=pod =head1 NAME EVP_BytesToKey - password based encryption routine =head1 SYNOPSIS #include 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); =head1 DESCRIPTION EVP_BytesToKey() derives a key and IV from various parameters. B is the cipher to derive the key and IV for. B is the message digest to use. The B paramter is used as a salt in the derivation: it should point to an 8 byte buffer or NULL if no salt is used. B is a buffer containing B bytes which is used to derive the keying data. B is the iteration count to use. The derived key and IV will be written to B and B respectively. =head1 NOTES A typical application of this function is to derive keying material for an encryption algorithm from a password in the B parameter. Increasing the B 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 B 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. =head1 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. =head1 RETURN VALUES EVP_BytesToKey() returns the size of the derived key in bytes. =head1 SEE ALSO L, L, L =head1 HISTORY =cut