.\"	$OpenBSD: EVP_BytesToKey.3,v 1.3 2016/11/21 09:56:35 schwarze Exp $
.\"
.Dd $Mdocdate: November 21 2016 $
.Dt EVP_BYTESTOKEY 3
.Os
.Sh NAME
.Nm EVP_BytesToKey
.Nd password based encryption routine
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_BytesToKey
.Fa "const EVP_CIPHER *type"
.Fa "const EVP_MD *md"
.Fa "const unsigned char *salt"
.Fa "const unsigned char *data"
.Fa "int datal"
.Fa "int count"
.Fa "unsigned char *key"
.Fa "unsigned char *iv"
.Fc
.Sh DESCRIPTION
.Fn EVP_BytesToKey
derives a key and IV from various parameters.
.Fa type
is the cipher to derive the key and IV for.
.Fa md
is the message digest to use.
The
.Fa salt
parameter is used as a salt in the derivation: it should point to an 8
byte buffer or
.Dv NULL
if no salt is used.
.Fa data
is a buffer containing
.Fa datal
bytes which is used to derive the keying data.
.Fa count
is the iteration count to use.
The derived key and IV will be written to
.Fa key
and
.Fa iv ,
respectively.
.Pp
A typical application of this function is to derive keying material for
an encryption algorithm from a password in the
.Fa data
parameter.
.Pp
Increasing the
.Fa count
parameter slows down the algorithm which makes it harder for an attacker
to perform a brute force attack using a large number of candidate
passwords.
.Pp
If the total key and IV length is less than the digest length and MD5
is used, then the derivation algorithm is compatible with PKCS#5 v1.5.
Otherwise, a non standard extension is used to derive the extra data.
.Pp
Newer applications should use more standard algorithms such as PBKDF2 as
defined in PKCS#5v2.1 for key derivation.
.Sh KEY DERIVATION ALGORITHM
The key and IV is derived by concatenating D_1, D_2, etc until enough
data is available for the key and IV.
D_i is defined recursively as:
.Pp
.Dl D_i = HASH^count(D_(i-1) || data || salt)
.Pp
where || denotes concatenation, D_0 is empty, HASH is the digest
algorithm in use, HASH^1(data) is simply HASH(data), HASH^2(data) is
HASH(HASH(data)) and so on.
.Pp
The initial bytes are used for the key and the subsequent bytes for the
IV.
.Sh RETURN VALUES
.Fn EVP_BytesToKey
returns the size of the derived key in bytes.
.Sh SEE ALSO
.Xr evp 3 ,
.Xr EVP_EncryptInit 3 ,
.Xr PKCS5_PBKDF2_HMAC 3 ,
.Xr RAND_bytes 3