Section: OpenSSL (3SSL)
Return to Main Contents
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. type
the cipher to derive the key and IV
is the message digest to use.
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
bytes which is used to derive the keying data. count
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 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
returns the size of the derived key in bytes.
- KEY DERIVATION ALGORITHM
- RETURN VALUES
- SEE ALSO