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133 .\" ========================================================================
135 .IX Title "EVP_BYTESTOKEY 3"
136 .TH EVP_BYTESTOKEY 3 "2021-08-24" "1.1.1l" "OpenSSL"
137 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
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142 EVP_BytesToKey \- password based encryption routine
144 .IX Header "SYNOPSIS"
146 \& #include <openssl/evp.h>
148 \& int EVP_BytesToKey(const EVP_CIPHER *type, const EVP_MD *md,
149 \& const unsigned char *salt,
150 \& const unsigned char *data, int datal, int count,
151 \& unsigned char *key, unsigned char *iv);
154 .IX Header "DESCRIPTION"
155 \&\fBEVP_BytesToKey()\fR derives a key and \s-1IV\s0 from various parameters. \fBtype\fR is
156 the cipher to derive the key and \s-1IV\s0 for. \fBmd\fR is the message digest to use.
157 The \fBsalt\fR parameter is used as a salt in the derivation: it should point to
158 an 8 byte buffer or \s-1NULL\s0 if no salt is used. \fBdata\fR is a buffer containing
159 \&\fBdatal\fR bytes which is used to derive the keying data. \fBcount\fR is the
160 iteration count to use. The derived key and \s-1IV\s0 will be written to \fBkey\fR
161 and \fBiv\fR respectively.
164 A typical application of this function is to derive keying material for an
165 encryption algorithm from a password in the \fBdata\fR parameter.
167 Increasing the \fBcount\fR parameter slows down the algorithm which makes it
168 harder for an attacker to perform a brute force attack using a large number
169 of candidate passwords.
171 If the total key and \s-1IV\s0 length is less than the digest length and
172 \&\fB\s-1MD5\s0\fR is used then the derivation algorithm is compatible with PKCS#5 v1.5
173 otherwise a non standard extension is used to derive the extra data.
175 Newer applications should use a more modern algorithm such as \s-1PBKDF2\s0 as
176 defined in PKCS#5v2.1 and provided by \s-1PKCS5_PBKDF2_HMAC.\s0
177 .SH "KEY DERIVATION ALGORITHM"
178 .IX Header "KEY DERIVATION ALGORITHM"
179 The key and \s-1IV\s0 is derived by concatenating D_1, D_2, etc until
180 enough data is available for the key and \s-1IV.\s0 D_i is defined as:
183 \& D_i = HASH^count(D_(i\-1) || data || salt)
186 where || denotes concatenation, D_0 is empty, \s-1HASH\s0 is the digest
187 algorithm in use, HASH^1(data) is simply \s-1HASH\s0(data), HASH^2(data)
188 is \s-1HASH\s0(\s-1HASH\s0(data)) and so on.
190 The initial bytes are used for the key and the subsequent bytes for
193 .IX Header "RETURN VALUES"
194 If \fBdata\fR is \s-1NULL,\s0 then \fBEVP_BytesToKey()\fR returns the number of bytes
195 needed to store the derived key.
196 Otherwise, \fBEVP_BytesToKey()\fR returns the size of the derived key in bytes,
199 .IX Header "SEE ALSO"
200 \&\fBevp\fR\|(7), \fBRAND_bytes\fR\|(3),
201 \&\s-1\fBPKCS5_PBKDF2_HMAC\s0\fR\|(3),
202 \&\fBEVP_EncryptInit\fR\|(3)
204 .IX Header "COPYRIGHT"
205 Copyright 2001\-2016 The OpenSSL Project Authors. All Rights Reserved.
207 Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use
208 this file except in compliance with the License. You can obtain a copy
209 in the file \s-1LICENSE\s0 in the source distribution or at
210 <https://www.openssl.org/source/license.html>.