PKCS7_sign(3) OpenSSL PKCS7_sign(3)
NAME
PKCS7_sign - create a PKCS#7 signedData structure
SYNOPSIS
PKCS7 *PKCS7_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs, BIO *data, int flags);
DESCRIPTION
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.
NOTES
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.
BUGS
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.
RETURN VALUES
PKCS7_sign() returns either a valid PKCS7 structure or NULL if an error occurred. The error can be
obtained from ERR_get_error(3).
SEE ALSO
ERR_get_error(3), PKCS7_verify(3)
HISTORY
PKCS7_sign() was added to OpenSSL 0.9.5
0.9.7l 2002-10-09 PKCS7_sign(3)
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