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 sig-
nature 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
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.
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
3rd Berkeley Distribution 0.9.7d PKCS7_sign(3)