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.\" $OpenBSD: PKCS7_verify.3,v 1.2 2016/11/06 15:52:50 jmc Exp $
.\"
.Dd $Mdocdate: November 6 2016 $
.Dt PKCS7_VERIFY 3
.Os
.Sh NAME
.Nm PKCS7_verify ,
.Nm PKCS7_get0_signers
.Nd verify a PKCS#7 signedData structure
.Sh SYNOPSIS
.In openssl/pkcs7.h
.Ft int
.Fo PKCS7_verify
.Fa "PKCS7 *p7"
.Fa "STACK_OF(X509) *certs"
.Fa "X509_STORE *store"
.Fa "BIO *indata"
.Fa "BIO *out"
.Fa "int flags"
.Fc
.Ft STACK_OF(X509) *
.Fo PKCS7_get0_signers
.Fa "PKCS7 *p7"
.Fa "STACK_OF(X509) *certs"
.Fa "int flags"
.Fc
.Sh DESCRIPTION
.Fn PKCS7_verify
verifies a PKCS#7 signedData structure.
.Fa p7
is the
.Vt PKCS7
structure to verify.
.Fa certs
is a set of certificates in which to search for the signer's
certificate.
.Fa store
is a trusted certificate store (used for chain verification).
.Fa indata
is the signed data if the content is not present in
.Fa p7 ,
that is if it is detached.
The content is written to
.Fa out
if it is not
.Dv NULL .
.Pp
.Fa flags
is an optional set of flags, which can be used to modify the verify
operation.
.Pp
.Fn PKCS7_get0_signers
retrieves the signer's certificates from
.Fa p7 .
It does
.Sy not
check their validity or whether any signatures are valid.
The
.Fa certs
and
.Fa flags
parameters have the same meanings as in
.Fn PKCS7_verify .
.Pp
Normally the verify process proceeds as follows.
.Pp
Initially some sanity checks are performed on
.Fa p7 .
The type of
.Fa p7
must be signedData.
There must be at least one signature on the data and if the content
is detached,
.Fa indata
cannot be
.Dv NULL .
.Pp
An attempt is made to locate all the signer's certificates, first
looking in the
.Fa certs
parameter (if it is not
.Dv NULL )
and then looking in any certificates contained in the
.Fa p7
structure itself.
If any signer's certificates cannot be located the operation fails.
.Pp
Each signer's certificate is chain verified using the
.Sy 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.
.Pp
Finally, the signed content is read (and written to
.Fa out
is it is not
.Dv NULL )
and the signature's checked.
.Pp
If all signature's verify correctly then the function is successful.
.Pp
Any of the following flags (OR'ed together) can be passed in the
.Fa flags
parameter to change the default verify behaviour.
Only the flag
.Dv PKCS7_NOINTERN
is meaningful to
.Fn PKCS7_get0_signers .
.Pp
If
.Dv PKCS7_NOINTERN
is set, the certificates in the message itself are not searched when
locating the signer's certificate.
This means that all the signer's certificates must be in the
.Fa certs
parameter.
.Pp
If the
.Dv PKCS7_TEXT
flag is set, MIME headers for type
.Sy text/plain
are deleted from the content.
If the content is not of type
.Sy text/plain ,
then an error is returned.
.Pp
If
.Dv PKCS7_NOVERIFY
is set, the signer's certificates are not chain verified.
.Pp
If
.Dv 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.
.Pp
If
.Dv PKCS7_NOSIGS
is set, then the signatures on the data are not checked.
.Pp
One application of
.Dv PKCS7_NOINTERN
is to only accept messages signed by a small number of certificates.
The acceptable certificates would be passed in the
.Fa certs
parameter.
In this case, if the signer is not one of the certificates supplied in
.Fa certs ,
then the verify will fail because the signer cannot be found.
.Pp
Care should be taken when modifying the default verify behaviour, for
example setting
.Dv 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
.Fa out
and its validity is not considered important.
.Pp
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).
.Sh RETURN VALUES
.Fn PKCS7_verify
returns 1 for a successful verification and 0 or a negative value if
an error occurs.
.Pp
.Fn PKCS7_get0_signers
returns all signers or
.Dv NULL
if an error occurred.
.Pp
The error can be obtained from
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr ERR_get_error 3 ,
.Xr PKCS7_sign 3
.Sh HISTORY
.Fn PKCS7_verify
was added to OpenSSL 0.9.5 .
.Sh BUGS
The trusted certificate store is not searched for the signer's
certificate.
This is primarily due to the inadequacies of the current
.Vt X509_STORE
functionality.
.Pp
The lack of single pass processing and the need to hold all data
in memory as mentioned in
.Xr PKCS7_sign 3
also applies to
.Fn PKCS7_verify .
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