NAME

       PKCS7_verify - verify a PKCS#7 signedData structure

SYNOPSIS

        #include <openssl/pkcs7.h>

        int PKCS7_verify(PKCS7 *p7, STACK_OF(X509) *certs, X509_STORE *store, BIO *indata, BIO *out, int flags);

        STACK_OF(X509) *PKCS7_get0_signers(PKCS7 *p7, STACK_OF(X509) *certs, int flags);

DESCRIPTION

       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 signer's certificate.
       store is a trusted certficate 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 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().

VERIFY PROCESS

       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 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.

NOTES

       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 timestamp).

RETURN VALUES

       PKCS7_verify() returns 1 for a successful verification and zero or a negative value if an
       error occurs.

       PKCS7_get0_signers() returns all signers or NULL if an error occurred.

       The error can be obtained from ERR_get_error(3)

BUGS

       The trusted certificate store is not searched for the signers certificate, this is
       primarily due to the inadequacies of the current X509_STORE functionality.

       The lack of single pass processing and need to hold all data in memory as mentioned in
       PKCS7_sign() also applies to PKCS7_verify().

SEE ALSO

       ERR_get_error(3), PKCS7_sign(3)

HISTORY

       PKCS7_verify() was added to OpenSSL 0.9.5