mirror of https://github.com/openssl/openssl
79 lines
2.6 KiB
Plaintext
79 lines
2.6 KiB
Plaintext
=pod
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=head1 NAME
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EVP_BytesToKey - password based encryption routine
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=head1 SYNOPSIS
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#include <openssl/evp.h>
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int EVP_BytesToKey(const EVP_CIPHER *type, const EVP_MD *md,
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const unsigned char *salt,
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const unsigned char *data, int datal, int count,
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unsigned char *key, unsigned char *iv);
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=head1 DESCRIPTION
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EVP_BytesToKey() derives a key and IV from various parameters. B<type> is
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the cipher to derive the key and IV for. B<md> is the message digest to use.
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The B<salt> parameter is used as a salt in the derivation: it should point to
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an 8 byte buffer or NULL if no salt is used. B<data> is a buffer containing
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B<datal> bytes which is used to derive the keying data. B<count> is the
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iteration count to use. The derived key and IV will be written to B<key>
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and B<iv> respectively.
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=head1 NOTES
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A typical application of this function is to derive keying material for an
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encryption algorithm from a password in the B<data> parameter.
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Increasing the B<count> parameter slows down the algorithm which makes it
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harder for an attacker to perform a brute force attack using a large number
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of candidate passwords.
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If the total key and IV length is less than the digest length and
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B<MD5> is used then the derivation algorithm is compatible with PKCS#5 v1.5
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otherwise a non standard extension is used to derive the extra data.
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Newer applications should use a more modern algorithm such as PBKDF2 as
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defined in PKCS#5v2.1 and provided by PKCS5_PBKDF2_HMAC.
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=head1 KEY DERIVATION ALGORITHM
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The key and IV is derived by concatenating D_1, D_2, etc until
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enough data is available for the key and IV. D_i is defined as:
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D_i = HASH^count(D_(i-1) || data || salt)
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where || denotes concatenation, D_0 is empty, HASH is the digest
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algorithm in use, HASH^1(data) is simply HASH(data), HASH^2(data)
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is HASH(HASH(data)) and so on.
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The initial bytes are used for the key and the subsequent bytes for
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the IV.
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=head1 RETURN VALUES
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If B<data> is NULL, then EVP_BytesToKey() returns the number of bytes
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needed to store the derived key.
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Otherwise, EVP_BytesToKey() returns the size of the derived key in bytes,
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or 0 on error.
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=head1 SEE ALSO
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L<evp(7)>, L<RAND_bytes(3)>,
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L<PKCS5_PBKDF2_HMAC(3)>,
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L<EVP_EncryptInit(3)>
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=head1 COPYRIGHT
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Copyright 2001-2016 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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