EVP_BYTESTOKEY(3) | Library Functions Manual | EVP_BYTESTOKEY(3)

# NAME

`EVP_BytesToKey` — password based encryption routine

# SYNOPSIS

library “libcrypto”
`#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 parameter is used as a salt in the derivation: it should point to a buffer containing `PKCS5_SALT_LEN` = 8 bytes 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.

# RETURN VALUES

If data is `NULL`, `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.

# SEE ALSO

evp(3), EVP_EncryptInit(3), PKCS5_PBKDF2_HMAC(3)

# HISTORY

`EVP_BytesToKey`() first appeared in SSLeay 0.5.1 and has been available since OpenBSD 2.4.

June 8, 2025 | Debian