Am 26.04.16 um 16:25 schrieb Stephan Mühlstrasser:
Hi,
I'm trying to plug my own digest algorithm implementation into the PKCS7
functions for creating a signature (using OpenSSL 1.0.2). The hash
computation shall be performed on a hardware device.
For that purpose I wanted to supply my own EVP_MD data structure to
PKCS7_add_signature(). A rough sketch of my code for replacing the
standard SHA-256 implementation looks like this:
static const EVP_MD my_digest_impl =
{
NID_sha256,
...
/* contains function pointers for my own implementation */
};
PKCS7 *p7 = PKCS7_new();
PKCS7_set_type(p7, NID_pkcs7_signed);
PKCS7_SIGNER_INFO *si = PKCS7_add_signature(p7, cert, pkey,
&my_digest_impl);
PKCS7_content_new(sig_parms->p7, NID_pkcs7_data);
PKCS7_set_detached(p7, 1);
BIO *p7bio = PKCS7_dataInit(p7, NULL);
...
...
How can I plug in my own digest implementation? Do I need to implement a
full OpenSSL engine for this purpose?
I was able to implement this requirement now by calling BIO_set_md() on
the BIO that is created by PKCS7_dataInit(). The code for replacing the
digest function looks like this (error checking omitted):
static const EVP_MD my_digest_impl =
{
NID_sha256,
...
/* contains function pointers for my own implementation */
};
EVP_MD_CTX *ctx;
BIO *p7bio = PKCS7_dataInit(p7, NULL);
BIO_get_md_ctx(p7bio, &ctx)
ctx->flags |= EVP_MD_CTX_FLAG_NO_INIT;
BIO_set_md(sig_parms->p7bio, &my_digest_impl);
ctx->update = my_digest_impl.update;
ctx->md_data = OPENSSL_malloc(my_digest_impl.ctx_size);
/* ... Now the ctx->md_data member is initialized with data specific to
the hardware device ... */
my_digest_impl.init(ctx);
The use of the EVP_MD_CTX_FLAG_NO_INIT flag is necessary, because
otherwise the digests init() function would be called from BIO_set_md()
without the necessary information for initializing the hardware device.
With the flag being set the data can be assigned to the md_data member
after the call to BIO_set_md() and then the digest's init() function can
be called.
I'd appreciate any comments if there's a problem with this approach. So
far this seems to be working fine.
--
Stephan
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