password based encryption
*type, const EVP_MD *md, const
unsigned char *salt, const unsigned char *data,
int datal, int count,
unsigned char *key, unsigned char
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.
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 perform 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 PBKDF2 as defined in PKCS#5v2.1 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 recursively as:
D_i = HASH^count(D_(i-1) || data || salt)
where || denotes concatenation, 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.
If data is
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.
evp(3), EVP_EncryptInit(3), PKCS5_PBKDF2_HMAC(3)
EVP_BytesToKey() first appeared in SSLeay
0.5.1 and has been available since OpenBSD 2.4.