EVP_KDF-TLS13_KDF - The TLS 1.3 EVP_KDF implementation
Support for computing the TLS 1.3 version of the HKDF KDF through the
  EVP_KDF API.
The EVP_KDF-TLS13_KDF algorithm implements the HKDF key derivation
    function as used by TLS 1.3.
"TLS13-KDF" is the name for this implementation; it can be used with
  the EVP_KDF_fetch() function.
The supported parameters are:
  - "properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8
    string>
- "digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string>
- "key" (OSSL_KDF_PARAM_KEY) <octet string>
- "salt" (OSSL_KDF_PARAM_SALT) <octet string>
- These parameters work as described in "PARAMETERS" in
      EVP_KDF(3).
- "prefix" (OSSL_KDF_PARAM_PREFIX) <octet
    string>
- This parameter sets the label prefix on the specified TLS 1.3 KDF context.
      For TLS 1.3 this should be set to the ASCII string "tls13 "
      without a trailing zero byte. Refer to RFC 8446 section 7.1 "Key
      Schedule" for details.
- "label" (OSSL_KDF_PARAM_LABEL) <octet string>
- This parameter sets the label on the specified TLS 1.3 KDF context. Refer
      to RFC 8446 section 7.1 "Key Schedule" for details.
- "data" (OSSL_KDF_PARAM_DATA) <octet string>
- This parameter sets the context data on the specified TLS 1.3 KDF context.
      Refer to RFC 8446 section 7.1 "Key Schedule" for details.
- "mode" (OSSL_KDF_PARAM_MODE) <UTF8 string> or
    <integer>
- This parameter sets the mode for the TLS 1.3 KDF operation. There are two
      modes that are currently defined:
  - "EXTRACT_ONLY" or EVP_KDF_HKDF_MODE_EXTRACT_ONLY
- In this mode calling EVP_KDF_derive(3) will just perform the
      extract operation. The value returned will be the intermediate
      fixed-length pseudorandom key K. The keylen parameter must match
      the size of K, which can be looked up by calling
      EVP_KDF_CTX_get_kdf_size() after setting the mode and digest.
    The digest, key and salt values must be set before a key is
        derived otherwise an error will occur. 
- "EXPAND_ONLY" or EVP_KDF_HKDF_MODE_EXPAND_ONLY
- In this mode calling EVP_KDF_derive(3) will just perform the expand
      operation. The input key should be set to the intermediate fixed-length
      pseudorandom key K returned from a previous extract operation.
    The digest, key and info values must be set before a key is
        derived otherwise an error will occur. 
 
This KDF is intended for use by the TLS 1.3 implementation in libssl. It does
  not support all the options and capabilities that HKDF does.
The OSSL_PARAM array passed to EVP_KDF_derive(3) or
    EVP_KDF_CTX_set_params(3) must specify all of the parameters
    required. This KDF does not support a piecemeal approach to providing
  these.
A context for a TLS 1.3 KDF can be obtained by calling:
 EVP_KDF *kdf = EVP_KDF_fetch(NULL, "TLS13-KDF", NULL);
 EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
The output length of a TLS 1.3 KDF expand operation is specified
    via the keylen parameter to the EVP_KDF_derive(3) function.
    When using EVP_KDF_HKDF_MODE_EXTRACT_ONLY the keylen parameter must
    equal the size of the intermediate fixed-length pseudorandom key otherwise
    an error will occur. For that mode, the fixed output size can be looked up
    by calling EVP_KDF_CTX_get_kdf_size() after setting the mode and
    digest on the EVP_KDF_CTX.
EVP_KDF(3), EVP_KDF_CTX_new(3), EVP_KDF_CTX_free(3),
  EVP_KDF_CTX_get_kdf_size(3), EVP_KDF_CTX_set_params(3),
  EVP_KDF_derive(3), "PARAMETERS" in EVP_KDF(3),
  EVP_KDF-HKDF(7)
This functionality was added in OpenSSL 3.0.
Copyright 2021 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (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
    <https://www.openssl.org/source/license.html>.