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135 .\" ========================================================================
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137 .IX Title "CMS_decrypt 3"
138 .TH CMS_decrypt 3 "2014-06-05" "1.0.1n" "OpenSSL"
139 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
140 .\" way too many mistakes in technical documents.
141 .if n .ad l
142 .nh
143 .SH "NAME"
144 .Vb 1
145 \& CMS_decrypt \- decrypt content from a CMS envelopedData structure
146 .Ve
147 .SH "LIBRARY"
148 libcrypto, -lcrypto
149 .SH "SYNOPSIS"
150 .IX Header "SYNOPSIS"
151 .Vb 1
152 \& #include <openssl/cms.h>
153 \&
154 \& int CMS_decrypt(CMS_ContentInfo *cms, EVP_PKEY *pkey, X509 *cert, BIO *dcont, BIO *out, unsigned int flags);
155 .Ve
156 .SH "DESCRIPTION"
157 .IX Header "DESCRIPTION"
158 \&\fICMS_decrypt()\fR extracts and decrypts the content from a \s-1CMS\s0 EnvelopedData
159 structure. \fBpkey\fR is the private key of the recipient, \fBcert\fR is the
160 recipient's certificate, \fBout\fR is a \s-1BIO\s0 to write the content to and
161 \&\fBflags\fR is an optional set of flags.
162 .PP
163 The \fBdcont\fR parameter is used in the rare case where the encrypted content
164 is detached. It will normally be set to \s-1NULL.\s0
165 .SH "NOTES"
166 .IX Header "NOTES"
167 \&\fIOpenSSL_add_all_algorithms()\fR (or equivalent) should be called before using this
168 function or errors about unknown algorithms will occur.
169 .PP
170 Although the recipients certificate is not needed to decrypt the data it is
171 needed to locate the appropriate (of possible several) recipients in the \s-1CMS\s0
172 structure.
173 .PP
174 If \fBcert\fR is set to \s-1NULL\s0 all possible recipients are tried. This case however
175 is problematic. To thwart the \s-1MMA\s0 attack (Bleichenbacher's attack on
176 \&\s-1PKCS\s0 #1 v1.5 \s-1RSA\s0 padding) all recipients are tried whether they succeed or
177 not. If no recipient succeeds then a random symmetric key is used to decrypt
178 the content: this will typically output garbage and may (but is not guaranteed
179 to) ultimately return a padding error only. If \fICMS_decrypt()\fR just returned an
180 error when all recipient encrypted keys failed to decrypt an attacker could
181 use this in a timing attack. If the special flag \fB\s-1CMS_DEBUG_DECRYPT\s0\fR is set
182 then the above behaviour is modified and an error \fBis\fR returned if no
183 recipient encrypted key can be decrypted \fBwithout\fR generating a random
184 content encryption key. Applications should use this flag with
185 \&\fBextreme caution\fR especially in automated gateways as it can leave them
186 open to attack.
187 .PP
188 It is possible to determine the correct recipient key by other means (for
189 example looking them up in a database) and setting them in the \s-1CMS\s0 structure
190 in advance using the \s-1CMS\s0 utility functions such as \fICMS_set1_pkey()\fR. In this
191 case both \fBcert\fR and \fBpkey\fR should be set to \s-1NULL.\s0
192 .PP
193 To process KEKRecipientInfo types \fICMS_set1_key()\fR or \fICMS_RecipientInfo_set0_key()\fR
194 and \fICMS_ReceipientInfo_decrypt()\fR should be called before \fICMS_decrypt()\fR and
195 \&\fBcert\fR and \fBpkey\fR set to \s-1NULL.\s0
196 .PP
197 The following flags can be passed in the \fBflags\fR parameter.
198 .PP
199 If the \fB\s-1CMS_TEXT\s0\fR flag is set \s-1MIME\s0 headers for type \fBtext/plain\fR are deleted
200 from the content. If the content is not of type \fBtext/plain\fR then an error is
201 returned.
202 .SH "RETURN VALUES"
203 .IX Header "RETURN VALUES"
204 \&\fICMS_decrypt()\fR returns either 1 for success or 0 for failure.
205 The error can be obtained from \fIERR_get_error\fR\|(3)
206 .SH "BUGS"
207 .IX Header "BUGS"
208 The lack of single pass processing and the need to hold all data in memory as
209 mentioned in \fICMS_verify()\fR also applies to \fICMS_decrypt()\fR.
210 .SH "SEE ALSO"
211 .IX Header "SEE ALSO"
212 \&\fIERR_get_error\fR\|(3), \fICMS_encrypt\fR\|(3)
213 .SH "HISTORY"
214 .IX Header "HISTORY"
215 \&\fICMS_decrypt()\fR was added to OpenSSL 0.9.8