mirror of https://github.com/openssl/openssl.git
123 lines
5.0 KiB
Plaintext
123 lines
5.0 KiB
Plaintext
=pod
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=head1 NAME
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CMS_decrypt, CMS_decrypt_set1_pkey_and_peer,
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CMS_decrypt_set1_pkey, CMS_decrypt_set1_password
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- 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,
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BIO *dcont, BIO *out, unsigned int flags);
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int CMS_decrypt_set1_pkey_and_peer(CMS_ContentInfo *cms,
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EVP_PKEY *pk, X509 *cert, X509 *peer);
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int CMS_decrypt_set1_pkey(CMS_ContentInfo *cms, EVP_PKEY *pk, X509 *cert);
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int CMS_decrypt_set1_password(CMS_ContentInfo *cms,
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unsigned char *pass, ossl_ssize_t passlen);
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=head1 DESCRIPTION
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CMS_decrypt() extracts the decrypted content from a CMS EnvelopedData
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or AuthEnvelopedData structure.
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It uses CMS_decrypt_set1_pkey() to decrypt the content
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with the recipient private key I<pkey> if I<pkey> is not NULL.
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In this case, the associated certificate is recommended to provide in I<cert> -
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see the NOTES below.
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I<out> is a BIO to write the content to and
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I<flags> is an optional set of flags.
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If I<pkey> is NULL the function assumes that decryption was already done
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(e.g., using CMS_decrypt_set1_pkey() or CMS_decrypt_set1_password()) and just
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provides the content unless I<cert>, I<dcont>, and I<out> are NULL as well.
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The I<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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CMS_decrypt_set1_pkey_and_peer() decrypts the CMS_ContentInfo structure I<cms>
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using the private key I<pkey>, the corresponding certificate I<cert>, which is
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recommended but may be NULL, and the (optional) originator certificate I<peer>.
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On success, it also records in I<cms> the decryption key I<pkey>, and then
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should be followed by C<CMS_decrypt(cms, NULL, NULL, dcont, out, flags)>.
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This call deallocates any decryption key stored in I<cms>.
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CMS_decrypt_set1_pkey() is the same as
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CMS_decrypt_set1_pkey_and_peer() with I<peer> being NULL.
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CMS_decrypt_set1_password() decrypts the CMS_ContentInfo structure I<cms>
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using the secret I<pass> of length I<passlen>.
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On success, it also records in I<cms> the decryption key used, and then
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should be followed by C<CMS_decrypt(cms, NULL, NULL, dcont, out, flags)>.
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This call deallocates any decryption key stored in I<cms>.
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=head1 NOTES
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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 I<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(),
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or use CMS_decrypt_set1_password() if the recipient has a symmetric key.
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In these cases both I<cert> and I<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_RecipientInfo_decrypt() should be called before CMS_decrypt() and
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I<cert> and I<pkey> set to NULL.
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The following flags can be passed in the I<flags> parameter.
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If the B<CMS_TEXT> flag is set MIME headers for type C<text/plain> are deleted
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from the content. If the content is not of type C<text/plain> then an error is
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returned.
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=head1 RETURN VALUES
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CMS_decrypt(), CMS_decrypt_set1_pkey_and_peer(),
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CMS_decrypt_set1_pkey(), and CMS_decrypt_set1_password()
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return 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 B<set1_> part of these function names is misleading
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and should better read: B<with_>.
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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)>, L<CMS_encrypt(3)>
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=head1 HISTORY
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CMS_decrypt_set1_pkey_and_peer() and CMS_decrypt_set1_password()
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were added in OpenSSL 3.0.
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=head1 COPYRIGHT
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Copyright 2008-2020 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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