EVP_BytesToKey - password based encryption routine
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);
derives a key and IV from various parameters.
is the cipher to derive the key and IV for. md
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.
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
A typical application of this function is to derive keying material for an
encryption algorithm from a password in the data
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
If the total key and IV length is less than the digest length and MD5
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 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
The initial bytes are used for the key and the subsequent bytes for the IV.
returns the size of the derived key in bytes.