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.\" ========================================================================
.\"
.IX Title "EVP_BytesToKey 3"
.TH EVP_BytesToKey 3 "2019-12-20" "1.0.2u" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
EVP_BytesToKey \- password based encryption routine
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/evp.h>
\&
\& int EVP_BytesToKey(const EVP_CIPHER *type,const EVP_MD *md,
\&                       const unsigned char *salt,
\&                       const unsigned char *data, int datal, int count,
\&                       unsigned char *key,unsigned char *iv);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBEVP_BytesToKey()\fR derives a key and \s-1IV\s0 from various parameters. \fBtype\fR is
the cipher to derive the key and \s-1IV\s0 for. \fBmd\fR is the message digest to use.
The \fBsalt\fR parameter is used as a salt in the derivation: it should point to
an 8 byte buffer or \s-1NULL\s0 if no salt is used. \fBdata\fR is a buffer containing
\&\fBdatal\fR bytes which is used to derive the keying data. \fBcount\fR is the
iteration count to use. The derived key and \s-1IV\s0 will be written to \fBkey\fR
and \fBiv\fR respectively.
.SH "NOTES"
.IX Header "NOTES"
A typical application of this function is to derive keying material for an
encryption algorithm from a password in the \fBdata\fR parameter.
.PP
Increasing the \fBcount\fR parameter slows down the algorithm which makes it
harder for an attacker to peform a brute force attack using a large number
of candidate passwords.
.PP
If the total key and \s-1IV\s0 length is less than the digest length and
\&\fB\s-1MD5\s0\fR 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 a more modern algorithm such as \s-1PBKDF2\s0 as
defined in PKCS#5v2.1 and provided by \s-1PKCS5_PBKDF2_HMAC.\s0
.SH "KEY DERIVATION ALGORITHM"
.IX Header "KEY DERIVATION ALGORITHM"
The key and \s-1IV\s0 is derived by concatenating D_1, D_2, etc until
enough data is available for the key and \s-1IV.\s0 D_i is defined as:
.PP
.Vb 1
\&        D_i = HASH^count(D_(i\-1) || data || salt)
.Ve
.PP
where || denotes concatentaion, D_0 is empty, \s-1HASH\s0 is the digest
algorithm in use, HASH^1(data) is simply \s-1HASH\s0(data), HASH^2(data)
is \s-1HASH\s0(\s-1HASH\s0(data)) and so on.
.PP
The initial bytes are used for the key and the subsequent bytes for
the \s-1IV.\s0
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
If \fBdata\fR is \s-1NULL,\s0 then \fBEVP_BytesToKey()\fR returns the number of bytes
needed to store the derived key.
Otherwise, \fBEVP_BytesToKey()\fR returns the size of the derived key in bytes,
or 0 on error.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBevp\fR\|(3), \fBrand\fR\|(3),
\&\fBEVP_EncryptInit\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"