Question

I wanted to extract the Signer Informations from PKCS#7 Signed Code Image using C/CPP. I wanted to know the openssl API’s. I am Able to extract Using bouncy castle (CMSSignedData).

Please let me know the openssl API’s which I can use in C/CPP to extract the each signers and signer informations and verify the Signers.

is there any API like X509_LOOKUP_buffer() instead of X509_LOOKUP_file() ???

Thanks in advance opensid

Answer 1

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;
}