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 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_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.
If the PKCS7_PARTIAL flag is set a partial PKCS7 structure is output to which additional
signers and capabilities can be added before finalization.
If the flag PKCS7_STREAM 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.
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
In OpenSSL 1.0.0 if signcert and pkey are NULL then a certificates only PKCS#7 structure
In versions of OpenSSL before 1.0.0 the signcert and pkey parameters must NOT be NULL.
Some advanced attributes such as counter signatures are not supported.
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_PARTIAL flag was added in OpenSSL 1.0.0
The PKCS7_STREAM flag was added in OpenSSL 1.0.0
1.0.1i 2010-01-06 PKCS7_sign(3)