man PKCS7_sign () - create a PKCS#7 signedData structure


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 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 included.

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 structure.


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