CMS_ENCRYPT(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

💌 CMS_ENCRYPT(3ossl) — aka the "wrap it up and seal it" page

okay so bestie, this is CMS_ENCRYPT(3ossl) from the OpenSSL fam's man pages, and it's literally about taking your content and locking it inside a CMS EnvelopedData structure so only your chosen recipients can peek. it's giving ✨sealed envelope✨ energy and i'm obsessed. let's get into it 🫶

✨ NAME ✨

CMS_encrypt_ex, CMS_encrypt — the duo that creates a CMS envelopedData structure. yes, both of them. they're besties 👯‍♀️

🌸 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() creates and returns a CMS EnvelopedData or AuthEnvelopedData structure — depending on your cipher vibes, more on that in a sec. 😌

let's break down the parameters, no gatekeeping here:

  • certs — a list of recipient certificates. like, the people you're sending the sealed envelope to 🎀
  • in — the content to be encrypted. the actual goods.
  • cipher — the symmetric cipher to use 🔐
  • flags — an optional set of flags. yes, optional, and yes, we'll list them all below, don't worry.
  • libctx — the library context, used internally when retrieving algorithms from providers
  • propq — the property query, same deal, used internally when fetching algorithms from providers
lowkey important: only certificates carrying RSA, Diffie-Hellman or EC keys are supported by this function. like, that's the whole list. no cap. 💀

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 vintage but it works everywhere, and honestly? iconic for compatibility 👑

the algorithm passed in the cipher parameter must support ASN1 encoding of its parameters. non-negotiable bestie. and here's the cute part — if the cipher mode is GCM, then an AuthEnvelopedData structure containing a MAC is used. otherwise, an EnvelopedData structure is used. currently the AES variants with GCM mode are the only supported AEAD algorithms. periodt. ✨

many browsers implement a "sign and encrypt" option which is simply an S/MIME envelopedData containing an S/MIME signed message. so cute, so composed 🥹. this can be readily produced by storing the S/MIME signed message in a memory BIO and passing it to CMS_encrypt().

🦋 the flags, one by one 🦋

the following flags can be passed in the flags parameter. buckle up, there's a bunch, but every single one matters:

  • CMS_TEXT — if set, MIME headers for type text/plain are prepended to the data. cute little header moment 🎀
  • CMS_BINARY — 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 😭. and — big note — if CMS_BINARY is set then CMS_TEXT is ignored. they do not co-exist, bestie.
  • 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. all of them. every single one.
  • 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. very "work in progress, do not disturb" energy 🚧
  • 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(). 🙈

🧚 streaming & finalization tea 🧚

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. like, chaotic. do not do this. 💀

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(). slay, options 💖

👑 recipient structure lore 👑

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(). iykyk 🔥

and yes — 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(). so flexible. so main character. ⭐

🌈 the family connection 🌈

CMS_encrypt() is similar to CMS_encrypt_ex() but uses default values of NULL for the library context libctx and the property query propq. literally the low-effort sibling and we love them both equally 🫶

🎀 RETURN VALUES 🎀

CMS_encrypt_ex() and CMS_encrypt() return either a CMS_ContentInfo structure or NULL if an error occurred. 😭 if you got NULL, the error can be obtained from ERR_get_error(3) — go check, bestie, the answer is in there.

🔥 SEE ALSO 🔥

(the reverse operation, if you were wondering. go read it next 👀)

⭐ HISTORY ⭐

  • CMS_encrypt_ex() was added in OpenSSL 3.0. a newer addition, a glow-up era ✨
  • the CMS_STREAM flag was first supported in OpenSSL 1.0.0. vintage, but still serving 💅

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! 💖 mwah 💋

this was CMS_ENCRYPT(3ossl) — OpenSSL version 3.5.0, dated 2026-09-25. go encrypt something cute, and remember: check your flags, finalize your streams, and never set CMS_BINARY and CMS_TEXT together expecting both to work 🎀✨

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