---
name: "libressl-CMS_sign"
section: "3"
platform: "linux"
package: "libressl"
version: "4.4.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/libressl-CMS_sign"
---

# libressl-CMS\_sign(3)

create a CMS SignedData structure

## NAME

**CMS\_sign** – create a CMS SignedData structure

## SYNOPSIS

Crypto Library (libcrypto, -lcrypto)

**#include \<openssl/cms.h\>**

*CMS\_ContentInfo* *\**\

**CMS\_sign**(*X509 \*signcert*, *EVP\_PKEY \*pkey*, *STACK\_OF(X509) \*certs*, *BIO \*data*, *unsigned int flags*);

## DESCRIPTION

**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.

The data to be signed is read from *data*.

Any of the following flags (OR'ed together) can be passed in the *flags* argument:

- CMS\_TEXT

  Prepend MIME headers for the type text/plain to the data. Many S/MIME clients expect the signed content to include valid MIME headers.

- CMS\_NOCERTS

  Do not include the signer's certificate 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 signer's certificate can be obtained by other means, for example from a previously signed message.

- CMS\_DETACHED

  Omit the data being signed from the *CMS\_ContentInfo* structure. This is used for *CMS\_ContentInfo* detached signatures which are used in S/MIME plaintext signed messages for example.

- CMS\_BINARY

  Do not translate the supplied content into MIME canonical format even though that is required by the S/MIME specifications. This option should be used if the supplied data is in binary format. Otherwise the translation will corrupt it.

- CMS\_NOATTR

  Do not add any *SignedAttributes*. By default, the *signerInfos* field includes several CMS *SignedAttributes* including the signing time, the CMS content type, and the supported list of ciphers in an *SMIMECapabilities* attribute.

- CMS\_NOSMIMECAP

  Omit just the *SMIMECapabilities*. If present, the SMIMECapabilities attribute indicates support for the following algorithms in preference order: 256-bit AES, 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.

- CMS\_USE\_KEYID

  Use the subject key identifier value to identify signing certificates. An error occurs if the signing certificate does not have a subject key identifier extension. By default, issuer name and serial number are used instead.

- CMS\_STREAM

  Only initialize the returned *CMS\_ContentInfo* structure to prepare it for performing 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. 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 results. Several functions including SMIME\_write\_CMS(3), i2d\_CMS\_bio\_stream(3), or PEM\_write\_bio\_CMS\_stream(3) finalize the structure. Alternatively, finalization can be performed by obtaining the streaming ASN1 *BIO* directly using BIO\_new\_CMS(3).

- CMS\_PARTIAL

  Output a partial *CMS\_ContentInfo* structure to which additional signers and capabilities can be added before finalization.

If a signer is specified, it will use the default digest for the signing algorithm. This is SHA-1 for both RSA and DSA keys.

If *signcert* and *pkey* are NULL, then a certificates only CMS structure is output.

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\_add1\_signer(3), non default digests can be used and custom attributes added. CMS\_final(3) must then be called to finalize the structure if streaming is not enabled.

## RETURN VALUES

**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).

## SEE ALSO

CMS\_add0\_cert(3), CMS\_add1\_signer(3), CMS\_ContentInfo\_new(3), CMS\_final(3), CMS\_sign\_receipt(3), CMS\_verify(3)

## STANDARDS

RFC 5652: Cryptographic Message Syntax (CMS)

- section 5.1: SignedData Type

- section 5.3: SignerInfo Type

RFC 8419: Use of Edwards-Curve Digital Signature Algorithm (EdDSA) Signatures in the Cryptographic Message Syntax (CMS)

RFC 8551: Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Message Specification, section 2.5.2: SMIMECapabilities Attribute

## HISTORY

**CMS\_sign**() first appeared in OpenSSL 0.9.8h and has been available since OpenBSD 6.7.

## BUGS

Some attributes such as counter signatures are not supported.