EVP_BYTESTOKEY(3ossl)

💖 EVP_BYTESTOKEY — bestie, it's the password-based encryption routine 💖

okay so this is EVP_BytesToKey(3ossl), a lil OpenSSL page, and we're gonna get you fluent in it. grab a boba, we're doing crypto lore 🧚✨

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🌸 NAME 🌸

EVP_BytesToKey — the password based encryption routine. that's it, that's the whole vibe check 💅

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✨ 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);

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🦋 DESCRIPTION 🦋

so like, EVP_BytesToKey() derives a key and an IV out of a bunch of parameters. iconique behaviour, honestly.

• type — this is the cipher you're deriving the key and IV for. it's giving "which lock are we making a key for".
• md — the message digest to use. the hash girlie of the operation.
• salt — used as a salt in the derivation. it should point to an 8 byte buffer, or be NULL if no salt is used. no salt = no seasoning, lowkey sad.
• data — a buffer containing datal bytes, and it's what gets used to derive the keying data.
• count — the iteration count to use. the more times we go around, the slower it gets (which is a flex, we'll get to that).
• key and iv — the derived key and IV get written here respectively. they're the output besties 🫶

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🎀 NOTES 🎀

• a typical application of this function is deriving keying material for an encryption algorithm from a password sitting in the data parameter. classic move, very iykyk.
• increasing the count parameter slows down the algorithm, which makes it harder for an attacker to brute force attack using a large number of candidate passwords. yes bestie, slow is a feature here, not a bug 😌
• 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. so close to standard, so far from god 💀
• newer applications should use a more modern algorithm such as PBKDF2 as defined in PKCS#5v2.1 and provided by PKCS5_PBKDF2_HMAC. bestie, the glow-up is right there. take it. ✨

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👑 KEY DERIVATION ALGORITHM 👑

the key and IV get derived by concatenating D_1, D_2, etc, until enough data is available for the key and IV. and D_i is defined 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

and then: the initial bytes are used for the key and the subsequent bytes for the IV. first come first served, literally 💅

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🌈 RETURN VALUES 🌈

• if data is NULL, then EVP_BytesToKey() returns the number of bytes needed to store the derived key. just window shopping, no deriving.
• otherwise, EVP_BytesToKey() returns the size of the derived key in bytes, or 0 on error. zero. the number that says "we failed, bestie 😭".

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⭐ SEE ALSO ⭐

• evp(7)
• RAND_bytes(3)
• PKCS5_PBKDF2_HMAC(3)
• EVP_EncryptInit(3)

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💜 COPYRIGHT 💜

Copyright 2001-2016 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

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and that's the whole page, bestie 💖 this was OpenSSL 3.5.9, dated 2026-09-29, page EVP_BYTESTOKEY(3ossl) — go forth and derive responsibly, mwah 🎀✨💅