Updated: 2021/Apr/14

CMS_sign(3)                         OpenSSL                        CMS_sign(3)

       CMS_sign - create a CMS SignedData structure

       libcrypto, -lcrypto

        #include <openssl/cms.h>

        CMS_ContentInfo *CMS_sign(X509 *signcert, EVP_PKEY *pkey, STACK_OF(X509) *certs,
                                  BIO *data, unsigned int flags);

       CMS_sign() creates and returns a CMS SignedData structure. signcert is
       the certificate to sign with, pkey is the corresponding private key.
       certs is an optional additional set of certificates to include in the
       CMS structure (for example any intermediate CAs in the chain). Any or
       all of these parameters can be NULL, see NOTES below.

       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

       Many S/MIME clients expect the signed content to include valid MIME
       headers. If the CMS_TEXT flag is set MIME headers for type text/plain
       are prepended to the data.

       If CMS_NOCERTS is set the signer's certificate will not be included in
       the CMS_ContentInfo 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 CMS_ContentInfo structure,
       unless CMS_DETACHED is set in which case it is omitted. This is used
       for CMS_ContentInfo 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 CMS_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 CMS signedAttributes
       including the signing time, the CMS content type and the supported list
       of ciphers in an SMIMECapabilities attribute. If CMS_NOATTR is set then
       no signedAttributes will be used. If CMS_NOSMIMECAP is set then just
       the SMIMECapabilities are omitted.

       If present the SMIMECapabilities attribute indicates support for the
       following algorithms in preference order: 256 bit AES, Gost R3411-94,
       Gost 28147-89, 192 bit AES, 128 bit AES, triple DES, 128 bit RC2, 64
       bit RC2, DES and 40 bit RC2. If any of these algorithms is not
       available then it will not be included: for example the GOST algorithms
       will not be included if the GOST ENGINE is not loaded.

       OpenSSL will by default identify signing certificates using issuer name
       and serial number. If CMS_USE_KEYID is set it will use the subject key
       identifier value instead. An error occurs if the signing certificate
       does not have a subject key identifier extension.

       If the flags CMS_STREAM is set then the returned CMS_ContentInfo
       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 CMS_PARTIAL flag is set a partial CMS_ContentInfo structure is
       output to which additional signers and capabilities can be added before

       If the flag CMS_STREAM is set the returned CMS_ContentInfo structure is
       not complete and outputting its contents via a function that does not
       properly finalize the CMS_ContentInfo structure will give unpredictable

       Several functions including SMIME_write_CMS(), i2d_CMS_bio_stream(),
       PEM_write_bio_CMS_stream() finalize the structure. Alternatively
       finalization can be performed by obtaining the streaming ASN1 BIO
       directly using BIO_new_CMS().

       If a signer is specified it will use the default digest for the signing
       algorithm. This is SHA1 for both RSA and DSA keys.

       If signcert and pkey are NULL then a certificates only CMS structure is

       The function CMS_sign() is a basic CMS signing function whose output
       will be suitable for many purposes. For finer control of the output
       format the certs, signcert and pkey parameters can all be NULL and the
       CMS_PARTIAL flag set. Then one or more signers can be added using the
       function CMS_sign_add1_signer(), non default digests can be used and
       custom attributes added. CMS_final() must then be called to finalize
       the structure if streaming is not enabled.

       Some attributes such as counter signatures are not supported.

       CMS_sign() returns either a valid CMS_ContentInfo structure or NULL if
       an error occurred. The error can be obtained from ERR_get_error(3).

       ERR_get_error(3), CMS_verify(3)

       The CMS_STREAM flag is only supported for detached data in OpenSSL
       0.9.8, it is supported for embedded data in OpenSSL 1.0.0 and later.

       Copyright 2008-2016 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at

1.1.1i                            2018-09-23                       CMS_sign(3)