1.\" $OpenBSD: EVP_BytesToKey.3,v 1.9 2024/12/05 15:12:37 schwarze Exp $ 2.\" full merge up to: 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: December 5 2024 $ 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 a buffer containing 82.Dv PKCS5_SALT_LEN No = 8 83bytes or 84.Dv NULL 85if no salt is used. 86.Fa data 87is a buffer containing 88.Fa datal 89bytes which is used to derive the keying data. 90.Fa count 91is the iteration count to use. 92The derived key and IV will be written to 93.Fa key 94and 95.Fa iv , 96respectively. 97.Pp 98A typical application of this function is to derive keying material for 99an encryption algorithm from a password in the 100.Fa data 101parameter. 102.Pp 103Increasing the 104.Fa count 105parameter slows down the algorithm, which makes it harder for an attacker 106to perform a brute force attack using a large number of candidate 107passwords. 108.Pp 109If the total key and IV length is less than the digest length and MD5 110is used, then the derivation algorithm is compatible with PKCS#5 v1.5. 111Otherwise, a non-standard extension is used to derive the extra data. 112.Pp 113Newer applications should use more standard algorithms such as PBKDF2 as 114defined in PKCS#5v2.1 for key derivation. 115.Sh KEY DERIVATION ALGORITHM 116The key and IV is derived by concatenating D_1, D_2, etc. until enough 117data is available for the key and IV. 118D_i is defined recursively as: 119.Pp 120.Dl D_i = HASH^count(D_(i-1) || data || salt) 121.Pp 122where || denotes concatenation, D_0 is empty, HASH is the digest 123algorithm in use, HASH^1(data) is simply HASH(data), HASH^2(data) is 124HASH(HASH(data)) and so on. 125.Pp 126The initial bytes are used for the key and the subsequent bytes for the 127IV. 128.Sh RETURN VALUES 129If 130.Fa data 131is 132.Dv NULL , 133.Fn EVP_BytesToKey 134returns the number of bytes needed to store the derived key. 135Otherwise, 136.Fn EVP_BytesToKey 137returns the size of the derived key in bytes or 0 on error. 138.Sh SEE ALSO 139.Xr evp 3 , 140.Xr EVP_EncryptInit 3 , 141.Xr PKCS5_PBKDF2_HMAC 3 142.Sh HISTORY 143.Fn EVP_BytesToKey 144first appeared in SSLeay 0.5.1 and has been available since 145.Ox 2.4 . 146