Chapter 4 Using Crypto-C 137
Using Cryptographic Hardware
surrender context (private key operations are more susceptible to a timing attack
when you use a surrender context, for instance), you must pass in
NULL_PTR. If you
want one operation to use a surrender context and another not to, you must create
two choosers.
When, later on, you call the Crypto-C function that will actually call down to the
token (such as
B_GenerateKeypair or B_SignFinal), Crypto-C will ignore any
surrenderContext
argument you pass at that time. PKCS #11 does not allow you to
have a surrender context associated with an individual function.
Suppose we were going to sign with the private key created.
We are passing in the same
B_PKCS11_SESSION struct. This time, though, it has a
sessionHandle
and a pointer to the cryptokiFunctions (set by Crypto-C during the last
call to
B_CreateHardwareChooser) Now, Crypto-C will not load the library, initialize
or create a session. We could have reset the
sessionHandle field to 0 but leave the
cryptokiFunctions field to the address given. In that case, Crypto-C would have
used the same token, but created a new session.
Both the generating chooser (
hwChooserGen
) and the signing chooser (
hwChooserSign
)
must be destroyed later. You should destroy choosers in the reverse order that they
were created. It will not be necessary in every situation, but there can be cases when it
is required. So it is simply a good idea always to destroy them in reverse order.
In both choosers, we have software backups. That is, if Crypto-C cannot create the
hardware chooser (for example, if the token is not in its slot), it will examine the
software replacement argument and see if there is a suitable AM in that array. If you
B_ALGORITHM_METHOD *RSA_SIGN_HW_CHOOSER[] = {
&AM_MD5,
(B_ALGORITHM_METHOD *)&AM_PKCS11_RSA_PRIVATE_SIGN,
(B_ALGORITHM_METHOD *)NULL_PTR
};
B_ALGORITHM_METHOD *RSA_SIGN_SW_CHOOSER[] = {
&AM_RSA_CRT_ENCRYPT,
(B_ALGORITHM_METHOD *)NULL_PTR
};
B_ALGORITHM_CHOOSER *hwChooserSign =
(B_ALGORITHM_CHOOSER)NULL_PTR;
if ((status = B_CreateHardwareChooser
(RSA_SIGN_HW_CHOOSER, &hwChooserSign,
RSA_SIGN_SW_CHOOSER, HI_PKCS11Session,
(POINTER)&p11Session)) != 0)
break;
