Edit File: EVP_BytesToKey.3
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Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l .nh .SH "NAME" EVP_BytesToKey \- password based encryption routine .SH "SYNOPSIS" .IX Header "SYNOPSIS" .Vb 1 \& #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); .Ve .SH "DESCRIPTION" .IX Header "DESCRIPTION" \&\fBEVP_BytesToKey()\fR derives a key and \s-1IV\s0 from various parameters. \fBtype\fR is the cipher to derive the key and \s-1IV\s0 for. \fBmd\fR is the message digest to use. The \fBsalt\fR parameter is used as a salt in the derivation: it should point to an 8 byte buffer or \s-1NULL\s0 if no salt is used. \fBdata\fR is a buffer containing \&\fBdatal\fR bytes which is used to derive the keying data. \fBcount\fR is the iteration count to use. The derived key and \s-1IV\s0 will be written to \fBkey\fR and \fBiv\fR respectively. .SH "NOTES" .IX Header "NOTES" A typical application of this function is to derive keying material for an encryption algorithm from a password in the \fBdata\fR parameter. .PP Increasing the \fBcount\fR 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. .PP If the total key and \s-1IV\s0 length is less than the digest length and \&\fB\s-1MD5\s0\fR 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. .PP Newer applications should use a more modern algorithm such as \s-1PBKDF2\s0 as defined in PKCS#5v2.1 and provided by \s-1PKCS5_PBKDF2_HMAC.\s0 .SH "KEY DERIVATION ALGORITHM" .IX Header "KEY DERIVATION ALGORITHM" The key and \s-1IV\s0 is derived by concatenating D_1, D_2, etc until enough data is available for the key and \s-1IV.\s0 D_i is defined as: .PP .Vb 1 \& D_i = HASH^count(D_(i\-1) || data || salt) .Ve .PP where || denotes concatentaion, D_0 is empty, \s-1HASH\s0 is the digest algorithm in use, HASH^1(data) is simply \s-1HASH\s0(data), HASH^2(data) is \s-1HASH\s0(\s-1HASH\s0(data)) and so on. .PP The initial bytes are used for the key and the subsequent bytes for the \s-1IV.\s0 .SH "RETURN VALUES" .IX Header "RETURN VALUES" If \fBdata\fR is \s-1NULL,\s0 then \fBEVP_BytesToKey()\fR returns the number of bytes needed to store the derived key. Otherwise, \fBEVP_BytesToKey()\fR returns the size of the derived key in bytes, or 0 on error. .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fBevp\fR\|(3), \fBrand\fR\|(3), \&\fBEVP_EncryptInit\fR\|(3) .SH "HISTORY" .IX Header "HISTORY"