1.\" $OpenBSD: EVP_BytesToKey.3,v 1.8 2019/06/07 20:46:25 schwarze Exp $ 2.\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100 3.\" 4.\" This file was written by Dr. Stephen Henson <steve@openssl.org>. 5.\" Copyright (c) 2001, 2011, 2013, 2014, 2015 The OpenSSL Project. 6.\" All rights reserved. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 15.\" 2. Redistributions in binary form must reproduce the above copyright 16.\" notice, this list of conditions and the following disclaimer in 17.\" the documentation and/or other materials provided with the 18.\" distribution. 19.\" 20.\" 3. All advertising materials mentioning features or use of this 21.\" software must display the following acknowledgment: 22.\" "This product includes software developed by the OpenSSL Project 23.\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)" 24.\" 25.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to 26.\" endorse or promote products derived from this software without 27.\" prior written permission. For written permission, please contact 28.\" openssl-core@openssl.org. 29.\" 30.\" 5. Products derived from this software may not be called "OpenSSL" 31.\" nor may "OpenSSL" appear in their names without prior written 32.\" permission of the OpenSSL Project. 33.\" 34.\" 6. Redistributions of any form whatsoever must retain the following 35.\" acknowledgment: 36.\" "This product includes software developed by the OpenSSL Project 37.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" 38.\" 39.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY 40.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 41.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 42.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR 43.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 44.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 45.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 46.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 47.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 48.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 49.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED 50.\" OF THE POSSIBILITY OF SUCH DAMAGE. 51.\" 52.Dd $Mdocdate: June 7 2019 $ 53.Dt EVP_BYTESTOKEY 3 54.Os 55.Sh NAME 56.Nm EVP_BytesToKey 57.Nd password based encryption routine 58.Sh SYNOPSIS 59.In openssl/evp.h 60.Ft int 61.Fo EVP_BytesToKey 62.Fa "const EVP_CIPHER *type" 63.Fa "const EVP_MD *md" 64.Fa "const unsigned char *salt" 65.Fa "const unsigned char *data" 66.Fa "int datal" 67.Fa "int count" 68.Fa "unsigned char *key" 69.Fa "unsigned char *iv" 70.Fc 71.Sh DESCRIPTION 72.Fn EVP_BytesToKey 73derives a key and IV from various parameters. 74.Fa type 75is the cipher to derive the key and IV for. 76.Fa md 77is the message digest to use. 78The 79.Fa salt 80parameter is used as a salt in the derivation: 81it should point to an 8-byte buffer or 82.Dv NULL 83if no salt is used. 84.Fa data 85is a buffer containing 86.Fa datal 87bytes which is used to derive the keying data. 88.Fa count 89is the iteration count to use. 90The derived key and IV will be written to 91.Fa key 92and 93.Fa iv , 94respectively. 95.Pp 96A typical application of this function is to derive keying material for 97an encryption algorithm from a password in the 98.Fa data 99parameter. 100.Pp 101Increasing the 102.Fa count 103parameter slows down the algorithm, which makes it harder for an attacker 104to perform a brute force attack using a large number of candidate 105passwords. 106.Pp 107If the total key and IV length is less than the digest length and MD5 108is used, then the derivation algorithm is compatible with PKCS#5 v1.5. 109Otherwise, a non-standard extension is used to derive the extra data. 110.Pp 111Newer applications should use more standard algorithms such as PBKDF2 as 112defined in PKCS#5v2.1 for key derivation. 113.Sh KEY DERIVATION ALGORITHM 114The key and IV is derived by concatenating D_1, D_2, etc. until enough 115data is available for the key and IV. 116D_i is defined recursively as: 117.Pp 118.Dl D_i = HASH^count(D_(i-1) || data || salt) 119.Pp 120where || denotes concatenation, D_0 is empty, HASH is the digest 121algorithm in use, HASH^1(data) is simply HASH(data), HASH^2(data) is 122HASH(HASH(data)) and so on. 123.Pp 124The initial bytes are used for the key and the subsequent bytes for the 125IV. 126.Sh RETURN VALUES 127If 128.Fa data 129is 130.Dv NULL , 131.Fn EVP_BytesToKey 132returns the number of bytes needed to store the derived key. 133Otherwise, 134.Fn EVP_BytesToKey 135returns the size of the derived key in bytes or 0 on error. 136.Sh SEE ALSO 137.Xr evp 3 , 138.Xr EVP_EncryptInit 3 , 139.Xr PKCS5_PBKDF2_HMAC 3 140.Sh HISTORY 141.Fn EVP_BytesToKey 142first appeared in SSLeay 0.5.1 and has been available since 143.Ox 2.4 . 144