EVP_BytesToKey (3)
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NAME
EVP_BytesToKey - password based encryption routineSYNOPSIS
#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 andIV
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 a more modern algorithm such as
PBKDF2
as
defined in PKCS#5v2.1 and provided by PKCS5_PBKDF2_HMAC.
KEY DERIVATION ALGORITHM
The key andIV
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
If data isNULL,
then EVP_BytesToKey() returns the number of bytes
needed to store the derived key.
Otherwise, EVP_BytesToKey() returns the size of the derived key in bytes,
or 0 on error.