NAME¶
EVP_BytesToKey - password based encryption routine
SYNOPSIS¶
#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);
DESCRIPTION¶
EVP_BytesToKey() derives a key and IV from various parameters.
type is the cipher to derive the key and IV for.
md is the
message digest to use. The
salt parameter is used as a salt in the
derivation: it should point to an 8 byte buffer or NULL if no salt is used.
data is a buffer containing
datal bytes which is used to derive
the keying data.
count is the iteration count to use. The derived key
and IV will be written to
key and
iv respectively.
NOTES¶
A typical application of this function is to derive keying material for an
encryption algorithm from a password in the
data parameter.
Increasing the
count 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.
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.
Newer applications should use more standard algorithms such as PKCS#5 v2.0 for
key derivation.
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 as:
D_i = HASH^count(D_(i-1) || data || salt)
where || denotes concatentaion, 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.
The initial bytes are used for the key and the subsequent bytes for the IV.
RETURN VALUES¶
EVP_BytesToKey() returns the size of the derived key in bytes.
SEE ALSO¶
evp(3),
rand(3),
EVP_EncryptInit(3)
HISTORY¶