Add documentation for EVP_PKEY_TLS1_PRF

=pod

=head1 NAME

EVP_PKEY_TLS1_PRF, EVP_PKEY_CTX_set_tls1_prf_md,

EVP_PKEY_CTX_set1_tls1_prf_secret, EVP_PKEY_CTX_add1_tls1_prf_seed -

TLS PRF key derivation algorithm

=head1 SYNOPSIS

 #include <openssl/kdf.h>

 int EVP_PKEY_CTX_set_tls1_prf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);

 int EVP_PKEY_CTX_set1_tls1_prf_secret(EVP_PKEY_CTX *pctx,

                                       unsigned char *sec, int seclen);

 int EVP_PKEY_CTX_add1_tls1_prf_seed(EVP_PKEY_CTX *pctx, seed, seedlen)

                                     unsigned char *seed, int seedlen);

=head1 DESCRIPTION

The EVP_PKEY_TLS1_PRF algorithm implements the PRF key derivation function for

TLS. It has no associated private key and only implements key derivation

using EVP_PKEY_derive().

EVP_PKEY_set_tls1_prf_md() sets the message digest associated with the

TLS PRF. EVP_md5_sha1() is treated as a special case which uses the PRF

algorithm using both B<MD5> and B<SHA1> as used in TLS 1.0 and 1.1.

EVP_PKEY_CTX_set_tls1_prf_secret() sets the secret value of the TLS PRF

to B<seclen> bytes of the buffer B<sec>. Any existing secret value is replaced

and any seed is reset.

EVP_PKEY_CTX_add1_tls1_prf_seed() sets the seed to B<seedlen> bytes of B<seed>.

If a seed is already set it is appended to the existing value.

=head1 NOTES

All these functions are implemented as macros.

A context for the TLS PRF can be obtained by calling:

 EVP_PKEY_CTX *pctx = EVP_PKEY_new_id(EVP_PKEY_TLS1_PRF, NULL);

The digest, secret value and seed must be set before a key is derived or an

error occurs.

The total length of all seeds cannot exceed 1024 bytes in length: this should

be more than enough for any normal use of the TLS PRF.

The output length of the PRF is specified by the length parameter in the

EVP_PKEY_derive() function. Since the output length is variable, setting

the buffer to B<NULL> is not meaningful for the TLS PRF.

Optimised versions of the TLS PRF can be implemented in an ENGINE.

=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 10 bytes using SHA-256 with the secret key "secret"

and seed value "seed":

 EVP_PKEY_CTX *pctx;

 unsigned char out[10];

 size_t outlen = sizeof(out);

 pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_TLS1_PRF, NULL);

 if (EVP_PKEY_derive_init(pctx) <= 0)

    /* Error */

 if (EVP_PKEY_CTX_set_tls1_prf_md(pctx, EVP_sha256()) <= 0)

    /* Error */

 if (EVP_PKEY_CTX_set1_tls1_prf_secret(pctx, "secret", 6) <= 0)

    /* Error */

 if (EVP_PKEY_CTX_add1_tls1_prf_seed(pctx, "seed", 4) <= 0)

    /* Error */

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

    /* Error */

=head1 SEE ALSO

L<EVP_PKEY_CTX_new(3)>,

L<EVP_PKEY_derive(3)>,

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