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- =pod
- =head1 NAME
- EVP_BytesToKey - password based encryption routine
- =head1 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);
- =head1 DESCRIPTION
- EVP_BytesToKey() derives a key and IV from various parameters. B<type> is
- the cipher to derive the key and IV for. B<md> is the message digest to use.
- The B<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. B<data> is a buffer containing
- B<datal> bytes which is used to derive the keying data. B<count> is the
- iteration count to use. The derived key and IV will be written to B<key>
- and B<iv> respectively.
- =head1 NOTES
- A typical application of this function is to derive keying material for an
- encryption algorithm from a password in the B<data> parameter.
- Increasing the B<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
- B<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.
- =head1 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.
- =head1 RETURN VALUES
- EVP_BytesToKey() returns the size of the derived key in bytes.
- =head1 SEE ALSO
- L<evp(3)|evp(3)>, L<rand(3)|rand(3)>,
- L<EVP_EncryptInit(3)|EVP_EncryptInit(3)>
- =head1 HISTORY
- =cut
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