PKCS7_verify() verifies a PKCS#7 signedData structure. p7 is the PKCS7
structure to verify. certs is a set of certificates in which to search for
the signers certificate. store is a trusted certificate store (used for
chain verification). indata is the signed data if the content is not
present in p7 (that is it is detached). The content is written to out
if it is not NULL.
flags is an optional set of flags, which can be used to modify the verify
PKCS7_get0_signers() retrieves the signers 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().
Normally the verify process proceeds as follows.
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 signers 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 signers certificates cannot be
located the operation fails.
Each signers 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 signatures checked.
If all signatures 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 signers 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
If PKCS7_NOVERIFY is set the signers 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 signers certificate) must be contained in the trusted store.
If PKCS7_NOSIGS is set then the signatures on the data are not checked.
One application of PKCS7_NOINTERN is to only accept messages signed by
a small number of certificates. The acceptable certificates would be passed
in the certs parameter. In this case if the signer is not one of the
certificates supplied in certs then the verify will fail because the
signer cannot be found.
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