79 lines
2.9 KiB
Text
79 lines
2.9 KiB
Text
=pod
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=head1 NAME
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CMS_decrypt - decrypt content from a CMS envelopedData structure
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=head1 SYNOPSIS
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#include <openssl/cms.h>
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int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert, BIO *dcont, BIO *out, unsigned int flags);
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=head1 DESCRIPTION
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CMS_decrypt() extracts and decrypts the content from a CMS EnvelopedData
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structure. B<pkey> is the private key of the recipient, B<cert> is the
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recipient's certificate, B<out> is a BIO to write the content to and
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B<flags> is an optional set of flags.
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The B<dcont> parameter is used in the rare case where the encrypted content
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is detached. It will normally be set to NULL.
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=head1 NOTES
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OpenSSL_add_all_algorithms() (or equivalent) should be called before using this
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function or errors about unknown algorithms will occur.
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Although the recipients certificate is not needed to decrypt the data it is
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needed to locate the appropriate (of possible several) recipients in the CMS
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structure.
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If B<cert> is set to NULL all possible recipients are tried. This case however
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is problematic. To thwart the MMA attack (Bleichenbacher's attack on
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PKCS #1 v1.5 RSA padding) all recipients are tried whether they succeed or
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not. If no recipient succeeds then a random symmetric key is used to decrypt
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the content: this will typically output garbage and may (but is not guaranteed
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to) ultimately return a padding error only. If CMS_decrypt() just returned an
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error when all recipient encrypted keys failed to decrypt an attacker could
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use this in a timing attack. If the special flag B<CMS_DEBUG_DECRYPT> is set
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then the above behaviour is modified and an error B<is> returned if no
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recipient encrypted key can be decrypted B<without> generating a random
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content encryption key. Applications should use this flag with
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B<extreme caution> especially in automated gateways as it can leave them
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open to attack.
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It is possible to determine the correct recipient key by other means (for
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example looking them up in a database) and setting them in the CMS structure
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in advance using the CMS utility functions such as CMS_set1_pkey(). In this
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case both B<cert> and B<pkey> should be set to NULL.
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To process KEKRecipientInfo types CMS_set1_key() or CMS_RecipientInfo_set0_key()
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and CMS_ReceipientInfo_decrypt() should be called before CMS_decrypt() and
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B<cert> and B<pkey> set to NULL.
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The following flags can be passed in the B<flags> parameter.
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If the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are deleted
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from the content. If the content is not of type B<text/plain> then an error is
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returned.
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=head1 RETURN VALUES
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CMS_decrypt() returns either 1 for success or 0 for failure.
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The error can be obtained from ERR_get_error(3)
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=head1 BUGS
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The lack of single pass processing and the need to hold all data in memory as
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mentioned in CMS_verify() also applies to CMS_decrypt().
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=head1 SEE ALSO
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L<ERR_get_error(3)|ERR_get_error(3)>, L<CMS_encrypt(3)|CMS_encrypt(3)>
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=head1 HISTORY
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CMS_decrypt() was added to OpenSSL 0.9.8
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=cut
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