CMS_ENCRYPT(3ossl)

📦
version 15.1 · 3 versions
, in freebsd 15.1

💌 CMS_ENCRYPT — create a CMS envelopedData structure ✨

Okay bestie, so this one's from OpenSSL and it's literally the page about CMS_encrypt_ex and CMS_encrypt, which is how you build a CMS EnvelopedData structure. Think "wrap up your data all cozy for a bunch of recipients" energy. 🎀

🌸 SYNOPSIS 🌸

#include <openssl/cms.h>

CMS_ContentInfo *CMS_encrypt_ex(STACK_OF(X509) *certs, BIO *in,
                                const EVP_CIPHER *cipher, unsigned int flags,
                                OSSL_LIB_CTX *libctx, const char *propq);
CMS_ContentInfo *CMS_encrypt(STACK_OF(X509) *certs, BIO *in,
                             const EVP_CIPHER *cipher, unsigned int flags);

💖 DESCRIPTION 💖

So CMS_encrypt_ex() is the main character here: it creates and returns a CMS EnvelopedData or AuthEnvelopedData structure. Iconic.

Here's the cast:

  • certs — a list of recipient certificates 🫶
  • in — the content that's getting encrypted
  • cipher — the symmetric cipher to use
  • flags — an optional set of flags

And quietly doing work in the background: the library context libctx and the property query propq, which are used internally when retrieving algorithms from providers. 🧚

💀 Heads up: only certificates carrying RSA, Diffie-Hellman or EC keys are supported by this function. Like, that's the whole guest list.

EVP_des_ede3_cbc() (triple DES) is the algorithm of choice for S/MIME use because most clients will support it. Yes, triple DES. It's giving old money. 💅

The algorithm passed in the cipher parameter must support ASN1 encoding of its parameters — no exceptions. If the cipher mode is GCM, then an AuthEnvelopedData structure containing MAC is used. Otherwise an EnvelopedData structure is used. Currently the AES variants with GCM mode are the only supported AEAD algorithms.

Many browsers implement a "sign and encrypt" option which is simply an S/MIME envelopedData containing an S/MIME signed message. Want that? Easiest slay ever: store the S/MIME signed message in a memory BIO and pass it to CMS_encrypt(). 🦋

The following flags can be passed in the flags parameter. 👇

  • CMS_TEXT — if this is set, MIME headers for type text/plain are prepended to the data.
  • CMS_BINARY — normally the supplied content gets translated into MIME canonical format (as required by the S/MIME specifications). If CMS_BINARY is set, no translation occurs. Use this one if your supplied data is in binary format, otherwise the translation will corrupt it. And lowkey important: if CMS_BINARY is set then CMS_TEXT is ignored. 🙈
  • CMS_USE_KEYID — OpenSSL will by default identify recipient 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 all recipient certificates do not have a subject key identifier extension.
  • CMS_STREAM — if set, a partial CMS_ContentInfo structure is returned suitable for streaming I/O: no data is read from the BIO in.
  • CMS_PARTIAL — if set, a partial CMS_ContentInfo structure is returned to which additional recipients and attributes can be added before finalization. 🧩
  • CMS_DETACHED — the data being encrypted is included in the CMS_ContentInfo structure, unless CMS_DETACHED is set, in which case it is omitted. This is rarely used in practice and is not supported by SMIME_write_CMS(). 💀
⚠️ Real talk: 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 results. Do not do that. 😭

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(). ✨

The recipients specified in certs use a CMS KeyTransRecipientInfo info structure. KEKRecipientInfo is also supported using the flag CMS_PARTIAL and CMS_add0_recipient_key().

Fun fact: the parameter certs may be NULL if CMS_PARTIAL is set and recipients added later using CMS_add1_recipient_cert() or CMS_add0_recipient_key(). Flexible king. 👑

And CMS_encrypt() is similar to CMS_encrypt_ex() but uses default values of NULL for the library context libctx and the property query propq. Basically the chill, no-frills version. 🍓

🌈 RETURN VALUES 🌈

CMS_encrypt_ex() and CMS_encrypt() return either a CMS_ContentInfo structure or NULL if an error occurred. If it's NULL, bestie, the error can be obtained from ERR_get_error(3). Go check. 🕵️‍♀️

🔥 SEE ALSO 🔥

⭐ HISTORY ⭐

  • The function CMS_encrypt_ex() was added in OpenSSL 3.0.
  • The CMS_STREAM flag was first supported in OpenSSL 1.0.0.

Copyright 2008-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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 <https://www.openssl.org/source/license.html>.

And that's the whole page, bestie — brought to you by 3.5.6, dated 2026-04-07, filed under CMS_ENCRYPT(3ossl). Go encrypt something iconic. 💅✨

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