Add documentation for the scrypt PKEY_METHOD

=pod

=head1 NAME

scrypt - EVP_PKEY scrypt KDF support

=head1 SYNOPSIS

 #include <openssl/kdf.h>

 int EVP_PKEY_CTX_set1_pbe_pass(EVP_PKEY_CTX *pctx, unsigned char *pass,

                                int passlen);

 int EVP_PKEY_CTX_set1_scrypt_salt(EVP_PKEY_CTX *pctx, unsigned char *salt,

                                   int saltlen);

 int EVP_PKEY_CTX_set_scrypt_N(EVP_PKEY_CTX *pctx, uint64_t N);

 int EVP_PKEY_CTX_set_scrypt_r(EVP_PKEY_CTX *pctx, uint64_t r);

 int EVP_PKEY_CTX_set_scrypt_p(EVP_PKEY_CTX *pctx, uint64_t p);

 int EVP_PKEY_CTX_set_scrypt_maxmem_bytes(EVP_PKEY_CTX *pctx, uint64_t maxmem);

=head1 DESCRIPTION

The EVP_PKEY_SCRYPT algorithm implements the scrypt password based key

derivation function, as described in RFC 7914.  It is memory-hard in the sense

that it deliberately requires a significant amount of RAM for efficient

computation. The intention of this is to render brute forcing of passwords on

systems that lack large amounts of main memory (such as GPUs or ASICs)

computationally infeasible.

scrypt provides three work factors that can be customized: N, r and p. N, which

has to be a positive power of two, is the general work factor and scales CPU

time in an approximately linear fashion. r is the block size of the internally

used hash function and p is the parallelization factor. Both r and p need to be

greater than zero. The amount of RAM that scrypt requires for its computation

is roughly (128 * N * r * p) bytes.

In the original paper of Colin Percival ("Stronger Key Derivation via

Sequential Memory-Hard Functions", 2009), the suggested values that give a

computation time of less than 5 seconds on a 2.5 GHz Intel Core 2 Duo are N =

2^20 = 1048576, r = 8, p = 1. Consequently, the required amount of memory for

this computation is roughly 1 GiB. On a more recent CPU (Intel i7-5930K at 3.5

GHz), this computation takes about 3 seconds. When N, r or p are not specified,

they default to 1048576, 8, and 1, respectively. The default amount of RAM that

may be used by scrypt defaults to 1025 MiB.

EVP_PKEY_CTX_set1_pbe_pass() sets the B<passlen> bytes long password.

EVP_PKEY_CTX_set1_scrypt_salt() sets the B<saltlen> bytes long salt value.

EVP_PKEY_CTX_set_scrypt_N(), EVP_PKEY_CTX_set_scrypt_r() and

EVP_PKEY_CTX_set_scrypt_p() configure the work factors N, r and p.

EVP_PKEY_CTX_set_scrypt_maxmem_bytes() sets how much RAM key derivation may

maximally use, given in bytes. If RAM is exceeded because the load factors are

chosen too high, the key derivation will fail.

=head1 STRING CTRLS

scrypt also supports string based control operations via

L<EVP_PKEY_CTX_ctrl_str(3)>.

The B<password> can be directly specified using the B<type> parameter "pass" or

given in hex encoding using the "hexpass" parameter. Similarly, the B<salt> can

either be specified using the B<type> parameter "salt" or in hex encoding by

using the "hexsalt" parameter. The work factors B<N>, B<r> and B<p> as well as

B<maxmem_bytes> can be set by using the parameters "N", "r", "p" and

"maxmem_bytes", respectively.

=head1 NOTES

All these functions are implemented as macros.

A context for scrypt can be obtained by calling:

 EVP_PKEY_CTX *pctx = EVP_PKEY_new_id(EVP_PKEY_SCRYPT, NULL);

The output length of an scrypt key derivation is specified via the length

parameter to the L<EVP_PKEY_derive(3)> function.

=head1 RETURN VALUES

All these functions return 1 for success and 0 or a negative value for failure.

In particular a return value of -2 indicates the operation is not supported by

the public key algorithm.

=head1 EXAMPLE

This example derives a 64-byte long test vector using scrypt using the password

"password", salt "NaCl" and N = 1024, r = 8, p = 16.

 EVP_PKEY_CTX *pctx;

 unsigned char out[64];

 size_t outlen = sizeof(out);

 pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_SCRYPT, NULL);

 if (EVP_PKEY_derive_init(pctx) <= 0) {

     error("EVP_PKEY_derive_init");

 }

 if (EVP_PKEY_CTX_set1_pbe_pass(pctx, "password", 8) <= 0) {

     error("EVP_PKEY_CTX_set1_pbe_pass");

 }

 if (EVP_PKEY_CTX_set1_scrypt_salt(pctx, "NaCl", 4) <= 0) {

     error("EVP_PKEY_CTX_set1_scrypt_salt");

 }

 if (EVP_PKEY_CTX_set_scrypt_N(pctx, 1024) <= 0) {

     error("EVP_PKEY_CTX_set_scrypt_N");

 }

 if (EVP_PKEY_CTX_set_scrypt_r(pctx, 8) <= 0) {

     error("EVP_PKEY_CTX_set_scrypt_r");

 }

 if (EVP_PKEY_CTX_set_scrypt_p(pctx, 16) <= 0) {

     error("EVP_PKEY_CTX_set_scrypt_p");

 }

 if (EVP_PKEY_derive(pctx, out, &outlen) <= 0) {

     error("EVP_PKEY_derive");

 }

 {

     const unsigned char expected[sizeof(out)] = {

         0xfd, 0xba, 0xbe, 0x1c, 0x9d, 0x34, 0x72, 0x00,

         0x78, 0x56, 0xe7, 0x19, 0x0d, 0x01, 0xe9, 0xfe,

         0x7c, 0x6a, 0xd7, 0xcb, 0xc8, 0x23, 0x78, 0x30,

         0xe7, 0x73, 0x76, 0x63, 0x4b, 0x37, 0x31, 0x62,

         0x2e, 0xaf, 0x30, 0xd9, 0x2e, 0x22, 0xa3, 0x88,

         0x6f, 0xf1, 0x09, 0x27, 0x9d, 0x98, 0x30, 0xda,

         0xc7, 0x27, 0xaf, 0xb9, 0x4a, 0x83, 0xee, 0x6d,

         0x83, 0x60, 0xcb, 0xdf, 0xa2, 0xcc, 0x06, 0x40

     };

     assert(!memcmp(out, expected, sizeof(out)));

 }

 EVP_PKEY_CTX_free(pctx);

=head1 CONFORMING TO

RFC 7914

=head1 SEE ALSO

L<EVP_PKEY_CTX_new(3)>,

L<EVP_PKEY_CTX_ctrl_str(3)>,

L<EVP_PKEY_derive(3)>

=head1 COPYRIGHT

Copyright 2017 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License").  You may not use

this file except in compliance with the License.  You can obtain a copy

in the file LICENSE in the source distribution or at

L<https://www.openssl.org/source/license.html>.

=cut