| 1 | =pod
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| 2 |
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| 3 | =head1 NAME
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| 4 |
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| 5 | CMS_decrypt - decrypt content from a CMS envelopedData structure
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| 6 |
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| 7 | =head1 SYNOPSIS
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| 8 |
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| 9 | #include <openssl/cms.h>
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| 10 |
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| 11 | int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert,
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| 12 | BIO *dcont, BIO *out, unsigned int flags);
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| 13 |
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| 14 | =head1 DESCRIPTION
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| 15 |
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| 16 | CMS_decrypt() extracts and decrypts the content from a CMS EnvelopedData
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| 17 | structure. B<pkey> is the private key of the recipient, B<cert> is the
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| 18 | recipient's certificate, B<out> is a BIO to write the content to and
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| 19 | B<flags> is an optional set of flags.
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| 20 |
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| 21 | The B<dcont> parameter is used in the rare case where the encrypted content
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| 22 | is detached. It will normally be set to NULL.
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| 23 |
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| 24 | =head1 NOTES
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| 25 |
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| 26 | Although the recipients certificate is not needed to decrypt the data it is
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| 27 | needed to locate the appropriate (of possible several) recipients in the CMS
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| 28 | structure.
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| 29 |
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| 30 | If B<cert> is set to NULL all possible recipients are tried. This case however
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| 31 | is problematic. To thwart the MMA attack (Bleichenbacher's attack on
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| 32 | PKCS #1 v1.5 RSA padding) all recipients are tried whether they succeed or
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| 33 | not. If no recipient succeeds then a random symmetric key is used to decrypt
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| 34 | the content: this will typically output garbage and may (but is not guaranteed
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| 35 | to) ultimately return a padding error only. If CMS_decrypt() just returned an
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| 36 | error when all recipient encrypted keys failed to decrypt an attacker could
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| 37 | use this in a timing attack. If the special flag B<CMS_DEBUG_DECRYPT> is set
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| 38 | then the above behaviour is modified and an error B<is> returned if no
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| 39 | recipient encrypted key can be decrypted B<without> generating a random
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| 40 | content encryption key. Applications should use this flag with
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| 41 | B<extreme caution> especially in automated gateways as it can leave them
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| 42 | open to attack.
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| 43 |
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| 44 | It is possible to determine the correct recipient key by other means (for
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| 45 | example looking them up in a database) and setting them in the CMS structure
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| 46 | in advance using the CMS utility functions such as CMS_set1_pkey(). In this
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| 47 | case both B<cert> and B<pkey> should be set to NULL.
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| 48 |
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| 49 | To process KEKRecipientInfo types CMS_set1_key() or CMS_RecipientInfo_set0_key()
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| 50 | and CMS_RecipientInfo_decrypt() should be called before CMS_decrypt() and
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| 51 | B<cert> and B<pkey> set to NULL.
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| 52 |
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| 53 | The following flags can be passed in the B<flags> parameter.
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| 54 |
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| 55 | If the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are deleted
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| 56 | from the content. If the content is not of type B<text/plain> then an error is
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| 57 | returned.
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| 58 |
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| 59 | =head1 RETURN VALUES
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| 60 |
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| 61 | CMS_decrypt() returns either 1 for success or 0 for failure.
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| 62 | The error can be obtained from ERR_get_error(3)
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| 63 |
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| 64 | =head1 BUGS
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| 65 |
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| 66 | The lack of single pass processing and the need to hold all data in memory as
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| 67 | mentioned in CMS_verify() also applies to CMS_decrypt().
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| 68 |
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| 69 | =head1 SEE ALSO
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| 70 |
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| 71 | L<ERR_get_error(3)>, L<CMS_encrypt(3)>
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| 72 |
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| 73 | =head1 COPYRIGHT
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| 74 |
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| 75 | Copyright 2008-2016 The OpenSSL Project Authors. All Rights Reserved.
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| 76 |
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| 77 | Licensed under the OpenSSL license (the "License"). You may not use
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| 78 | this file except in compliance with the License. You can obtain a copy
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| 79 | in the file LICENSE in the source distribution or at
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| 80 | L<https://www.openssl.org/source/license.html>.
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| 81 |
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| 82 | =cut
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