Add documentation for EVP_PKEY_TLS1_PRF

Reviewed-by: NMatt Caswell <matt@openssl.org>

doc/crypto/EVP_PKEY_TLS1_PRF.pod

+=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)>,
+
+=cut