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124 .\" ========================================================================
126 .IX Title "EVP_BytesToKey 3"
127 .TH EVP_BytesToKey 3 "2012-01-18" "1.0.0g" "OpenSSL"
128 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
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133 EVP_BytesToKey \- password based encryption routine
135 .IX Header "SYNOPSIS"
137 \& #include <openssl/evp.h>
139 \& int EVP_BytesToKey(const EVP_CIPHER *type,const EVP_MD *md,
140 \& const unsigned char *salt,
141 \& const unsigned char *data, int datal, int count,
142 \& unsigned char *key,unsigned char *iv);
145 .IX Header "DESCRIPTION"
146 \&\fIEVP_BytesToKey()\fR derives a key and \s-1IV\s0 from various parameters. \fBtype\fR is
147 the cipher to derive the key and \s-1IV\s0 for. \fBmd\fR is the message digest to use.
148 The \fBsalt\fR paramter is used as a salt in the derivation: it should point to
149 an 8 byte buffer or \s-1NULL\s0 if no salt is used. \fBdata\fR is a buffer containing
150 \&\fBdatal\fR bytes which is used to derive the keying data. \fBcount\fR is the
151 iteration count to use. The derived key and \s-1IV\s0 will be written to \fBkey\fR
152 and \fBiv\fR respectively.
155 A typical application of this function is to derive keying material for an
156 encryption algorithm from a password in the \fBdata\fR parameter.
158 Increasing the \fBcount\fR parameter slows down the algorithm which makes it
159 harder for an attacker to peform a brute force attack using a large number
160 of candidate passwords.
162 If the total key and \s-1IV\s0 length is less than the digest length and
163 \&\fB\s-1MD5\s0\fR is used then the derivation algorithm is compatible with PKCS#5 v1.5
164 otherwise a non standard extension is used to derive the extra data.
166 Newer applications should use more standard algorithms such as PKCS#5
167 v2.0 for key derivation.
168 .SH "KEY DERIVATION ALGORITHM"
169 .IX Header "KEY DERIVATION ALGORITHM"
170 The key and \s-1IV\s0 is derived by concatenating D_1, D_2, etc until
171 enough data is available for the key and \s-1IV\s0. D_i is defined as:
174 \& D_i = HASH^count(D_(i\-1) || data || salt)
177 where || denotes concatentaion, D_0 is empty, \s-1HASH\s0 is the digest
178 algorithm in use, HASH^1(data) is simply \s-1HASH\s0(data), HASH^2(data)
179 is \s-1HASH\s0(\s-1HASH\s0(data)) and so on.
181 The initial bytes are used for the key and the subsequent bytes for
184 .IX Header "RETURN VALUES"
185 \&\fIEVP_BytesToKey()\fR returns the size of the derived key in bytes.
187 .IX Header "SEE ALSO"
188 \&\fIevp\fR\|(3), \fIrand\fR\|(3),
189 \&\fIEVP_EncryptInit\fR\|(3)