PKCS7_verify(3)

     NAME
          PKCS7_verify - verify a PKCS#7 signedData structure

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

          int 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