.\" $OpenBSD: CMS_decrypt.3,v 1.8 2019/11/02 15:39:46 schwarze Exp $ .\" full merge up to: OpenSSL e9b77246 Jan 20 19:58:49 2017 +0100 .\" .\" This file is a derived work. .\" The changes are covered by the following Copyright and license: .\" .\" Copyright (c) 2019 Ingo Schwarze .\" .\" Permission to use, copy, modify, and distribute this software for any .\" purpose with or without fee is hereby granted, provided that the above .\" copyright notice and this permission notice appear in all copies. .\" .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: November 2 2019 $ .Dt CMS_DECRYPT 3 .Os .Sh NAME .Nm CMS_decrypt , .Nm CMS_decrypt_set1_pkey , .Nm CMS_decrypt_set1_key .Nd decrypt content from a CMS EnvelopedData structure .Sh SYNOPSIS .In openssl/cms.h .Ft int .Fo CMS_decrypt .Fa "CMS_ContentInfo *cms" .Fa "EVP_PKEY *private_key" .Fa "X509 *certificate" .Fa "BIO *dcont" .Fa "BIO *out" .Fa "unsigned int flags" .Fc .Ft int .Fo CMS_decrypt_set1_pkey .Fa "CMS_ContentInfo *cms" .Fa "EVP_PKEY *private_key" .Fa "X509 *certificate" .Fc .Ft int .Fo CMS_decrypt_set1_key .Fa "CMS_ContentInfo *cms" .Fa "unsigned char *symmetric_key" .Fa "size_t keylen" .Fa "const unsigned char *id" .Fa "size_t idlen" .Fc .Sh DESCRIPTION .Fn CMS_decrypt extracts and decrypts the content from the CMS .Vt EnvelopedData structure .Fa cms using the .Fa private_key and the .Fa certificate of the recipient. It writes the decrypted content to .Fa out . .Pp In the rare case where the compressed content is detached, pass it in via .Fa dcont . For normal use, set .Fa dcont to .Dv NULL . .Pp Although the recipient's .Fa certificate is not needed to decrypt the data, it is needed to locate the appropriate (of possibly several) recipients in the CMS structure. .Pp If the .Fa certificate is set to .Dv NULL , all possible recipients are tried. This case however is problematic. To thwart the MMA attack (Bleichenbacher's attack on PKCS #1 v1.5 RSA padding), all recipients are tried whether they succeed or not. If no recipient succeeds, a random symmetric key is used to decrypt the content: this will typically output garbage and may (but is not guaranteed to) ultimately return a padding error only. If .Fn CMS_decrypt just returned an error when all recipient encrypted keys failed to decrypt, an attacker could use this in a timing attack. If the special flag .Dv CMS_DEBUG_DECRYPT is set, the above behaviour is modified and an error .Em is returned if no recipient encrypted key can be decrypted .Em without generating a random content encryption key. Applications should use this flag with extreme caution especially in automated gateways as it can leave them open to attack. .Pp It is possible to determine the correct recipient key by other means (for example by looking them up in a database) and setting them in the .Fa cms structure in advance using the CMS utility functions such as .Fn CMS_decrypt_set1_pkey . In this case both .Fa certificate and .Fa private_key should be set to .Dv NULL when calling .Fn CMS_decrypt later on. .Pp To process .Vt KEKRecipientInfo types, .Fn CMS_decrypt_set1_key or .Xr CMS_RecipientInfo_set0_key 3 and .Xr CMS_RecipientInfo_decrypt 3 should be called before .Fn CMS_decrypt and .Fa certificate and .Fa private_key set to .Dv NULL when calling .Fn CMS_decrypt later on. .Pp If the .Dv CMS_TEXT bit is set in .Fa flags , MIME headers for type text/plain are deleted from the content. If the content is not of type text/plain, an error occurs. .Sh RETURN VALUES .Fn CMS_decrypt , .Fn CMS_decrypt_set1_pkey , and .Fn CMS_decrypt_set1_key return 1 for success or 0 for failure. The error can be obtained from .Xr ERR_get_error 3 . .Sh SEE ALSO .Xr CMS_ContentInfo_new 3 , .Xr CMS_encrypt 3 , .Xr CMS_get0_RecipientInfos 3 .Sh STANDARDS RFC 5652: Cryptographic Message Syntax (CMS) .Bl -dash -compact -offset indent .It section 6.1: EnvelopedData Type .It section 6.2.3: KEKRecipientInfo Type .El .Sh HISTORY .Fn CMS_decrypt , .Fn CMS_decrypt_set1_pkey , and .Fn CMS_decrypt_set1_key first appeared in OpenSSL 0.9.8h and have been available since .Ox 6.7 . .Sh BUGS The lack of single pass processing and the need to hold all data in memory as mentioned in .Xr CMS_verify 3 also applies to .Fn CMS_decrypt .