199 lines
6.2 KiB
Groff
199 lines
6.2 KiB
Groff
.\" $NetBSD: EVP_BytesToKey.3,v 1.6 2005/11/25 21:09:34 christos Exp $
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.\" ========================================================================
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.\"
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.IX Title "EVP_BytesToKey 3"
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.TH EVP_BytesToKey 3 "2005-11-24" "0.9.8a" "OpenSSL"
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.SH "NAME"
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EVP_BytesToKey \- password based encryption routine
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.SH "LIBRARY"
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libcrypto, -lcrypto
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/evp.h>
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.Ve
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.PP
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.Vb 4
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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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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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\&\fIEVP_BytesToKey()\fR derives a key and \s-1IV\s0 from various parameters. \fBtype\fR is
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the cipher to derive the key and \s-1IV\s0 for. \fBmd\fR is the message digest to use.
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The \fBsalt\fR paramter is used as a salt in the derivation: it should point to
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an 8 byte buffer or \s-1NULL\s0 if no salt is used. \fBdata\fR is a buffer containing
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\&\fBdatal\fR bytes which is used to derive the keying data. \fBcount\fR is the
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iteration count to use. The derived key and \s-1IV\s0 will be written to \fBkey\fR
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and \fBiv\fR respectively.
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.SH "NOTES"
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.IX Header "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 \fBdata\fR parameter.
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.PP
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Increasing the \fBcount\fR parameter slows down the algorithm which makes it
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harder for an attacker to peform a brute force attack using a large number
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of candidate passwords.
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.PP
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If the total key and \s-1IV\s0 length is less than the digest length and
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\&\fB\s-1MD5\s0\fR 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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.PP
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Newer applications should use more standard algorithms such as PKCS#5
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v2.0 for key derivation.
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.SH "KEY DERIVATION ALGORITHM"
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.IX Header "KEY DERIVATION ALGORITHM"
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The key and \s-1IV\s0 is derived by concatenating D_1, D_2, etc until
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enough data is available for the key and \s-1IV\s0. D_i is defined as:
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.PP
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.Vb 1
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\& D_i = HASH^count(D_(i-1) || data || salt)
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.Ve
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.PP
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where || denotes concatentaion, D_0 is empty, \s-1HASH\s0 is the digest
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algorithm in use, HASH^1(data) is simply \s-1HASH\s0(data), HASH^2(data)
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is \s-1HASH\s0(\s-1HASH\s0(data)) and so on.
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.PP
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The initial bytes are used for the key and the subsequent bytes for
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the \s-1IV\s0.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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\&\fIEVP_BytesToKey()\fR returns the size of the derived key in bytes.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fIevp\fR\|(3), \fIrand\fR\|(3),
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\&\fIEVP_EncryptInit\fR\|(3)
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.SH "HISTORY"
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.IX Header "HISTORY"
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