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Title "CMS_DECRYPT 3" For nroff, turn off justification. Always turn off hyphenation; it makes
way too many mistakes in technical documents.
CMS_decrypt, CMS_decrypt_set1_pkey_and_peer, CMS_decrypt_set1_pkey, CMS_decrypt_set1_password \- decrypt content from a CMS envelopedData structure Header "SYNOPSIS" .Vb 1 #include <openssl/cms.h> \& int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert, BIO *dcont, BIO *out, unsigned int flags); int CMS_decrypt_set1_pkey_and_peer(CMS_ContentInfo *cms, EVP_PKEY *pk, X509 *cert, X509 *peer); int CMS_decrypt_set1_pkey(CMS_ContentInfo *cms, EVP_PKEY *pk, X509 *cert); int CMS_decrypt_set1_password(CMS_ContentInfo *cms, unsigned char *pass, ossl_ssize_t passlen); .Ve Header "DESCRIPTION" \fBCMS_decrypt() extracts the decrypted content from a \s-1CMS\s0 EnvelopedData or AuthEnvelopedData structure. It uses CMS_decrypt_set1_pkey() to decrypt the content with the recipient private key pkey if pkey is not \s-1NULL.\s0 In this case, it is recommended to provide the associated certificate in cert - see the \s-1NOTES\s0 below. \fIout is a \s-1BIO\s0 to write the content to and \fIflags is an optional set of flags. If pkey is \s-1NULL\s0 the function assumes that decryption was already done (e.g., using CMS_decrypt_set1_pkey() or CMS_decrypt_set1_password()) and just provides the content unless cert, dcont, and out are \s-1NULL\s0 as well. The dcont parameter is used in the rare case where the encrypted content is detached. It will normally be set to \s-1NULL.\s0

\fBCMS_decrypt_set1_pkey_and_peer() decrypts the CMS_ContentInfo structure cms using the private key pkey, the corresponding certificate cert, which is recommended to be supplied but may be \s-1NULL,\s0 and the (optional) originator certificate peer. On success, it also records in cms the decryption key pkey, and this should be followed by \*(C`CMS_decrypt(cms, NULL, NULL, dcont, out, flags)\*(C'. This call deallocates any decryption key stored in cms.

\fBCMS_decrypt_set1_pkey() is the same as \fBCMS_decrypt_set1_pkey_and_peer() with peer being \s-1NULL.\s0

\fBCMS_decrypt_set1_password() decrypts the CMS_ContentInfo structure cms using the secret pass of length passlen. On success, it also records in cms the decryption key used, and this should be followed by \*(C`CMS_decrypt(cms, NULL, NULL, dcont, out, flags)\*(C'. This call deallocates any decryption key stored in cms.

Header "NOTES" Although the recipients certificate is not needed to decrypt the data it is needed to locate the appropriate (of possible several) recipients in the \s-1CMS\s0 structure.

If cert is set to \s-1NULL\s0 all possible recipients are tried. This case however is problematic. To thwart the \s-1MMA\s0 attack (Bleichenbacher's attack on \s-1PKCS\s0 #1 v1.5 \s-1RSA\s0 padding) all recipients are tried whether they succeed or not. If no recipient succeeds then 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 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 \s-1CMS_DEBUG_DECRYPT\s0 is set then the above behaviour is modified and an error is returned if no recipient encrypted key can be decrypted without generating a random content encryption key. Applications should use this flag with \fBextreme caution especially in automated gateways as it can leave them open to attack.

It is possible to determine the correct recipient key by other means (for example looking them up in a database) and setting them in the \s-1CMS\s0 structure in advance using the \s-1CMS\s0 utility functions such as CMS_set1_pkey(), or use CMS_decrypt_set1_password() if the recipient has a symmetric key. In these cases both cert and pkey should be set to \s-1NULL.\s0

To process KEKRecipientInfo types CMS_set1_key() or CMS_RecipientInfo_set0_key() and CMS_RecipientInfo_decrypt() should be called before CMS_decrypt() and \fIcert and pkey set to \s-1NULL.\s0

The following flags can be passed in the flags parameter.

If the \s-1CMS_TEXT\s0 flag is set \s-1MIME\s0 headers for type \*(C`text/plain\*(C' are deleted from the content. If the content is not of type \*(C`text/plain\*(C' then an error is returned.

Header "RETURN VALUES" \fBCMS_decrypt(), CMS_decrypt_set1_pkey_and_peer(), \fBCMS_decrypt_set1_pkey(), and CMS_decrypt_set1_password() return either 1 for success or 0 for failure. The error can be obtained from ERR_get_error\|(3). Header "BUGS" The set1_ part of these function names is misleading and should better read: with_.

The lack of single pass processing and the need to hold all data in memory as mentioned in CMS_verify() also applies to CMS_decrypt().

Header "SEE ALSO" \fBERR_get_error\|(3), CMS_encrypt\|(3) Header "HISTORY" \fBCMS_decrypt_set1_pkey_and_peer() and CMS_decrypt_set1_password() were added in OpenSSL 3.0. Header "COPYRIGHT" Copyright 2008-2023 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use this file except in compliance with the License. You can obtain a copy in the file \s-1LICENSE\s0 in the source distribution or at <https://www.openssl.org/source/license.html>.