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 chain).
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 parameter.
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
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
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 included.
PKCS7_sign() is somewhat limited. It does not support multiple
signers, some advanced attributes such as counter signatures are
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.
Clear text signing does not store the content in memory but
the way PKCS7_sign() operates means that two passes of the data
must typically be made: one to compute the signatures and a second
to output the data along with the signature. There should be a way
to process the data with only a single pass.
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