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EVP_BytesToKey(3)                                  OpenSSL                                 EVP_BytesToKey(3)



NAME
       EVP_BytesToKey - password based encryption routine

SYNOPSIS
        #include <openssl/evp.h>

        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);

DESCRIPTION
       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.

NOTES
       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.

RETURN VALUES
       EVP_BytesToKey() returns the size of the derived key in bytes.

SEE ALSO
       evp(3), rand(3), EVP_EncryptInit(3),

HISTORY
0.9.7l                                           2003-09-30                                EVP_BytesToKey(3)