EVP_BytesToKey.pod revision 279264 
1=pod
2
3=head1 NAME
4
5EVP_BytesToKey - password based encryption routine
6
7=head1 SYNOPSIS
8
9 #include <openssl/evp.h>
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11 int EVP_BytesToKey(const EVP_CIPHER *type,const EVP_MD *md,
12                       const unsigned char *salt,
13                       const unsigned char *data, int datal, int count,
14                       unsigned char *key,unsigned char *iv);
15
16=head1 DESCRIPTION
17
18EVP_BytesToKey() derives a key and IV from various parameters. B<type> is
19the cipher to derive the key and IV for. B<md> is the message digest to use.
20The B<salt> parameter is used as a salt in the derivation: it should point to
21an 8 byte buffer or NULL if no salt is used. B<data> is a buffer containing
22B<datal> bytes which is used to derive the keying data. B<count> is the
23iteration count to use. The derived key and IV will be written to B<key>
24and B<iv> respectively.
25
26=head1 NOTES
27
28A typical application of this function is to derive keying material for an
29encryption algorithm from a password in the B<data> parameter.
30
31Increasing the B<count> parameter slows down the algorithm which makes it
32harder for an attacker to peform a brute force attack using a large number
33of candidate passwords.
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35If the total key and IV length is less than the digest length and
36B<MD5> is used then the derivation algorithm is compatible with PKCS#5 v1.5
37otherwise a non standard extension is used to derive the extra data.
38
39Newer applications should use more standard algorithms such as PKCS#5
40v2.0 for key derivation.
41
42=head1 KEY DERIVATION ALGORITHM
43
44The key and IV is derived by concatenating D_1, D_2, etc until
45enough data is available for the key and IV. D_i is defined as:
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47	D_i = HASH^count(D_(i-1) || data || salt)
48
49where || denotes concatentaion, D_0 is empty, HASH is the digest
50algorithm in use, HASH^1(data) is simply HASH(data), HASH^2(data)
51is HASH(HASH(data)) and so on.
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53The initial bytes are used for the key and the subsequent bytes for
54the IV.
55
56=head1 RETURN VALUES
57
58EVP_BytesToKey() returns the size of the derived key in bytes.
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60=head1 SEE ALSO
61
62L<evp(3)|evp(3)>, L<rand(3)|rand(3)>,
63L<EVP_EncryptInit(3)|EVP_EncryptInit(3)>
64
65=head1 HISTORY
66
67=cut
68