EVP_BytesToKey 3 2005-11-24 0.9.9-dev OpenSSL

NAME

EVP_BytesToKey - password based encryption routine

LIBRARY

libcrypto, -lcrypto

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);

DESCRIPTION

_E_V_P___B_y_t_e_s_T_o_K_e_y_(_) derives a key and IV from various parameters. ttyyppee is the cipher to derive the key and IV for. mmdd is the message digest to use. The ssaalltt paramter is used as a salt in the derivation: it should point to an 8 byte buffer or NULL if no salt is used. ddaattaa is a buffer containing ddaattaall bytes which is used to derive the keying data. ccoouunntt is the iteration count to use. The derived key and IV will be written to kkeeyy and iivv respectively.

NOTES

A typical application of this function is to derive keying material for an encryption algorithm from a password in the ddaattaa parameter.

Increasing the ccoouunntt 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 MMDD55 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

_E_V_P___B_y_t_e_s_T_o_K_e_y_(_) returns the size of the derived key in bytes.

SEE ALSO

_o_p_e_n_s_s_l___e_v_p(3), _o_p_e_n_s_s_l___r_a_n_d(3), _E_V_P___E_n_c_r_y_p_t_I_n_i_t(3)

HISTORY