM, you really don't want to override the
default methods, all you want to do is load and engine based key and
override only specific key methods which are useful.  The reason is the
TPM is really slow, so you want the native system to do all the crypto
operations it can rather than routing them through the TPM.  So, for
instance, if you look at the elliptic curve overrides for the tpm2
engine,

https://git.kernel.org/pub/scm/linux/kernel/git/jejb/openssl_tpm2_engine.git/

all it does is change the EC_KEY methods for sign and nothing else (so
verify still uses non-TPM methods).

James