EVP_BytesToKey — password based encryption routine
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);
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 paramter
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 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.
EVP_BytesToKey() returns the size of the derived key in bytes.
evp(3), rand(3), EVP_EncryptInit(3),