PKCS7_sign(3) OpenSSL PKCS7_sign(3)
PKCS7_sign - create a PKCS#7 signedData structure
PKCS7 *PKCS7_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs, BIO *data, int flags);
PKCS7_sign() creates and returns a PKCS#7 signedData structure.
signcert is the certificate to sign with, pkey is the corresponsding
private key. certs is an optional additional set of certificates to
include in the PKCS#7 structure (for example any intermediate CAs in
The data to be signed is read from BIO data.
flags is an optional set of flags.
Any of the following flags (ored together) can be passed in the flags
Many S/MIME clients expect the signed content to include valid MIME
headers. If the PKCS7_TEXT flag is set MIME headers for type text/plain
are prepended to the data.
If PKCS7_NOCERTS is set the signer's certificate will not be included
in the PKCS7 structure, the signer's certificate must still be supplied
in the signcert parameter though. This can reduce the size of the
signature if the signers certificate can be obtained by other means:
for example a previously signed message.
The data being signed is included in the PKCS7 structure, unless
PKCS7_DETACHED is set in which case it is omitted. This is used for
PKCS7 detached signatures which are used in S/MIME plaintext signed
messages for example.
Normally the supplied content is translated into MIME canonical format
(as required by the S/MIME specifications) if PKCS7_BINARY is set no
translation occurs. This option should be used if the supplied data is
in binary format otherwise the translation will corrupt it.
The signedData structure includes several PKCS#7 autenticatedAttributes
including the signing time, the PKCS#7 content type and the supported
list of ciphers in an SMIMECapabilities attribute. If PKCS7_NOATTR is
set then no authenticatedAttributes will be used. If PKCS7_NOSMIMECAP
is set then just the SMIMECapabilities are omitted.
If present the SMIMECapabilities attribute indicates support for the
following algorithms: triple DES, 128 bit RC2, 64 bit RC2, DES and 40
bit RC2. If any of these algorithms is disabled then it will not be
If the flags PKCS7_STREAM is set then the returned PKCS7 structure is
just initialized ready to perform the signing operation. The signing is
however not performed and the data to be signed is not read from the
data parameter. Signing is deferred until after the data has been
written. In this way data can be signed in a single pass.
If the PKCS7_PARTIAL flag is set a partial PKCS7 structure is output to
which additional signers and capabilities can be added before
If the flag PKCS7_STREAM is set the returned PKCS7 structure is not
complete and outputting its contents via a function that does not
properly finalize the PKCS7 structure will give unpredictable results.
Several functions including SMIME_write_PKCS7(),
i2d_PKCS7_bio_stream(), PEM_write_bio_PKCS7_stream() finalize the
structure. Alternatively finalization can be performed by obtaining the
streaming ASN1 BIO directly using BIO_new_PKCS7().
If a signer is specified it will use the default digest for the signing
algorithm. This is SHA1 for both RSA and DSA keys.
In OpenSSL 1.0.0 the certs, signcert and pkey parameters can all be
NULL if the PKCS7_PARTIAL flag is set. One or more signers can be added
using the function PKCS7_sign_add_signer(). PKCS7_final() must also be
called to finalize the structure if streaming is not enabled.
Alternative signing digests can also be specified using this method.
In OpenSSL 1.0.0 if signcert and pkey are NULL then a certificates only
PKCS#7 structure is output.
In versions of OpenSSL before 1.0.0 the signcert and pkey parameters
must NOT be NULL.
Some advanced attributes such as counter signatures are not supported.
PKCS7_sign() returns either a valid PKCS7 structure or NULL if an error
occurred. The error can be obtained from ERR_get_error(3).
PKCS7_sign() was added to OpenSSL 0.9.5
The PKCS7_PARTIAL flag was added in OpenSSL 1.0.0
The PKCS7_STREAM flag was added in OpenSSL 1.0.0
1.0.1i 2010-01-06 PKCS7_sign(3)