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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
PKCS7_sign() creates and returns a PKCS#7 signedData structure. signcert is the certifi-
cate to sign with, pkey is the corresponsding private key. certs is an optional addi-
tional set of certificates to include in the PKCS#7 structure (for example any intermedi-
ate 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 struc-
ture, 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 SMIMECapa-
bilities 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.
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 struc-
ture. 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.
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
0.9.7a 2002-10-09 PKCS7_sign(3)
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