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.\" $OpenBSD: PKCS5_PBKDF2_HMAC.3,v 1.2 2016/11/06 15:52:50 jmc Exp $
.\"
.Dd $Mdocdate: November 6 2016 $
.Dt PKCS5_PBKDF2_HMAC 3
.Os
.Sh NAME
.Nm PKCS5_PBKDF2_HMAC ,
.Nm PKCS5_PBKDF2_HMAC_SHA1
.Nd password based derivation routines with salt and iteration count
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo PKCS5_PBKDF2_HMAC
.Fa "const char *pass"
.Fa "int passlen"
.Fa "const unsigned char *salt"
.Fa "int saltlen"
.Fa "int iter"
.Fa "const EVP_MD *digest"
.Fa "int keylen"
.Fa "unsigned char *out"
.Fc
.Ft int
.Fo PKCS5_PBKDF2_HMAC_SHA1
.Fa "const char *pass"
.Fa "int passlen"
.Fa "const unsigned char *salt"
.Fa "int saltlen"
.Fa "int iter"
.Fa "int keylen"
.Fa "unsigned char *out"
.Fc
.Sh DESCRIPTION
.Fn PKCS5_PBKDF2_HMAC
derives a key from a password using a salt and iteration count as
specified in RFC 2898.
.Pp
.Fa pass
is the password used in the derivation of length
.Fa passlen .
.Fa pass
is an optional parameter and can be
.Dv NULL .
If
.Fa passlen
is -1, then the function will calculate the length of
.Fa pass
using
.Xr strlen 3 .
.Pp
.Fa salt
is the salt used in the derivation of length
.Fa saltlen .
If the
.Fa salt
is
.Dv NULL ,
then
.Fa saltlen
must be 0.
The function will not attempt to calculate the length of the
.Fa salt
because it is not assumed to be NUL terminated.
.Pp
.Fa iter
is the iteration count and its value should be greater than or equal to 1.
RFC 2898 suggests an iteration count of at least 1000.
Any
.Fa iter
less than 1 is treated as a single iteration.
.Pp
.Fa digest
is the message digest function used in the derivation.
Values include any of the EVP_* message digests.
.Fn PKCS5_PBKDF2_HMAC_SHA1
calls
.Fn PKCS5_PBKDF2_HMAC
with
.Xr EVP_sha1 3 .
.Pp
The derived key will be written to
.Fa out .
The size of the
.Fa out
buffer is specified via
.Fa keylen .
.Pp
A typical application of this function is to derive keying material for
an encryption algorithm from a password in the
.Fa pass ,
a salt in
.Fa salt ,
and an iteration count.
.Pp
Increasing the
.Fa iter
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.
.Sh RETURN VALUES
.Fn PKCS5_PBKDF2_HMAC
and
.Fn PBKCS5_PBKDF2_HMAC_SHA1
return 1 on success or 0 on error.
.Sh SEE ALSO
.Xr evp 3 ,
.Xr EVP_BytesToKey 3 ,
.Xr rand 3
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