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 struc-
ture. 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 canon-
ical 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 oth-
erwise the translation will corrupt it.
The signedData structure includes several PKCS#7 autenti-
catedAttributes including the signing time, the PKCS#7 con-
tent type and the supported list of ciphers in an SMIMECapa-
bilities attribute. If PKCS7_NOATTR is set then no authenti-
catedAttributes will be used. If PKCS7_NOSMIMECAP is set
then just the SMIMECapabilities are omitted.
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PKCS7_SIGN(3) OpenSSL PKCS7_SIGN(3)
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 multi-
ple signers, some advanced attributes such as counter signa-
tures 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 res-
traints. 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).
ERR_get_error(3), PKCS7_verify(3)
PKCS7_sign() was added to OpenSSL 0.9.5
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