Hi,
I had a similar problem. I had to extract signingTime attribute from a PKCS#7 signature. I couldn't find the ultimate solution on the Internet but I could pick up bits and parts from various places and came up to this. Maybe there is a nicer/better/safer way, it's the first time I amd doing this but it seems to work.
In a function I have p_pkcs7SigSize bytes of PKCS#7 signature in a buffer pointing by const void *p_pkcs7Sig. I got signing time by this. I've removed the error handling, do not use this code verbose!
BIO *v_in = NULL;
PKCS7 *v_p7 = NULL;
STACK_OF(PKCS7_SIGNER_INFO) *v_signerInfos = NULL;
PKCS7_SIGNER_INFO *v_signerInfo = NULL;
ASN1_TYPE *v_asn1SigningTime = NULL;
/* make BIO for input buffer */
v_in = BIO_new_mem_buf( (void*)(uintptr_t) p_pkcs7Sig, p_pkcs7SigSize );
/* make a PKCS7 object of it */
v_p7 = d2i_PKCS7_bio( v_in, NULL);
/* get all signer infos */
v_signerInfos = PKCS7_get_signer_info( v_p7 );
/* if you need all signer infos then loop through all,
* count you get by k_PKCS7_SIGNER_INFO_num(v_signerInfos)
*/
/* get the first signer info */
v_signerInfo = sk_PKCS7_SIGNER_INFO_value(v_signerInfos,0);
/* get signing time */
v_asn1SigningTime = PKCS7_get_signed_attribute( v_signerInfo, NID_pkcs9_signingTime );
/* You should got a v_asn1SigningTime->type == V_ASN1_UTCTIME,
* if yes then the actual value is in the string buffer at
* v_asn1SigningTime->value.utctime->data
*/
if ( v_in )
{
BIO_free_all( v_in );
v_in = NULL;
}