#include <openssl/pkcs7.h> PKCS7 *PKCS7_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs, BIO *data, int flags);
The data to be signed is read from BIO data.
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_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.
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.
The PKCS7_PARTIAL flag was added in OpenSSL 1.0.0
The PKCS7_STREAM flag was added in OpenSSL 1.0.0