NAME

       EVP_KDF-TLS13_KDF - The TLS 1.3 EVP_KDF implementation

DESCRIPTION

       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.

   Identity
       "TLS13-KDF" is the name for this implementation; it can be used with the EVP_KDF_fetch() function.

   Supported parameters
       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.

NOTES

       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.

CONFORMING TO

       RFC 8446

SEE ALSO

       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)

HISTORY

       This functionality was added in OpenSSL 3.0.

COPYRIGHT

       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>.