PKCS7_sign(3SSL) OpenSSL PKCS7_sign(3SSL)
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_PARTSIGN 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. Currently the
flag PKCS7_DETACHED must also be set.
Currently the flag PKCS7_PARTSIGN is only supported for detached data.
If this flag 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.
At present only the SMIME_write_PKCS7() function properly finalizes the
PKCS7_sign() is somewhat limited. It does not support multiple signers,
some advanced attributes such as counter signatures are not supported.
The SHA1 digest algorithm is currently always used.
When the signed data is not detached it will be stored in memory within
the PKCS7 structure. This effectively limits the size of messages which
can be signed due to memory restraints. There should be a way to sign
data without having to hold it all in memory, this would however
require fairly major revisions of the OpenSSL ASN1 code.
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_PARTSIGN flag was added in OpenSSL 0.9.8
0.9.8g 2003-02-15 PKCS7_sign(3SSL)