204 lines
7.1 KiB
Groff
204 lines
7.1 KiB
Groff
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.\" ========================================================================
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.\"
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.IX Title "CMS_DECRYPT 3"
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.TH CMS_DECRYPT 3 "2020-03-02" "3.0.0-dev" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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CMS_decrypt \- decrypt content from a CMS envelopedData structure
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/cms.h>
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\&
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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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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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\&\fICMS_decrypt()\fR extracts and decrypts the content from a \s-1CMS\s0 EnvelopedData
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structure. \fBpkey\fR is the private key of the recipient, \fBcert\fR is the
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recipient's certificate, \fBout\fR is a \s-1BIO\s0 to write the content to and
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\&\fBflags\fR is an optional set of flags.
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.PP
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The \fBdcont\fR parameter is used in the rare case where the encrypted content
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is detached. It will normally be set to \s-1NULL\s0.
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.SH "NOTES"
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.IX Header "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 \s-1CMS\s0
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structure.
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.PP
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If \fBcert\fR is set to \s-1NULL\s0 all possible recipients are tried. This case however
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is problematic. To thwart the \s-1MMA\s0 attack (Bleichenbacher's attack on
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\&\s-1PKCS\s0 #1 v1.5 \s-1RSA\s0 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 \fICMS_decrypt()\fR 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 \fB\s-1CMS_DEBUG_DECRYPT\s0\fR is set
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then the above behaviour is modified and an error \fBis\fR returned if no
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recipient encrypted key can be decrypted \fBwithout\fR generating a random
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content encryption key. Applications should use this flag with
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\&\fBextreme caution\fR especially in automated gateways as it can leave them
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open to attack.
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.PP
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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 \s-1CMS\s0 structure
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in advance using the \s-1CMS\s0 utility functions such as \fICMS_set1_pkey()\fR. In this
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case both \fBcert\fR and \fBpkey\fR should be set to \s-1NULL\s0.
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.PP
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To process KEKRecipientInfo types \fICMS_set1_key()\fR or \fICMS_RecipientInfo_set0_key()\fR
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and \fICMS_RecipientInfo_decrypt()\fR should be called before \fICMS_decrypt()\fR and
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\&\fBcert\fR and \fBpkey\fR set to \s-1NULL\s0.
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.PP
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The following flags can be passed in the \fBflags\fR parameter.
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.PP
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If the \fB\s-1CMS_TEXT\s0\fR flag is set \s-1MIME\s0 headers for type \fBtext/plain\fR are deleted
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from the content. If the content is not of type \fBtext/plain\fR then an error is
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returned.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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\&\fICMS_decrypt()\fR returns either 1 for success or 0 for failure.
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The error can be obtained from \fIERR_get_error\fR\|(3)
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.SH "BUGS"
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.IX Header "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 \fICMS_verify()\fR also applies to \fICMS_decrypt()\fR.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fIERR_get_error\fR\|(3), \fICMS_encrypt\fR\|(3)
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.SH "COPYRIGHT"
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.IX Header "COPYRIGHT"
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Copyright 2008\-2016 The OpenSSL Project Authors. All Rights Reserved.
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.PP
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Licensed under the Apache License 2.0 (the \*(L"License\*(R"). 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 \s-1LICENSE\s0 in the source distribution or at
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<https://www.openssl.org/source/license.html>.
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