STACK_OF(X509) *PKCS7_get0_signers(PKCS7 *p7, STACK_OF(X509) *certs, int flags);
flags is an optional set of flags, which can be used to modify the verify operation.
PKCS7_get0_signers() retrieves the signer's certificates from p7, it does not check their validity or whether any signatures are valid. The certs and flags parameters have the same meanings as in PKCS7_verify().
Initially some sanity checks are performed on p7. The type of p7 must be signedData. There must be at least one signature on the data and if the content is detached indata cannot be NULL.
An attempt is made to locate all the signer's certificates, first looking in the certs parameter (if it is not NULL) and then looking in any certificates contained in the p7 structure itself. If any signer's certificates cannot be located the operation fails.
Each signer's certificate is chain verified using the smimesign purpose and the supplied trusted certificate store. Any internal certificates in the message are used as untrusted CAs. If any chain verify fails an error code is returned.
Finally the signed content is read (and written to out is it is not NULL) and the signature's checked.
If all signature's verify correctly then the function is successful.
Any of the following flags (ored together) can be passed in the flags parameter to change the default verify behaviour. Only the flag PKCS7_NOINTERN is meaningful to PKCS7_get0_signers().
If PKCS7_NOINTERN is set the certificates in the message itself are not searched when locating the signer's certificate. This means that all the signers certificates must be in the certs parameter.
If the PKCS7_TEXT flag is set MIME headers for type text/plain are deleted from the content. If the content is not of type text/plain then an error is returned.
If PKCS7_NOVERIFY is set the signer's certificates are not chain verified.
If PKCS7_NOCHAIN is set then the certificates contained in the message are not used as untrusted CAs. This means that the whole verify chain (apart from the signer's certificate) must be contained in the trusted store.
If PKCS7_NOSIGS is set then the signatures on the data are not checked.
Care should be taken when modifying the default verify behaviour, for example setting PKCS7_NOVERIFY|PKCS7_NOSIGS will totally disable all verification and any signed message will be considered valid. This combination is however useful if one merely wishes to write the content to out and its validity is not considered important.
Chain verification should arguably be performed using the signing time rather than the current time. However since the signing time is supplied by the signer it cannot be trusted without additional evidence (such as a trusted timestamp).
PKCS7_get0_signers() returns all signers or NULL if an error occurred.
The error can be obtained from ERR_get_error(3)
The lack of single pass processing and need to hold all data in memory as mentioned in PKCS7_sign() also applies to PKCS7_verify().